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. CHAPTER ‘I3

COMMUNICATIONS AND NAVIGATION SYSTEMS

GENERALCommunications and navigation are the two

major functionsiof airborne radio. Communicationsystems primarily involve voice transmission andreception between aircraft or aircraftand groundstations. Radios are used in aircraftas navigationalaids in a number. of applications. They range froma simple radio direction finder to navigational sys-tems which use computers and other advanced elec-tronic techniques to automatically solve the naviga-tional problems for an entire flight. Marker beaconreceivers, instrument landing systems (involvingradio signals for glide slope and direction), dis»tance measuring equipment, radar, area navigationsystems, and omnidirectional radio receivers are buta few basic applications of airborne radio naviga-tion systems available for installation and use inaircraft.

Safe aircraft operation is dependent to a largedegree upon the satisfactory performance of theairborne communications and navigation systems.Reliability and performance of the radio and radarsystem are directly related to the skills of those whoperform the maintenance.

Federal Aviation Regulations require an inspec-tion of radio equipment installations at regular "in-tervals. These inspections include a visual examina-tion for security of -attachment, condition of wiring,bonding, shock mounts, radio racks and supportingstructure. In addition, al functional checlt is usuallyperformed to determine that the equipment is opera-ting properly and that its operation does not inter-fere with the operation of other systems.

Aircraft mechanics’ responsibilities include the in-stallation and inspection of radios, antennas, navi-gation equipment, and associated wiring. In addi-tion, the FAA has certified radio repair facilities toperform maintenance on radios, antennas, naviga-tion, and radar equipment. Transmitting equipmentis calibrated by persons licensed by the FCC (Fed-eral Communications Commission).

To be in a more favorable position to inspect

system installations, a mechanic should possesssome basic knowledge and understanding of theprinciples, purposes, and operation of the radioequipment used in aircraft. V _ ‘

Because of the many different maltes and modelsof equipment and the various systems in use, it isnot feasible to describe each in this handbook. Theinformation presented in this chapter is general innature and provides a broad introduction to radio,its principles, and application to aircraft from atnechanic’s viewpoint.

BASIC RADIO PRINCIPLESThe principle of radio communication can be il-

lustrated by using a simple transformer. As shownin figure 13-1, closing the switch in the primarycircuit causes the lamp in the secondary circuit tobe illuminated. Opening the switch extinguishes thelight.

Alternating electromagnetic fieldPrimarycircuit mi

at —La-m

-. - 1@II=I= <= P_ \

Switch H Secondarycircuit. _

Iron core

FIOUlt£ I3-1. A simple transformer circuit.

There is no direct connection between the pri-mary and secondary circuits. The energy that illu-minates the light is transmitted by an alternatingelectromagnetic field in the core of the transformer.This is a simple form of wirelws control of onecircuit (the secondary) by another circuit (the pri-mary). V

The basic concept of radio communications in-volves the transmission and reception of electromag-netic (radio) energy waves through space. Alternat-ing current passing through a conductor createselectromagnetic fields around the conductor. Energy

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is alternately stored in these fields and returned tothe conductor. As the frequency of current alterna-tion increases, lcss and less of the energy stored inthe field returns to the conductor. Instead of retum-ing, the energy is radiated into space in the form ofelectromagnetic waves. A conductor radiating inthis manner is called the transmitting antenna.

For an antenna to radiate efiiciently a transmittermust supply it with an altemating current of theselected frequency. The frequency of the radio waveradiated will be equal to the frequency of the ap-plied current. When current flows through a trans-mitting antenna, radio waves are radiated in alldirections in much the same way that waves travelon the surface of a pond into which a rock has beenthrown. Radio waves travel ata speed of approxi-mately 186,000 miles per second.

If a radiated electromagnetic field passes througha conductor, some of the energy in the field will setelectrons in motion in the conductor. This electronflow constitutes a current that varies with changesin the electromagnetic field. Thus, a variation of thecurrent in a radiating antenna causes a similarvarying current in a conductor (receiving antenna)at a distant location. Anyintelligence being pro-duced as current in a transmitting antenna will bereproduced as current in a receiving antenna.

Frequency BandsThe radio frequency portion of the electromag-

netic spectrum extends from approximately 30 kHz(kilohertz) to 30,000 MHz (Megahertz). As a mat-ter of convenience, this part of the spectrum isdivided into frequency bands. Each band or fre-quency range produces difierent efiects in trans-

mission. The radio frequency hands proven mostuseful and presently in use are:

Frequency Range V BandLow frequency (L/F) ____ __30 to 300 kHzMedium frequency (M/F) __300 to 3,000 kHzHigh frequency (H/F) ___-_3,000 kHz to 30 MHzVery high frequency (VHF)_30 to 300 MHzUltra high frequency (UHF) _300 to 3,000 MHzSuperhigh frequency (SI-IF) _3,000 to 30,000 MHz

In practice, radio equipment usually covers onlya portion of the designated band, e.g., civil VI-IFequipment normally operates at frequencies between108.0 MHz and 135.95 MHz.

BASIC EQUIPMENT COMPONENTSThe basic components (figure 13—-2) of a commu-

nication system are: microphone, transmitter, trans-mitting antenna, receiving antenna, receiver, and 8headset or loudspeaker.

TransmittersA transmitter may be considered as a generator

which changes electrical power into radio waves. Atransmitter must perfonn these functions: (1) Gen-erate a RF (radio frequency) signal, (2) amplifythe RF signal, and (3) provide a means of placingintelligence on the signal.

The transmitter contains an osfiillflmr Birfillit t°generate the RF signal -(or a subhannonic of thetransmitter frequency, if frequency doublers or mul-tipliers are used) and amplifier circuits to increasethe output oithe oscillator to the power level re-quired for proper operation.

The voice (audio) intelligence is added i0 the RF

Transrnittin T7 Receiving

Microphone

Transmitter

Santenna imtelmfl

Headset

535555355»

Receiver

Loudspeaker

Ftctmn 13-2. Basic communication equipment.

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signal by a special circuit called the modulator. Themodulator uses the audio signal to vary the ampli-tude or frequency of the RF signal. If the amplitudeis varied, the process is called amplitude modulationor AM. If the frequency is varied, the process isknown as frequency modulation or FM.

Transmitters take many forms, have varying de-grees of complexity and develop various levels ofpower. The amount of power generated by a trans-mitter afiects the strengthvof the electromagneticfield radiating from the antenna. Thus, it followsthat the higher the power output from a transmitter,the greater the distance its signal may be received.

VHF transmitters used in single engine and lighttwin-engine aircraft vary in power output fro:n_1watt to 30 watts, depending on the particular modelradio. However, radios having 3 to 5 watt ratingsare used most frequently. Executive and large trans-port aircraft are usually equipped with VHF trans-mitters having a~ power output of 20 to 30 watts.

Aviation communication transmitters are crystal-controlled in order to meet the frequency tolerancerequirements of the FCC. Most transmitters are se-lectable for more than one frequency. The fre-quency of the channel selected is determined by acrystal. Transmitters may have from one to 680channels.

ReceiversThe communications receiver must select radio

frequency signals and convert the intelligence con-rained on these signals into a usable form; eitheraudible signals for communication and audible orvisual signal for navigation.

Radio waves of many frequencies are present inthe air. A receiver must be able to select the desiredfrequency from all those present and amplify thesmall a.c. signal voltage.

The receiver contains a demodulator circuit toremove the intelligence. If the demodulator circuitis sensitive to amplitude changes, it is used in AMsets and called a detector. A demodulator circuitthat is sensitive to frequency changes is used forFM reception and is known as a discriminator.

Amplifying circuits within the receiver increasesthe audio signal to a power level which will operatethe headset or loudspeaker properly.

AntennaAn antenna is a special type of electrical circuit

designed to radiate and receive electromagnetic en-ergy. As mentioned previously, a transmitting an-

tenna is-a conductor which radiates electromagneticwaves when a radio frequency current is passedthrough it. Antennas vary in shape and design(figure 13-3) depending upon the frequency to betransmitted, and specific purposes they must serve.In general, communication transmitting stations ra-diate signals in all directions. However, special an-tennas are designed that radiate only in certaindirections or certain beam patterns.

. . V F mm ' 'Communication, H co “mean”navigation

c“.::5JMarker beacon

Distance measuringequipment

P-

Q HQ1/°

Gudescope VHF communication

FIGURE 13-3. Antennas.

WQThe receiving antenna must intercept the electro-

magnetic waves that are present in the air. Theshape and size of the receiving antenna will alsovary according to the specific purpose for which itis intended. In airborne communications the sameantenna is normally used for both transmission andreception of signals.

MicrophonesA microphone is essentially an energy converter

that changes acoustical (sound) energy into corre-sponding electrical energy. When spoken into a mi-crophone, the audio pressure waves generated strikethe diaphragm of the microphone causing it tomove in and out in accordance with the instanta-neous pressure delivered to it. The diaphragm is

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attached to a device that causes current to flow inproportion to the pressure applied. ._

For good quality sound, the electrical wavesfrom a microphone must correspond closely in mag-nitude and frequency to the sound waves that causethem, so that no new frequencies are introduced. Adesirable characteristic is the ability of the micro-phone to favor sounds coming from a nearby sourceover random sounds coming from a relativelygreater distance. When talking into this type ofmicrophone, the lips must be held as close as possi-ble to the diaphragm.

Persons inexperienced in the use of the micro-phone are usually surprised at the quality of theirown transmissions when they are taped and playedback. Words quite clear when spoken to anotherperson can be almost unintelligible over the radio.Readable radio transmissions depend on the follow-ing factors: (1) Voice amplitude, (2) rate ofspeech, and (3) pronunciation and phrasing. Clar-ity increases with amplitude up to a level just shortof shouting. When using a microphone, speakloudly, without exerting extreme effort. Talk slowlyenough so that each word is spoken distinctly.Avoid using unnecessary words.

POWER SUPPLYThe power supply is a component that furnishes

the correct voltages and current needed to operatethe communication-equipment. The power supplycan be a separate component or‘ it may be containedwithin the equipment it supplies. Electromechanicaldevices used as electronic power supplies includedynamotors and inverters. '

The dynamotor performs the dual functions ofmotor and generator, changing the relatively lowvoltage of the aircraft electrical system into a muchhigher value. The multi-vibrator is another type ofvoltage supply used to obtain a high a.c. or d.c.voltage from a comparatively low d.c. voltage.

In many aircraft, the primary. source of electricpower is direct current. An inverter is used to sup-ply the required alternating current. Common air:craft inverters consist of a d.c. motor driving ana.c. generator. Static, or solid-state inverters arereplacing the electromechanical inverters in manyapplications. Static inverters have no moving parts,but use semiconductor devices and circuits that pe-riodically pulse d.c. current through the primary ofa transformer to obtain an a.c. output from thesecondary.

' 522

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COMMUNICATION SYSTEMSThe most common communication system in use

today is the VHF system. In addition to VHFequipment, large aircraft are usually equipped withHF communication systems.

Airborne communications systems vary consider-ably in size, weight, power requirements, quality ofOperation, and cost, depending upon the desiredoperation. I V

Many airborne VHF and HF communication sys-tems use transceivers. A transceiver is a self-con-tained transmitter and receiver which sharecommon circuits; i.e., power supply, antenna, andtuning.’The transmitter and receiver both operateon the same frequency, and the microphone buttondetermines when there is an output from the trans-mitter. In the absence of transmission, the receiveris sensitive to incoming signals. Since weight andspace are of great importance in aircraft, the trans-ceiver is widely used. Large aircraft may beequipped with transceivers or a communicationssystem that uses separate transmitters and receivers.

The operation of radio equipment is essentiallythe same whether installed on large aircraft or smallaircraft. In some radio installations the controls forfrequency selection, volume, and the “on-ofi” switchare integral with the radio main chassis. In otherinstallations, the controls are mounted on a panellocated in the cockpit and the radio equipment islocated in racks in another part of the aircraft.

Because of thegmany difierent types and modelsof radios in use, it is not possible to discuss thespecific techniques for operating each in this man-ual. However, there are various practices of a non-specific nature which apply to all radios. Thesegeneral practices will be described.

VHF (Very High Frequency) CommunicationsVHF airborne communication sets operate in the

frequency range from 108.0 MHz to 135.95 MHz.VHF receivers are manufactured that cover only thecommunications frequencies, or both comrmmica-tions and navigation frequencies. In general, theVHF radio waves follow approximately straightlines. Theoretically, the range of contact is the dis-tance to the horizon and this distance is determinedby the heights of the transmitting and receivingantennas. However, communication is sometimespossible many hundreds of miles beyond the as-sumed horizon range.

Many VHF radios have the transmitter, receiver,power supply, and operating controls built into a

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Power source

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VHF transceiver

_ "'\MlCIOPh0HO speaker

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-toEarphones G

FIGURE 13-4. VHF system diagram.

single unit. This unit is frequently installed in acutout in the instrument panel. A system diagram ofa typical panel-mounted VHF transceiver is shownin figure 13-4. Others have certain portions of the

/‘K\{\ ) Speaker\../

\ID Earphones Antenna

\ (f OutputPower W \ \‘ -('- Vsotllrce 1 ‘/63! Microphone ‘

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communication system mounted on the instrumentpanel and the remainder remotely installed in aradio or baggage compartment.

To perform an operational check of a VHF com-munication system, a_source of electric power mustbe available. After turning the radio control switch“on”, allow suflicient time for the equipment towarm up beforevbeginning the operational checks.Using the frequency selector, select the frequency ofthe ground station to be contacted. Adjust the vol-ume control to the desired level.

With the microphone held close to the mouth,press the microphone button and speak directly intothe microphone to transmit; when through talking,release the button. This action will return the com-munication receiver to operation. When the groundstation acknowledges the initial transmission, re-quest that an operational check be made on allfrequencies or channels. Prior to transmitting, makecertain that a station license is displayed intheaircraft. In addition, the person operating the trans-mitter must hold a current Restricted Radiotele-phone Operator’s permit. Both the station licenseand the operator’s permit are issued by the FCC.

(I)Antenna coupler

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HF SSB transceiver Control unit

FIGURE 13-5. HF system diagram.

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HF (High Frequency) Communications’A high frequency communication system (figure

13-5) is used for long-range communication. HFsystems operate essentially the same as a VHF sys-tem, but operate in the frequency range from 3MHz to 30 MHz. Communications over long dis-tances are possible with HF radio because of thelonger transmission range. HF transmitters havehigher power outputs than VHF transmitters.

The design of antennas used with HF communi-cation systems vary with the size and shape of theaircraft. Aircraft which cruise below 300 m.p.h.generally use a long wire antenna. Higher speedaircraft have specially designed antenna probes in-stalled in the vertical stabilizer. Regardless of thetype antenna, a tuner is used» to match theance of the transceiver to the antenna.

An operational check of an HF radio consists ofturning the control switch to “on,” adjusting theRF gain and volume controls, selecting the desiredchannel and transmitting the appropriate messageto the called station. Best adjustment of the gaincontrolcan be obtained with the volume control setat half range. The gain control is used to providethe strongest signal with the least amount of noise.The volume control is used to set sound level andafiects only the loudness of the signal.

AIRBORNE NAVIGATION EQUIPMENT“Airborne navigation equipment” is a phrase em-

bracing rnany systems and instrmnents. These sys-tems include VHF omnirange (VOR), instrumentlanding systems, distance-measuring equipment, au-tomatic direction finders, doppler systems, and iner-tial navigation systems. V V

When applied to navigation, the radio receiversand transmitters handle signals which are used todetermine bearing and in some cases distance, fromgeographical points or radio stations. I H

VHF QMNIRANGE SYSTEM if

The VHF VOR (omnidirectional range) is anelectronic navigation system. As the name implies,the omnidirectional or all-directional range stationprovides the pilot with courses from any pointwithin its service range. It produces 360 usableradials or courses, any one of which is a radio pathconnected to the station. The radials can be consid-ered as lines that extend from the transmitter an-tenna like spokes of a wheel. Operation is in‘ theVHF portion of the radio spectrum (frequencyrange of 108.0 MHz — 117.95 MHz) with the result

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that interference from atmospheric and precipita-tion static is negligible. The navigational informa-tion is visually displayed on an instrument in thecockpit.

The typical airborne VCR receiving system(figure 13-6) consists of a receiver, visual indica-tor, antennas, and a power supply. In addition, aunit frequency selector is required and in somecases located on the receiver imit front panel. Somemanufacturers design a remote control frequencyselector so the equipment may be installed in someother area of the aircraft. This frequency selector isused to tune the receiver to a selected VOR groundstation.

Power source

Audie output‘iV011/LDC receiver

Indicator

" Frequency selector

Flctms 13-6. VOR system diagram.

The VOR receiver, in addition to course naviga-tion, functions as a localizer receiver during ILS[instrument landing system) operation. Also, someVOR receivers include a glide slope receiver in asingle case. Regardless of how individual manufac-turers may design the VORlequipment, the intelli-gence from the VOR receiver is displayed on theCDI (course deviation indicator).

The CDI, figure 13-7, performs several functions.During VOR operation the vertical needle is usedas the course indicator. The vertical needle alsoindicates when the aircraft deviates from the courseand the direction the aircraft must be turned toattain the desired course. The “T0-FROM” indica-tor presents the direction to or from the stationalong the omniradial. The course deviation indica-tor also contains a “VOR-LOC” flag alarm. Nor-mally this is a small arm which extends into view

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Course pointerHorizontal pointer

\ Vertical\‘3 '/ pointer

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OFF/TO-FROMReciprocal course mdlcatcr

pointer

Fiouns 13—7. Course deviation indicator.

only in the case of a receiver malfunction or theloss of a transmitted signal. .

When localizer signals are selected on the re-ceiver, the indicator shows the position of the local-izer beam relative to the aircraft and the directionthe aircraft must be turned to intercept the local-izer. '

During VOB. operation the VOR radial to beused is selected by rotating the OBS [omnibearingselector). The OBS is generally located on the CD1;however, in some installationsiit is a part of thenavigation receiver. The OBS is graduated in de-grees from zero to 360. Each degree is a VURcourse to be flown in reference to a ground station.

The following steps are typical of those per-formed during a ground operational check. Whenchecking a VOR system, follow the specific proce-dures recommended by the equipment manufac-turer. The operational check can be performedusing appropriate test equipment, VOT (Very HighFrequency Omnirange Test) or the terminal VCRfacility.

(1) Place. the on/off switch in the “ON” posi-tion.

(2) Adjust the frequency selector to the de-sired station.

w (3) Allow suflicient time for equipment towarm up.

(4) The. VOR flag will disappear when theVOR station signal is received. -

(5) Adjust the volume control to the desiredlevel; assure that the selected VOR stationidentification is clear and correct.

(6) Check the CDI for vertical needle deflec-lion.

(7) Center the vertical needle by rotating theOBS.

(8) Check “TO-FROM” indicator for “TO”indication.

(9) Rotate the OBS to read 10° higher thanthe setting at which the vertical needlewas centered. The vertical needle shouldmove left and cover the last dot whichcorresponds to 10° course displacement.

(10) Retum the OBS to the original position.The vertical needle should return to thecenter position.

(11) Rotate the OBS to read 10° lower thanthe original setting. The pertical needleshould move right and cover the last dotwhich corresponds to 10° course displace-ment. .

(12) Assure that the vertical needle moves anequaldistance in both directions. Thistotal course width or course sensitivityshould be 20°.

NOTE: When “TO-FROM" indicator reads“FROM”, the vertical needle will deflect in thedirection opposite that stated in the above pro-cedures.

If the operational check is unsatisfactory, it willbe necessary to remove the VOR receiver and asso-ciated instruments from the aircraft and have themcalibrated.

INSTRUMENT LANDING SYSTEMThe ILS (instrument landing system), one of the

facilities of the Federal airways, operates in theVHF portion of the electromagnetic spectrum. TheILS can be visualized as a slide made of radiosignals on which the aircraft can be brought safelyto the runway.

The entire system consists of a runway localizer,a glide slope signal, and marker beacons for posi-tion location. The localizer equipment produces aradio course aligned with the center of an airportrunway. The on-course signals result from equalreception of two signals; one containing 90 Hzmodulation and the other containing 150 Hz modu-lation. On one side of the runway center line theradio receiver develops an output in which the I50Hz tone predominates. This area is called the blue

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sector. On the other side of the centerline the 90 Hzoutput is greater. This area is the yellow sector.

The localizer facility operates in the frequencyrange of 103.0 MHz to 112.0 MHz on the “oddtenths” of the megahertz steps. The VOR receiveralso operates in this ferquency range on the “eventenths” of the megahertz steps. The airborne VORreceiver functions as the Iocalizer receiver duringILS operation. _

The glide slope is a radio beam which providesvertical guidance to the pilot, assisting him in mak-ing the correct angle of descent to the runway.Glide slope signals are radiated from two antennaslocated adjacent to the touchdown point of the run-way. Each glide slope facility operates in the UHFfrequency range from 329.3 MHz to 335.0 MI-Iz.

The glide slope and VOR/localizer receivers maybe separate receivers or combined in a single case.The glide slope receiver is paired to the localizerand one frequency selector is used to tune bothreceivers. A component diagram of an ILS is shownin figure 13-8.

The information from both localize!‘ and glide-slope receivers is presented on the CDI; the verticalneedle displays localizer information and the hori-zontal needle displays glide slope information(figure 13—9). ~When both needles are centered, theaircraft is on course and descending at the proper

Runway _

\-on/|.oc ax-rsx.\'.\ ~l erI °

VOR/LOC receiver

Audio output V

Indicator

Frequency selector

Glideslope antenna

K-~ Power source

Clideslope receiver

FIGURE 13-8. Component diagram of an ILS.

Full-scale yellow, Full-scale blue,fly to rlght” V ' “fly to left”

Glide slope pointer

Fly down.

J Localizerl pointerI

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’ §/ \

/ \ y

“’"" ./ %'FIGURE 13-9. Glide slope information.

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rate. In addition the CDI contains a red warningflag for each system which comes into view whenthe receiver fails or the loss of a transmitted signaloccurs.

Two antennas are usually required for’lI.S opera-tion. One for the localizer receiver, also used forVOR navigation, and one for the glide slope. Someof the small aircraft use a single multi-element an-tenna for both glide slope and VOR/LDC opera-tion. The VCR/Iocalizer antenna is normally in-stalled on the top of the aircraft fuselage or flushmounted inthe vertical stabilizer. The glide slopeantenna is, in most cases, installed on the nose ofthe aircraft. On aircraft equipped with radomes, theglide slope antenna is installed under the radome.

Marker BeaconsMarker beacons are used, in connection with the

instrument landing system. ,The markers are signalswhich indicate the position of the aircraft" along theapproach to the runway. Two markers are used ineach installation. The location of each marker isidentified by both an aural tone and a signal lamp.The marker beacon transmitters," operating on afixed 75 MHz frequency, are placed at specific loca-tions along the approach pattern of an ILS facility.The antenna radiation‘ pattern is beamed straightup.

A marker receiver (figure 13410) installed in theaircraft receives the antenna signals and convertsthem into power to illuminate a signal lamp andproduce an audible tone in a headset. The outer

marker marks the beginning of the approach path.The outer marker signal is modulated by a 400-Hzsignal which produces a tone keyed in long dashes.In addition to providing aural identification, thesignal lights a purple lamp in the cockpit. The mid-dle marker is usually about 3,500 ft. from the endof the runway and is modulated at 1,300 Hz whichproduces a higher-pitched tone keyed with alternatedots and dashes. An amber lamp flashes to _-indicatethat the aircraft is passing over the middle marker.

Marlcerheaoon receivers vary. in designfrom sim-ple receivers that have no operating controls and noaural output to more sophisticated receivers thatproduce an aural tone and have an on/ofl switchand a volume control to adjust the sound level ofthe identification code.

Where three lights are used, a white light indi-cates the aircrait positions at various points alongthe airways. In addition to the light, a rapid seriesof tones (six dots per second) of 3,000 Hz is re-ceived in the headset. Distance-measuring equip-ment is rapidly replacing the “along-route” markersystem. A 3,000-Hz tone and white light marker arealso being used for inner markers (missed approachpoint) on some Category ll, ILS-equipped runways.

The ILS system cannot be ground tested fullywithout using test eqnipmentsimulating localizerand glide slope signals. ’

If an aircraft is located at an airport which hasan ILS-equipped runway, it may be possible to de-termine if the receiver is functioning by performing

Power source

noMarker receiver

Marker antenna

V n @ ' y 3 (NHi-Lo switch Q

Marker lights

Earphones

Ftcunlt 13-10. Marker receiver system diagram.

the following. Place the on/off switch (ii soequipped) in the “on” position and adjust the fre-quency selector to the proper ILS channel for theairport where the aircraft is located. Allow suflS-cient time for the equipment to warm up. In astrong signal area, both the localizer and glide slopewaming flags will either start to move or go com-pletely out of view. Observe that both cross pointersare deflected to their maximum displacement.

Some of the more sophisticated solid-state II-Sequipment contains self-monitoring circuits. Thesecircuits can be used for performing an operationaltest using the procedures in the aircraft or equip-ment manufacturer’s service manuals.

DISTANCE-MEASURING EQUIPMENT

The purpose of DME (distance-measuring equip-ment) is to provide a constant visual indication ofthe distance the aircraft is from a ground station. ADME reading is not a true indication of point-to-point distance ashmeasured over the ground. DMEindicates the slant range between the aircraft andthe ground station. Slant-range error increases asthe aircraft approaches the station. At a distance of30 to 60 nautical miles the slant range error isnegligible. up

DME operates in the UHF range of the radiofrequency spectrum. The transmitting frequenciesare in two groups, between 962 MHz to 1,024 MHzand 1,151 MHz to 1,212 MHz; the receiving fre-quencies are between 1,025 MI-Iz to 1,149 MHz.Transmitting and receiving frequencies are given achannel number which is paired with a VO_R chan-nel. In some aircraft installations the DME channelselector is ganged with the VOR channel selector tosimplify the radio operation. A typical DME controlpanel is shown in figure 13-11.

The aircraft is equipped with a DME transceiverwhich is tuned to a selected DME ground station.Usually DME ground stations are colocated with a

FIGURE I3-11. Typical navigation DME control.

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VOR facility (called VORTAC). The airbornetransceiver transmits a pair of spaced pulses to theground station. The pulse spacing serves to identifythe signal as a valid DME interrogation. After re-ception of the challenging pulses, the ground stationresponds with a pulse transmission on a separatefrequency to send a reply to the aircraft. Uponreception of the signal by the airborne transceiver,the elapsed time between the challenges and thereply is measured This time interval is a measureof the distance separating the aircraft and theground station. This distance is indicated in nauti-cal miles by a cockpit instrument similar to the oneshown in figure 13-12.

I

F[CURB 13-12. DME digital indicator.

A typical DME antenna is shown in figure 13-13.Most DME antennas have la cover installed to pro-tect them from damage. The DME antenna isusually a short, stub type mounted on the lowersurface of the aircraft. To prevent an interruptionin DME operation, the antenna must be located in aposition that will not be blanked by the wing whenthe aircraft is banked.

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FIGURE I3-13. Typical DME antenna.

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To determine if the DME operates, turn the on/all switch to the “on” position and select the appro-priate channel. Allow suflicient time for the equip-ment to warm up. During this period, the distanceindicator, both digital or pointer, will travel fromminimum to maximum readings (sweep or search).When the DME has locked on a station, the indica-tor will stop searching and the red wanting flag (ifthe indicator is equipped with one) will disappear.In most installations, no functional check can hemade on the ground withouta DME test set.

LUTOMATIC DIRECTION FINDERSADF (automatic direction finders) are radio re-

ceivers equipped with directional antennas whichare used to determine the direction from whichsignals are received. Most ADF receivers providecontrols for manual operation in addition to auto-matic direction finding. When an aircraft is withinreception range of a radio station, the ADF equip-ment provides a means of fixing the position withreasonable accuracy. The ADF operates in the lowand medium frequency spectrum from 190 kHzthrough 1,750 kHz. _The direction to the station isdisplayed, on an indicator located in the cockpit, asa relative bearing to the station.

The airborne equipment (figure 13-14), consist!of a receiver, loop antenna, sense or nondirectionalantenna, indicator, and control unit. Most ADF re~ceivers used in general aviation aircraft are panelmounted. Their operating controls appear on thefront of the radio ease.

l Sense antenna

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FIGURE 15-14. Typical ADP installation.

In one type ADF system, the loop antenna (figure13-15) rotates through 360° and receives maximumsignal strength when in a parallel position with thedirection of the transmitted signal. As the loop isrotated from this position, the signal becomesweaker and reaches a minimum when the plane ofthe loop is perpendicular to the direction of thetransmitted signal. This position of the loop iscalled the null position. The null position of theloop is used for direction finding. When the loop isrotated to a null position the radio station is beingreceived on a line perpendicular to the plane of theloop. However, the direction of the radio stationfrom the aircraft may be either of two directions180° apart. The inability of the loop antenna todetermine from which of the two directions thetransmitted signal is being received necessitates theinstallation of a sense antenna.

The loop and sense antenna are both connectedto the ADF receiver. When the signal strength ofthe sense antenna is superimposed on the signalreceived from the loop antenna, it results in only

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one null position of the loop. The one null positionalways indicates the direction to the transmittingfacility.

Another type ADF system uses fixed, ferrite coreloops in conjunction with a rotatable transformercalled a resolver or goniometer. It operates essen-tially the same as the rotating loop, except that oneof the windings of the goniometer rotates instead ofthe loop. .

A general procedure for performing an opera-tional check of the ADF system is as follows:

(1) Turn on/oif switch to the “on” positionand allow the radio to warm up. On in-stallations that use the RMI (Radio Mag-netic Indicator) pointer as an ADF indi-cator, assure tliat the switch has been po-sitioned to present ADF information.

(2) Tune to the desired station.(3) Adjust the volume control to an appropri-

ate level. _(4-) Rotate loop antenna and determine that

only one null is received.(5) Check that the ADF needle points towards

the station. If the aircraft is situatedamong buildings or any other large re-flecting surfaces, the ADF needle may in-dicate an error as a result of a reflectedsignal.

RADAR BEACON TRANSPONDER j

The radar beacon transponder system is used inconjunction with a ground base surveillance radarto provide positive aircraft identification directly onthe controller’s radar scope. '

The airborne equipment or transponder receivesa ground radar interrogation for each sweep of thesurveillance radar antenna and automatically dis-patches a coded response. Civil transponders oper-ate in two modes labeled “Mode A” and “ModeAC” which are switch controlled. The flight identi-fication code, a four-digit number, is assigned dur-ing the flight planning procedure. _

Some aircraft transponders are equipped with analtitude encoding feature. The aircraft’s altitutde istransmitted to the ground station through the trans-ponder. The mode selector switch is placed in the“AC” mode when it is necessary to transmit altitudeinformation.

There are several different aircraft transpondersin use. They all perform the same function and arebasically the same electrically. The major differ-

ences are in construction; either a single unit or acontrol unit for remotely operating the transponder.

A typical transponder is shown in figure 13-16.The “front panel of the illustrated transponder con-tains all the switches and dials needed for opera-tion.

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FIGURE 13-16. Typical transponder system.

A short stub or covered stub antenna is used fortransponder operation and is usually mounted onthe lower surface of the aircraft fuselage.

To ground checlt the radar beacon transponderthe appropriate test equipment must he used.

DOPPLER NAVIGATION SYSTEMSDoppler navigational radar automatically and

continuously computes and displays ground speedand drift angle of an aircraft in flight without theaid of ground stations, wind estimates, or true air-speed data. The Doppler radar does not sense direc-tion as search radar does. Instead, it is speed con-scious and drift-conscious. ltjuses continuous car-rier wave transmission energy and determines theforward and lateral velocity components of the air-craft by utilizing the principle known as Dopplereilect.

The Doppler effect, or frequency change of asignal, can be explained in terms of an approaching

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and departing sound. As shown in figure 13-17, thesound emitter is a siren located on a moving ambu-lance and the receiver is the ear of a stationaryperson. Notice the spacing between the emitterwhen it is approaching and when it is departingfrom the stationary receiver. When the sound wavesare closely spaced the listener hears a sound that ishigher in pitch. The reverse is true when the emitteris moving away from the listener. Doppler radaruses the frequency change phenomenon just de-scribed, except in the radio frequency range. ,

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Sound waves equally i Sound waves equallyand densely spaced I I I I I 1 l but widely spaced

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The Doppler radar emits narrow beams of energyat one frequency, and these waves of energy strikethe earth’s surface and are reflected. Energy wavesreturning from the earth are spaced differently thanthe waves striking the earth. The earth-returnedenergy is intercepted by the receiver and comparedwith the outgoing transmitter energy. The differ-ence, due to Doppler effect, is used to developground spced and wind drift angle information.

A ground operational check of a Doppler systemconsists of setting a precise airspeed, and a devia-tion angle which will give a distance-ofi-coursereading. Always refer to the equipment manufactur-

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INERTIAL ‘NAVIGATION SYSTEMThe intertial navigation system is presently being

used on large aircraft as a long-range navigationaid. It is a self-contained system and does not re-quire signal inputs from ground navigational facili-ties. The system derives attitude, velocity, and head-ing information from measurement of the aircraft’saccelerations. Two accelerometers are required, onereferenced to north and the other to east. The accel-rometers (figure 13-18) are mounted on a gyro-stabilized unit, called the stable platform, to avertthe introduction of errors resulting from the accel-eration due to gravity.

An inertial navigation system is a complex systemcontaining four basic components. They are:

(1) A stable platform which is oriented tomaintain accelerometers horizontal to theearth’s surface and provide azimuth orien-tation.

{2} Accelerometers arranged on the platformto supply specific components of accelera-

» tion.(3) Integrators which receive the output from

v the accelerometers‘ and furnish velocityand distance.

(4-) A computer which receives signals fromthe integrators and changes distance trav-eled to position in selected coordinates.

The diagram in figure 13-18 shows how thesecomponents are linked together to solve a naviga-tion problem. Initial conditions are set into thesystem and the navigation process is begun. In iner-tial navigation, the term initialization is used todenote the process of bringing the system to a set of

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FIGURE 13-18. A basic inertial navigation system.

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initial conditions from which it can proceed withthe navigation process. These conditions includeleveling the platform, aligning the azimuth refer-ence, setting initial velocity and position, and mak-ing any computations required to start the naviga-tion. '

Although all inertial navigation systems must beinitialized, the procedure varies according to theequipment and the type aircraft in which it is in-stalled. The prescribed initialization procedures aredetailed in the appropriate manufacturer's manuals.

From the diagram it can be seen that the acceler-ometers are maintained in a horizontal position tothe earth’s surface by a gyro-stabilized platform. Asthe aircraft accelerates, a signal from the accelerom-eter is sent to the integrators. The output from theintegrators, or distance, is then fed into the com-puter, where two operations are performed. First, aposition is determined in relation to the preset flightprofile, and second, a signal is sent back to theplatform to position the accelerometer horizontallyto the earth’s surface. The output from high-speedgyros and accelerometers, when connected to theflight controls of the aircraft, resists any changes inthe flight profile.

AIRBORNE WEATHER RADAR SYSTEMRadar (radio detection and ranging) is a device

used to see certain objects in darkness, fog, orstorms, as well as in clear weather. In addition tothe appearance of these objects on the radar scope,their range and relative position are also indicated.

Radar is an electronic system using a pulse trans-mission of radio energy to receive a reflected signalfrom a target. The received signal is known as anecho; the time between the transmitted pulse andreceived echo is computed electronically and is dis-played on the radar scope in terms of nauticalmiles.

A radar system (figure 13-19) consists of atransceiver and synchronizer, an antenna installedin the nose of the aircraft, a control unit installedin the cockpit, and an indicator or scope. A wave-guide connects the receiver/transmitter to the ‘an-tenna. '

In the operation of a typical weather radar sys-tem, the transmitter feeds short pulses of radio-irequency energy through a waveguide to the dishantenna in the nose of the aircraft. In one typicalinstallation the antenna radiates the energy in abeam 3.8° wide. Part of the transmitted energy isreflected from objects in the path of the beam and

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Ftclin 13-19. Weather radar system diagram.

is received by the dish anterma. Electronic switch-ing simultaneously connects the antenna to thetransmitter and disconnects the receiver duringpulse transmission. Following the completion ofpulse transmission, the antenna is switched from thetransmitter to the receiver. The switching cycle isperformed for each transmitted pulse.

The time required for radar waves to reach thetarget and reflect to the aircraft antenna is directlyproportional to the distance of the target from theaircraft. The receiver measures the time intervalbetween transmission of radar signals and receptionof reflected energy and uses this interval to repre-sent the distance, or range, of the target.

Rotation or sweep of the antenna and radar beamgives azimuth V indications. The indicator sweeptrace rotates in synchronization with the antenna.The indicator display shows the area and the rela-tive ‘size of targets, whose azimuthal position isshown relative to the lineof flight.

The weather radar increases safety in flight byenabling the operator to detect storms in the flightpath in order to chart la course around them. Theterrain-mapping facilities of the radar show shore-lines, islands, and other topographical featuresalong the flight path. These indications are pre-sented on the visual indicator in range and azimuthrelative to the heading of the aircraft.

An operational check consists of the following:(1) Tow or taxi the aircraft clear of all build-

ings and parked aircraft.

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(2) Apply power to the equipment, and allowsuflicient warmup time.

(3) Tilt the antenna to an upward position.(4) Check the scan on the radar scope for an

indication of targets. ‘

IADIQ ALTIMETER aRadio altimeters are used to measure the distance

from the aircraft to the ground. This is accom-plished by transmitting radio frequency energy tothe ground and receiving the reflected energy at theaircraft. Most modern day altimeters are pulse typeand the altitude is determined by measuring thetime‘ required for the transmitted pulse to hit theground and return. The indicating instrument willindicate the true altitude of the-aircraft, which is itsheight above water, mountains,»buildings, or otherobjects on the surface of the earth. ’

The present day generation of radio altimetersare primarily used during landing and are a Catc-gory II requirement. The altimeterprovides thepilot with the altitude of the aircraft during ap-proach. Altimeter indications determine the decisionpoint whether to continue to land, or execute aclimb-out. A

A radio altimeter system (figure 13-20) consists

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Radio altimeter P°w°’SOUIC6

Receiver Transmitterantenna antenna

FIGURE 13¢20. Typical radio altimeter system diagram.

of a transceiver, normally located in an equipmentrack, an indicator installed in the instrument paneland two antennas located on the belly of the air-craft.

EMERGENCY LOCATOR TRANSMITTER lEl.TlEmergency locator transmitters are self-contained,

self-powered radio transmitters designed to transmita signal on the international distress bands of121.5 MHz (civilian) and 243 MHz (military).

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Ficuns 13-21. Emergency locator transmitter {ELT).

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Operation is automatic on impact. The transmittermay also be activated by a remote switch in thecockpit or a switch integral with the unit. If the“G” force switch in the transmitter is activatedfrom impact it can be turned ofi only with theswitch on the case. (See figure 13-21.)Transmitter

The transmitter may be located anywhere withinthe aircraft, but the ideal location is, as far aft aspossible but just forward of the vertical fin. Itmust he accessible to permit monitoring the re-placement date of the battery and for arming ordisarming of the unit. A remote control arm/‘disarm switch may be installed in the cockpit.

The external antenna must be installed as far aspracticable from other antennas to prevent inter-action between avionics systems.Batteries

Batteries are the power supply for emergencylocator transmitters. When activated, the batterymust be capable of furnishing power for signaltransmission for at least 48 hours. The useful life ofthe battery is the length of time which the batterymay be stored without losing its ability to con-tinuously operate the ELT for 48 hours. Thisuseful life is established by the battery manu-facturer; batteries must be changed or rechargedas required at 50% of the battery’s useful life.This gives reasonable assurance that the ELT willoperate if activated. The battery replacement datemust be marked on the outside of the transmitter.This time is computedfrom the date of manufac-ture of the battery.

Batteries may be nickel-cadmium, lithium, “mag-nesium dioxide, dry-cell batteries. Wet cell bat-teries have an unlimited shelf life until liquid isadded. At that time their life in an ELT is regu-lated the same as dry cell batteries——-change at50% of shelf life. When replacing batteries useonly those recommended‘ by the manufacturer ofthe ELT. Do not use flashlight type batteries astheir condition and useful life are unknown.

Testing ATesting of ELT’s should be coordinated with .the

nearest FAA Tower or Flight Service Station andestablish coordination for the test. Tests shouldbe conducted only during the first five minutes ofany hour and should be restricted to 3 audiosweeps. Any time maintenance is performed inthe vicinity of the ELT, the VHF communicationreceiver should be tuned to 121.5 MHz and listenfor "ELT audio sweeps. If it is determined theELT is operating, it must be turned off imme-diately.

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False Alarms

False alarms have caused many of the problemswith ELT’s. Battery failures, with resulting cor-rosion of the unit results in either a completefailure or an unwanted transmission. Anothertype of unwanted transmission is the result ofcareless handling by the operators of the aircraft.

Test Equipment ITwo monitors are available for identifying

and/or locating unwanted ELT transmissions. Aminiature Scanning receiver may be mounted inthe cockpit to warn the pilot if his ELT is trans-mitting. The other is a small portable ELT locatorfor use at general aviation airports to assist infinding an aircraft whose transmitter has acci-dentally become activated.

‘The operation of an ELT can be verified by tun-ing a communication receiver to the civil emergencyfrequency (121.5 MHz) and activating the ELT.Turn the ELT ofi immediately upon receiving asignal in the communication receiver.

In _all maintenance and testing of ELT’s themanufacturers instructions must be followed.

INSTALLATION OF COMMUNICATION ANDNAVIGATION EQUIPMENT

There are" many factors which the mechanic mustconsider prior to altering an aircraft by the addi-tion of radio equipment. These factors include thespace available, ‘the size and weight of the equip-ment, and previously accomplished alterations. Inaddition, the power consumption of the addedequipment must be calculated to determine the max-imum continuous electrical load. Each installationshould be planned to allow easy access for inspec-tion, maintenance, and exchange of units.

The installation of radios is primarily mechani-cal, -involving sheet metal work in mounting theradios, racks, antennas, and controls. Routing ofthe interconnecting wires, cables, antenna leads,etc., isalso an important part of the installationprocess. When selecting a location for the equip-ment, first consider the areas designated by theairframe manufacturer. If such information is notavailable, or if the aircraft does not contain provi-sions for adding equipment, select an area that willcarry the loads imposed by the weight of the equip-ment, and which is capable of withstanding theadditional inertia forces.

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Vibration IsolationVibration is a continued motion caused by an

oscillating force. The amplitude and frequency ofP vibration of the aircraft structure will vary consid-

. Bear case support erably.wrth the type of aircraft.Radio equipment is sensitive to mechanical shock

and vibration and is normally “shock-mounted” toprovide some protection against in-flight vibrationand landing shock.

When special mounts (figure 13-23) are used toisolate radio equipment from vibrating structure,such mounts should provide adequate isolation over

Fmmt 13_22_ Typical radio installation in a the entire range of expected vibration frequencies.When installing shock mounts, assure that theequipment weight does not exceed the weight-carrying capabilities of the mounts.

5!’Shock mount

Machine screws andself-locking nuts.

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Rivets or machine screwsand self-locking nuts.

'Jill =="s{stationary instrument panel.

If the radio is to be mounted in the instrumentpanel and no provisions have been made for suchan installation, determine if the panel is primarystructure prior to making any cutouts. To minimizethe load on a stationary instrument panel, install asupport bracket (figure 13-22) between the rear ofthe radio case or rack and a nearby structuralmember of the aircraft.

The radio equipment must be securely mountedto the aircraft. All mounting bolts must be securedby locking devices to prevent loosening from vibra-tion. - - V V V

Adequate clearance between the radio equipmentand the adjacent structure must be provided to pre-vent mechanical damage to electric wiring or radioequipment from vibration, chafing, or shock land-ins- , ,

Do not locate radio equipment and Wiring nearunits containing combustible fluids. When separa-tion is impractical, install bafiies or shrouds to pre-vent contact of the combustible fluids with radioequipment in the event of a plumbing failure.

4.».FICURE 13-23. Typical shock-mounted base.

Radios installed in instrument panels do not ordi-narily require vibration protection, since the panel

Cooling and MoistureThe performance and service life of most radio

equipment is seriously limited by excessive ambienttemperatures. The -installation should be planned sothat the radio equipment candissipate its heat read-ily. In some installations it may be necessary toproduce an airflow over the radio equipment, eitherwith a blower or through the use of a venturi.

The presence oi Water in radio equipment pro-motes rapid deterioration of the exposed compo-nents. Some means must be provided to prevent theentry of water into the compartments housing theradio equipment.

tain that the added weight can be safely carried bythe existing mounts. In some cases it may be neces-sary to install larger capacity mounts or to increasethe number of mounting points.

Radio equipment installed on shock mounts musthave suliicient clearance from surrounding equip-ment and structure to allow for normal swaying of

Periodic inspection of the shock mounts is re~quired, and defective mounts should be replacedwith the proper type. The factors to observe duringthe inspection are: (1) Deterioration of the shock- n

535 1

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absorbing material, (2) stiffness and resiliency ofthe material, and (3) overall rigidity of the mount.If the mount is too stifi, it may not provide ade-quate protection against the shock of landing. If themount is not stiff enough, it may allow prolongedvibration following an initial shock.

Shock-absorbing materials commonly‘ used inshock mounts are usually electrical insulators. Forthis reason, each electronic unit mounted withshock mounts must be electrically bonded to a struc-tural member of the aircraft, similar to that shownin figure 13-24. This may also be accomplished byusing sheets of high-conductivity metal (copper oraluminum) where it is impossible to use a shortbond strap.

\Shock mount ' *Ti * —-n "i

Bonding

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FIGURE 13-24. Typical shock mount bonding jumper.

REDUCING RADIO INTERFERENCESuppression of radio interference is a task of first

importance. The problem has increased in propor-tion to the complexity of both the electrical systemand the electronic equipment. Almost every compo-nent of the aircraft is a possible source of radiointerference. Radio interference of any kind deteri-orates the performance and reliability of the radioand electronic systems.

Isolation is the easiest and most practical methodof radio noise suppression. This involves separatingthe source of radio noise from the input circuits ofthe affected equipment. In many cases, the noise ina receiver may be entirely eliminated simply bymoving the antenna lead-in wire just a few inchesaway from the noise source. Some of the sources ofradio interference in aircraft are rotating electricaldevices, switching devices, ignition systems, propel-ler control systems, a.c. powerlines, and voltage reg-ulators.

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An aircraft can become highly charged g withstatic electricity while in flight. If the aircraft isimproperly bonded, all metal parts will not have thesame amount of charge. A difierence of potentialwill exist between various metal surfaces. The neu-tralization of the charges flowing in paths of varia-ble resistance, due to such causes as intermittentcontact from vibration or the movement of the con-trol surfaces, will produce electrical disturbances(noise) in the radio receiver.

Bonding provides the necessary electrical connec-tion between metallic parts of an aircraft. Bondingjumpers and bonding clamps are examples of bond-ing connectors. Bonding also provides the l0W-res-istance return path for single-wire electrical sys-tems.

Bonding radio equipment to the airframe willprovide a low-impedance ground return and mini-mize radio interference from static electricitycharges. Bonding jumpers should be as short aspossible and installed in such a manner that theresistance does not exceed 0.003 ohm. When ajumper is used only to reduce radio noise and isnot for current-carrying purposes, a resistance of0.01 ohm is satisfactory.

The aircraft structure is also the ground for theradio. For the radio to function correctly, a properbalance must be maintained between the aircraftstructure arid the antenna. This means the surfacearea of the ground must be constant. Control sur-faces, for example, may at times become partiallyinsulated fromthe remaining structure. This wouldaffect radio operation if the condition was not alle-viated by bonding.

Shielding is one of the most effective methods ofsuppressing radio noise. The primary object ofshielding is to electrically contain the radio fre-quency noise energy. ln practical applications, thenoise energy is kept flowing along the inner surfaceof the shield to ground instead of radiating intospace. The use of shielding is particularly effectivein situations where filters cannot be used. A goodexample of this is where noise energy radiates froma source and is picked up by the various circuitsthat eventually connect to the receiver input cir-cuits. It would be ‘impractical to filter all of theleads or units that are affected by the radiated noiseenergy; thus the application of effective shielding atthe noise source itself is preferred, for it eliminatesthe radiated portion of the noise energy by confin-ing it within the shield at its source.

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Ignition wiring and spark plugs are usuallyshielded to minimize radio interference. If an intol-erable radio noise level is present despite shielding,it may be necessary to provide a filter between themagneto and magneto switch to reduce the noise.This may consist of a single bypass capacitor or acombination of capacitors and choke coils. Whenthis is done, the shielding between the filter andmagneto switch can usually be eliminated.

The size of a. filter may vary widely, dependingon the voltage and current requirements as well asthe degree of attenuation desired. Filters areusually incorporated in equipment known to gener-ate radio interference, but since these filters areoften inadequate it is frequently necessary to addexternal filters.

Static Dischurger WicksStatic dischargers are installed on aircraft to

reduce radio receiver interference. This interferenceis caused by corona discharge emitted from theaircraft as a result of precipitation static. Coronaoccurs in short pulses which produce noise at theradio frequency spectrum. Static dischargers, nor-mally mounted on the trailing edges of the controlsurfaces, wing tips, and vertical stabilizer, dis-charge the precipitation static at points a criticallength away from the wing and tail extremitieswhere there is little or no coupling of the static intothe radio antenna.

Three major types of static dischargers are inuse:

(1) Flexible vinyl-covered, silver- or carbon-impregnated braid.

(2) Semiflexible metallic braid.(3) Null-field.

Flexible and semiflexible dischargers are attachedto the aircraft by metal screws and should be peri-odically checked for tightness. At least 1 in. of the

Non-conductive yellow nylon cap ,1;

Nylon/resistive coating .

Set screw y/ Tungsten needle

Aluminum retainer (bonded)

I. . . .Typical wing trailing edge installation

FIGURE 13-25. Null-field static discharger.

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inner braid of vinyl covered dischargers should ex-tend beyond the vinyl covering. Null-field dischar-gers (figure 13-25) are riveted and epoxy bondedto the aircraft structure. A resistance measurementfrom the mount to the airfrace should not exceed0.1 ohm.

INSTALLATION OF AIRCRAFT ANTENNA SYS-TEMS

An introductory knowledge of radio equipment isa valuable asset to the aviation mechanic, especiallya knowledge of antenna installation and mainte-nance, since these tasks are often performed by themechanic.

Antennas take many forms and sizes dependentupon the job they are to perform. Airborne anten-nas should be mechanically secure, mounted in in-terference-free locations, have the same polarizationas the ground station, and be electrically matchedto the receiver or transmitter which they serve.

The following procedures describe the installa-tion of a. typical rigid antenna:

(1) Place a template similar to that shown infigure 13-26 on the fore-and-aft centerline at the desired location. Drill themounting holes and correct diameter holefor the transmission line cable in the fuse-lage skin. "

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FIGURE 13-26. Typical antenna motmting template.

(2) Install a reinforcing doubler of suflicientthickness to reinforce the aircraft skin.The length andwidth of the reinforcingplate should approximate the exampleshown in figure 13-27.

(3) Install the antenna on the fuselage, makingsure that the mounting bolts are tightenedfirmly against the reinforcing doubler and

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Antenna

Fuselage skin

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Reinforcing doublerAlclad .2024-T3

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Flctma 13-27. Typical antenna installation on ca skin panel.

the mast is drawn tight against the gasket.If a gasket is not used, seal between themast and the fuselage with a suitablesealer, such as zinc chromate paste, orsquat ‘

The mounting bases of antennas vary in shapeand sizes; however, the aforementioned installationprocedure is typical and may be used for mast-typeantenna installations.

Transmission lines .A transmitting or receiving antenna is connected

directly to its associated transmitter or receiver bywire(s) which are shielded. The interconnectingshielded wire(s) are called a coaxial cable whichconnects the antenna to the rcceiveror transmitter.The job of the transmission line (coaxial cable) isto get the energy to the place where it is to be usedand to accomplish this with minimumenergy loss.

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A transmission line connects the final power ampli-fier of a transmitter to the transmitting antenna. Thetransmission line for a receiver connects the antennato the first tuned circuit of the receiver. Transmis-sion lines may vary from only a few feet to severalfeet in length.

Transponders, DME, and other pulse type trans-ceivers require transmission lines that are precise inlength. The critical length of the transmission linesprovides minimum attenuation of the transmitted orreceived signal. Refer to the equipment manufactureinstallation manual for type and allowable length oftransmission lines. p V

Coaxial cable is utilized in most airborne installa-tions for transmission lines. Coaxial cable is anunbalanced line that functions with a balanced an-tenna. To provide the proper impedance matchingand the most efiicient power transfer a balun isused. The balun is an integral part of the antennaand is not visible without disassembling the an-tenna.

when installing coaxial cable (transmissionlines) secure the cables. firmly along their entirelength at intervals of approximately 2 ft. To assureoptimum operation, coaxial cables should not berouted or tied to other wire bundles. When bendingcoaxial cable, be sure that the bend is at least 10times the size of the cable diameter. »

Maintenance Procedures »Detailed instructions, procedures, and specifica-

tions for the servicing of radio equipment are con-tained in the manulacturefs operation and servicemanuals. In addition, instructions for removal andinstallation of the units are contained in the mainte-nance manual for the aircraft in which the equip-ment is installed. .

Although installation appears to he a simple pro-cedure, many radio troubles can be attributed tocarelessness or oversight when replacing radioeqnipment. Specific instances are loose cableconnections, switched cable terminations, improperbonding, lack of or improper safety wiring, or fail-ure to perform an operational check after installa-tion.

Two additional points conceming installation ofequipment needs emphasis. Prior to re-installingany unit, inspect its mounting for proper conditionof shock mounts and bonding straps. After installa-tion, safety wire as appropriate.

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