MILLER ANTENNA

1. How should a transmitting antenna be designed if a vertically polarized wave is to be

radiated, and how should be the receiving antenna be designed for best performance in

receiving the ground wave from this transmitting antenna?

Answer:

The transmit antenna must be vertically polarized. The receive antenna must also be

vertically polarized.

2. A ship radio-telephone transmitter operates on 2738 kHz. At a certain point distant from

the transmitter, the 2738 kHz has a measured field of 147mV/m. The second field harmonic

at the same point is measured as 405µV/m. To the nearest unit in decibels, how much has

the harmonic emission been attenuated below the 2738 kHz fundamental?

Solution:

𝑑𝐵 = 20𝑙𝑜𝑔𝑉1

𝑉2= 20𝑙𝑜𝑔

147𝑥10−3

405𝑥10−6= 51.2𝑑𝐵

Final Answer: 51.2dB

3. If a field intensity of 25 mV/m develops 2.7 V in a certain antenna, what is its effective

height?

Solution:

𝑑 =𝑉

𝐸=

2.7𝑉

25 mV /m = 108𝑚

Final Answer: 108 m

4. If a power of 500 kHz transmitter is increased from 150W to 300W, what would be the

percentage change in field intensity at a given distance from the transmitter? What would

be the decibel change in field intensity?

Solution:

𝑃∞𝐸I2

. :𝐸∞ 𝑃

𝑃change=2

. :𝐸𝑖𝑠𝑖𝑛𝑐𝑟𝑒𝑎𝑠𝑒𝑑𝑏𝑦 2 𝑜𝑟 1.414 𝑜𝑟 141.4% 𝑐𝑕𝑎𝑛𝑔𝑒

𝑑𝐵𝑐𝑕𝑎𝑛𝑔𝑒 = 20 log 1.414 = 3𝑑𝐵

Final Answer: 3dB

5. If a 500 kHz transmitter of constant power produces field strength of 100µV/m at a

distance of 100 mi from the transmitter, what would be the theoretical field strength at a

distance of 200 mi from the transmitter?

Solution:

𝐸 = 30𝑃𝑡

𝑟

𝑎𝑡 200𝑚𝑖:

𝐸 =100μV/m

2= 50μV/m

Final Answer: 50μV/m

6. If the antenna current at 500 kHz transmitter is reduced 50%, what would be the

percentage change in the field intensity at receiving point?

Solution:

𝑃∞i2

. :𝑃 = 1

2

2

𝑃𝑜𝑟𝑖𝑔𝑖𝑛𝑎𝑙 = 0.25

𝐸∞ 𝑃𝑡

. :𝐸∞ 0.25𝐸𝑜𝑟𝑖𝑔𝑖𝑛𝑎𝑙

. :𝐸∞0.5 𝐸𝑜𝑟𝑖𝑔𝑖𝑛𝑎𝑙 Or 50%

Final Answer: 50%

7. Define field intensity. Explain how it is measured.

Answer:

The intensity of radiation any point in a radiation field. This is measured in terms of field

strength which is inversely proportional to the distance from the transmitters.

8. What is effective radiated power? Given transmitter power output, antenna resistance,

antenna transmission line losses, transmitter efficiency, and power gain, show how ERP is

calculated.

Answer:

This refers to the fact that an antenna will work equally well for transmit and receive

given adequate power boundaries. ERP is the product of the antenna input power and antenna

power gain or the product of power fed to the antenna and its power gain,ERP=PinG

9. Define polarization as it refers to broadcast antennas.

Answer:

The direction in space of the electric field portion of the electromagnetic wave being

radiated by transmitting system.

10. Explain the development of Hertz antenna from a quarter-wavelength, open-circuited

transmission line.

Answer:

The two wire transmission line was considered to be an adequate transmission medium at

extremely high frequencies; it can become an effective antenna. The characteristic of the open-

ended line are such that the voltage of the line is maximum, and the current at the end is zero.

Since the current at the ends of the line is minimum, a quarter-wave (at the source) the current

must be maximum. The impedance at the sending end is low, and the impedance at the open

circuit is high. It is desired to have maximum radiation. This maximum radiation is not possible

with the two-wire transmission line because the magnetic field surrounding each conductor of

the line is in a direction that opposes the lines of the force about the other conductor. Under this

condition, the quarter-wave transmission line proves to be an unsatisfactory antenna; however,

with only slight physical modification, this section of the transmission line can be transformed

into a relatively efficient antenna. This transformation is accomplished by bending line outward

〖90〗^0to form a Hertz antenna.

11. Draw a diagram showing how a current varies along a half-wavelength Hertz antenna.

Answer:

the current is at maximum at the center and minimum at both ends.

12. Explain the voltage and current relationships in one-wavelength antenna, one-half-

wavelength (dipole) antenna, and one-quarter-wavelength grounded antenna.

Answer:

For one wavelength antenna the current is minimum at both ends and at the center. The

voltage is maximum at these points.

For half-wavelength antenna the current is at maximum at the center and minimum at

both ends. The voltage is maximum at both ends but minimum at the center.

For one-quarter wavelength the current is maximum at the ground point and minimum at

the top of the antenna. The voltage is maximum at the top of the antenna but minimum at the

ground point.

13. What effect does the magnitude of the voltage and the current at a point on a half-

wavelength antenna in free space (a dipole) have on the impedance at that point?

Answer:

The impedance value for half-wave dipole varies from about 2500Ω at the open ends to

73Ω at the source ends.

14. Can either of the two fields that emanate from an antenna produce an EMF in a

receiving antenna? If so, how?

ANSWER: Yes, This property of interchangeability for transmitting and receiving operations

is known as antenna reciprocity. It occurs because antenna characteristics are essentially the

same regardless of whether an antenna is sending or receiving EMF energy. And because of

that, antenna can also be treated as the same viewpoint of transmitting antenna when an

antenna is used for receving EMF energy.

15. Draw the three-dimensional radiation pattern for the Hertz antenna, and explain how it

is developed.

ANSWER:

The linear half-wave dipole is one of the most widely used antennas at frequencies above

2MHz. At frequencies below 2MHz, the physical length of a half-wavelength antenna is

prohibitive. The half-wave dipole is generally referred to as Hertz Antenna after Heinrich

Hertz, who was first demonstrate the existence of electromagnetic wave.

The radio signals radiated by an antenna form an electromagnetic field with a definite pattern,

depending on the type of antenna used. This radiation pattern shows the antenna‟s directional

characteristics. The pattern shows that maximum field strength for ½ wavelength dipole

occurs at the right angles of the antenna, while virtually zero energy is launched “off the

ends”. Its radiation pattern is spherical or it is circular or omnidirectional.

16. Define Antenna Beamwidth.

ANSWER:

Antenna Beamwidth is the angular separation between the half power points on its

radiation pattern.

Beamwidth is the angular separation between the two half power levels (-3dB) on the

major lobe of an antenna‟s plane radiation pattern.

17. What is the effective radiated power of a television broadcast station if the output of the

transmitter is 1000W, antenna transmission line loss is 50W, and the antenna power gain is

3? (2850W)

SOLUTION:

where

Po= output power of the transmitter

l = transmission line loss

Ap= Power gain

ERP= Po*Ap-l

ERP= ((1000)*(3))-50

ERP =2850W

18. A λ/2 dipole is driven with a 5-W signal at 225 MHz. A receiving dipole 100km away is

aligned such that its gain is cut in half. Calculate the received power and voltage into a

73-Ωreceiver. (7.57 pW, 23.5 μV)

ANSWER:

𝑃𝑡 = 5W

𝐺𝑡 = 1.64

𝐺𝑟 =1.64

2= 0.82

𝑑 = 100𝑥103 m

𝑅 = 73Ω

𝑃𝑟 =𝑃𝑡𝐺𝑡𝐺𝑟λ

2

16𝜋2𝑑2

𝑃𝑟 =5W 𝑥 1.64 𝑥 0.82 𝑥

3𝑥108m/s225𝑥106Hz

2

16𝜋2(100𝑥103 m)2

𝑷𝒓 = 𝟕. 𝟓𝟕 𝐩𝐖

𝑃 =𝑉2

𝑅;𝑉 = 𝑃𝑅

𝑉 = (7.57 pW)(73𝛀)

𝑽 = 𝟐𝟑.𝟓𝟏𝛍𝐕

19. Define radiation resistance and explain its significance.

ANSWER:

Radiation resistance is the value of resistance that, if it replaced the antenna, would

dissipate exactly the same amount of power that the antenna dissipates. In a sense, it is a

fictitious quantity. It seems apparent that radiation resistance is at times a rather nebulous

concept, as it is not always easily measured. It is a useful concept only when it is readily

measurable and has no meaning for antennas in which there is no clearly defined current

value to which it can be referred.

20. The ammeter connected at the base of a Marconi antenna has a certain reading. If this

reading is increased 2.77 times, what is the increase in output power? (7.67)

ANSWER:

𝑃𝑜𝑢𝑡 = 𝐼2𝑅

𝐿𝑒𝑡 I = 1𝐴 𝑎𝑛𝑑 𝑅 = 50Ω,

𝑃𝑜𝑢𝑡 = 1𝐴 2 50Ω = 50𝑊

𝑃𝑜𝑢𝑡2 = 2.77𝐴 2 50Ω = 383.645𝑊

𝐼𝑛𝑐𝑟𝑒𝑎𝑠𝑒 𝑖𝑛 𝑃𝑜𝑢𝑡 =𝑃𝑜𝑢𝑡2

𝑃𝑜𝑢𝑡=

383.645𝑊

50𝑊

𝑰𝒏𝒄𝒓𝒆𝒂𝒔𝒆 𝒊𝒏 𝑷𝒐𝒖𝒕 = 𝟕. 𝟔𝟕

21. How is the operating power of an AM transmitter determined using antenna resistance

and antenna current?

ANSWER:

It can be determined using the formula,

𝑹𝒓 =𝑷

𝑰𝟐; 𝑷 = 𝑰𝟐𝑹𝒓

where𝑅𝑟 = radiation resistance (Ω)

𝐼 = effective rms value of antenna current at the feed point (A)

𝑃 = 𝑡𝑜𝑡𝑎l power radiated from the antenna

22. Explain what happens to an antenna’s radiation resistance as its length is continuously

increased.

ANSWER:

The antenna‟s radiation resistance increases as its length is continuously increased.

23. Explain the effect that ground has on an antenna.

ANSWER:

The height of antenna above ground affects radiation resistance. Since earth is considered

to be a good conductor, changes in radiation resistance occur because of ground reflections

that intercept the antenna and alter the amount of antenna current flowing.

24. Calculate the efficiency of an antenna that has a radiation resistance of 73Ω and an

effective dissipation resistance of 5Ω. What factors could enter into the dissipation

resistance? (93.6%)

ANSWER:

𝑅𝑟 = 73Ω

𝑅𝑑 = 5Ω

𝜂 =𝑅𝑟

𝑅𝑟 + 𝑅𝑑 𝑥 100

𝜂 =73Ω

73Ω + 5Ω 𝑥 100

𝜼 = 𝟗𝟑.𝟔%

25. Explain the following terms with respect to antennas (transmission or reception):

(a) Field strength

(b) Power gain

(c) Physical length

(d) Electrical length

(e) Polarization

(f) Diversity reception

(g) Corona discharge

ANSWER:

(a) Field Strength of an antenna describes its received signal strength. It is inversely

proportional to the distance from the transmitter.

(b) Power Gain is the same as directive gain except that the total power fed to the antenna

is used. It is assumed that the given antenna and the reference antenna have the same

input power and that the reference antenna is lossless.

(c) Physical Length of an antenna is less by about 5% than the corresponding wavelength

in space. It can be approximated as about 95% of the calculated electrical length.

(d) Electrical Length of an antenna is determined by the ratio of speed of light with its

operating frequency. In other words, it is the wavelength in space.

(e) Polarization of an antenna refers to the orientation/direction of the electrical field

radiated from it with respect to earth.

(f) Diversity Reception is the process of transmitting and/or receiving a number of signals

and then either adding them all together at the receiver or selecting the best one at any

given instant. It is used to accommodate severe fading problems.

(g) Corona Discharge is the term coined to the arcing effects in high-powered transmitters

that causes dissipation of power in the actual antenna.

26. What is the relationship between the electrical and physical length of Hertz antenna?

ANSWER:

The physical length of Hertz antenna is one-half of its electrical length.

27. What factors determine the resonant frequency of any particular antenna?

ANSWER:

The factors that determine the resonant frequency of any particular antenna are

inductance and capacitance.

28. If a vertical antenna is 405 ft high and is operated at 1250 kHz, what is its physical

height expressed in wavelengths? (5.54 λ)

ANSWER:

29. What must be the height of a vertical radiator one-half wavelength high if the operating

frequency is 1100 kHz? (136 m)

ANSWER:

𝑓 = 1100𝑘𝐻𝑧

λ =𝑐

𝑓

λ =3x108 m/s

1100x103 Hz= 272.727 m

λ

2=

272.727 m

2

𝛌

𝟐= 𝟏𝟑𝟔.𝟑𝟔 𝐦

30. What is an antenna feed line? Explain the use of resonant antenna feed line including

advantages and disadvantages.

ANSWER:

Antenna feed line is the transmission line used to transfer energy from the generator

(transmitter) to the antenna.

Resonant antenna feed line is actually not widely used because it tends to be inefficient

and is very critical with respect to its length for a particular operating frequency. However,

in certain high-frequency applications, it sometimes proves convenient. A series resonant

circuit is used to develop the high currents needed to excite the line. The advantages of this

is that it makes impedance matching unnecessary and it makes it possible to compensate for

any irregularities in either the line or the antenna by providing the appropriate resonant

circuit at the input. Its disadvantages are increased power losses in the line due to high

standing waves of current, increased probability of arc-over because of high standing waves

of voltage, very critical length, and production of radiation fields by the line due to the

standing waves to it.

31. What is a non resonant feed line? Explain their advantages and disadvantages.

ANSWER: A non resonant feed line or flat, meaning that there are no standing waves

on the line. The value of the impedance will be the same in each point of the line. If the feed

line were lossless, the magnitude of the voltage and current would also be the same along

the line. The only thing that would change is the phase angle and that would be directly

proportional to the line length.

32. Explain the operation of a delta match. Under what conditions is it a convenient

matching system?

ANSWER: The delta matching system matches a high-impedance transmission line

to a lower impedance antenna by connecting the line to the driven element in two places

spaced a fraction of a wavelength each side of element center. It is mechanically simpler to

leave the antenna wire uncut, and to bridge the split line across a portion of the uncut

antenna. This “delta” method of matching is not as flexible as to frequency, and for that

reason, perhaps, is in little use among amateurs for low impedance lines. It is quite practical.

33. Draw a simple schematic diagram of a push pull, neutralized radio frequency amplifier

stage, couple to a Marconi type antenna system.

ANSWER:

34. Show by a diagram how a two wire radio frequency transmission line may be connected

to feed a Hertz antenna.

ANSWER:

35. Calculate the characteristic impedance of a quarter wavelength section used to connect

a 300- Ω antenna to a 75-Ω line. (150 Ω)

ANSWER: Z0‟= 75 (300) = 150 Ω

36. Which type of antenna has a minimum of directional characteristics in the horizontal

plane?

ANSWER:

37. If the resistance and the current at the base of a Marconi antenna are known, what

formula could be used to determine the power in the antenna?

ANSWER: I2R

38. What is the difference between a Hertz and a Marconi Antenna?

ANSWER: Hertz antenna, a vertical 1/2-wave dipole antenna, center fed, positioned

1/2 wavelength above the ground. Clearly this is not a very practical configuration at low

frequencies. Nevertheless, it would be possible to improvise a temporary low frequency

vertical dipole by suspending a ¼-wavelength section of wire beneath a large helium

balloon. The transmitter and battery, mounted in a lightweight box, is attached at the lower

end of this wire. A second ¼-wavelength section of wire is connected to the

transmitter. Finally the entire assembly is allowed to rise into the air, tethered by a long

nylon line. Insulators are inserted at the antenna ends for good measure. The transmitter

itself would be wirelessly remote controlled, and could be configured as a cross-band

repeater.

Next is the Marconi antenna, a vertical 1/4-wave ground-plane monopole antenna element

above a conducting surface, at or slightly above ground level. A vertical conductor with no

loading coil and no capacitive top loading is assumed. It is fed at its base by an RF power

supply plus an appropriate matching section, with the opposing terminal connected to an

elevated „counterpoise‟ constructed on insulating supports. As an alternative to the

counterpoise, a connection is made directly to the earth‟s surface. This shallow ground

connection is constructed so as to introduce the least possible resistance. Restrictions are

imposed by the depth and surface area of the buried conductors, and the local ground

conductivity.

39. Draw a sketch and discuss the horizontal and vertical radiation patterns of a quarter

wave vertical antenna. Would this also apply to a similar type of receiving antenna?

ANSWER:

40. What is an image antenna? Explain its relationship to the Marconi antenna.

ANSWER: An image antenna is an electrical mirror-image of an antenna element

formed by the radio waves reflecting from a conductive surface called a ground plane, such

as the surface of the earth. It is used as a geometrical technique in calculating the radiation

pattern of the antenna.

41. What would constitute the ground plane if a quarter wave-grounded (whip) antenna,

1m length, were mounted on the metal roof of an automobile? Mounted near the rear

bumper of an automobile?

ANSWER:

42. What is the importance of the ground radials associated with standard broadcast

antennas? What is likely to be the result of a large number of such radials becoming

broken or seriously corroded?

ANSWER: When the antenna is mounted close to the ground, the earth below takes

the place of the "missing" half of the dipole. If ground conductivity is fair to good, a short

metal stake or rod may provide a sufficiently good ground connection for resonant and low

SWR operation on the bands for which the antenna is designed.

43. What is the effect on the resonant frequency of connecting an inductor in series with an

antenna?

ANSWER: Indeed it can be a very good antenna. Look no further than the transistor

radios and AM band receivers. In those ubiquitous consumer goods the antenna consisted of

a piece of very low loss ferrite with a very high permittivity. This was wrapped in many

amp*turns of very fine copper wire. The high permittivity gave the antennas an effective

cross-sectional area -due to the permittivity- (If I recall correctly) of a square mile or so, thus

bringing the antenna's electrical size up to the dimensions of the wavelength that it was

receiving.

On a technical bent, you could consider that the antennae interacted with the magnetic field

portion of the radiating Pointing vector.

44. What is the effect on the resonant frequency of adding a capacitor in series with an

antenna?

ANSWER: An antenna slightly shorter than a half wavelength will appear to the source as a

capacitive circuit. Compensation for additional length can be made by cutting the antenna

down to proper length or by tuning out the inductive reactance by adding capacitance in

series. This added Xc will completely cancel the inductive reactance, and the source will then

see a pure resistance, provided the proper size capacitor is used.

45. If you desire to operate on a frequency lower than the resonant frequency of an

available Marconi antenna, how may this be accomplished?

ANSWER: Marconi type requires a conducting path to ground. The site location could be

moved to a location where the soil possesses a high conductivity.

46. What will be the effect on the resonant frequency if the physical length of a hertz

antenna is needed?

ANSWER: Since hertz antennas are predominately used with frequencies above 2 MHz, one

of its disadvantages was excessive radiation at high frequencies.

47. Why do some standard broadcast stations use top loaded antennas?

ANSWER: The metallic spoked wheel at the top adds shunt capacitance to the ground. This

additional capacitance reduces the antenna‟s capacitive reactance since C and Xc are

inversely related. The antenna can, therefore, be made nearly resonant with the proper

amount of top loading.

48. Explain why a loading coil is sometimes associated with an antenna. Under this

condition, uld absence of the coil mean a capacitive antenna impedance?

ANSWER: Antennas that have less than a quarter wavelength have an input impedance that

is highly capacitive and they become inefficient radiators. A highly reactive load cannot

accept energy from the transmitter. Series inductance is termed loading coil. It is used to tune

out the capacitive appearance of the antenna. The coil-antenna combination can thus be made

to appear resonant (resistive) so that it can absorb the full transmitter power.

49. Explain how the directional capabilities of the elementary antenna array shown in fig.

13-16 are developed.

ANSWER: It consists of a simple hertz half wave dipole and a non-driven halfwave element

located a quarter wavelength behind the dipole. The non-driven element is also termed a

parasitic element, since it is not electrically connected. The dipole radiates EM waves with

the usual bidirectional pattern.

50. Define the following terms:

ANSWER:

a. driven elements– is a conductive element and electrically connected to anything else. If

it is not electrically connected, it is called non-driven element.

b. parasitic elements - is a conductive element, typically a metal rod, which is not

electrically connected to anything else.

c. reflector – “reflects” energy from the driven element.

d. director – “direct” electromagnetic energy since it is in the direction of the propagated

energy with respect to the driven element.

51. Calculate the ERP from a Yagi-Uda antenna (illustrated in fig. 13-17) driven with

500W. (2500W)

ANSWER:

52. Calculate the F/B ratio for an antenna with

a) Forward gain of 7dB and reverse gain of -3dB

ANSWER:

F/B ratio = [7dB – (-3dB)]

F/B ration = 10dB

b) Forward gain of 18dB and reverse gain of 5dB

ANSWER:

F/B ratio = [18dB – (5dB)]

F/B ration = 13dB

53. Sketch two different Yagi-Uda configurations.

ANSWER:

54. Describe the physical configuration of a collinear array. What is the effect of adding

more elements to this antenna?

ANSWER: Collinear array is any combination of half wave elements in which all the

elements are placed end to end so as to form a straight line. Each element is excited so that

their fields are all in phase (additive) for points perpendicular to the array.

55. Describe the physical configuration of a broadside array. Explain the major advantage

they have as compared to collinear arrays.

ANSWER: Such an array provides greater directivity in both the vertical and horizontal

planes than the collinear array. The separation between each stack is a half wavelength. The

signal reversal shown in the connecting wires puts the voltage and current in each element of

each stack in phase. The net resulting radiation pattern is a directive pattern in the horizontal

plane (as with the collinear array) but also a directive pattern in the vertical plane (in contrast

to the collinear array).

56. What is the direction of maximum radiation from two vertical antennas spaced 180

degrees and having equal currents in phase?

ANSWER: It is omnidirectional in the ground plane but falls to zero off the antenna‟s top.

57. How does a directional antenna array at an AM broadcast station reduce radiation in

some directions and increase it in other directions?

ANSWER: The elements are physically placed in such a way that their radiation fields

interact with each other, producing a total radiation pattern that is a vector sum of the

individual fields. The purpose of an array is to increase the directivity of an antenna system

and concentrate the radiated power within a smaller geographic area.

58. What factors can cause the directional pattern of an AM station to change?

ANSWER: Radiation resistance, Wavelength, distance between antennas.

59. Define phased array.

ANSWER: Phased array antenna is a group of antennas or a group of antenna arrays that,

when connected together, function as a single antenna whose beamwitdh and direction can be

changed electronically without having to physically move any of the individual antennas or

antenna elements within the array.

60. Describe the major characteristics of a log-periodic antenna. What explains its

widespread use? Explain the significance of its shortest and longest elements.

ANSWER: A log-periodic antenna is independent of their radiation resistance and radiation

pattern to frequency. They have bandwidth ratio of 10:1 or greater. They can be

unidirectional or bidirectional and have low to moderate directive gain. Log-periodic

antennas are used mainly for HF and VHF communications. However, they do not have a

terminating resistor and therefore, more efficient. Very often, TV antennas advertised as

“high-gain” or “high-performance” antennas are log-periodic antennas.

61. Design a log-periodic antenna to cover the complete VHF TV band. (See Chapter 7 for

the frequencies involved.) Use a design factor (Ƭ) of 0.7 and provide a scaled sketch of

the antenna with all dimensions indicated.

ANSWER:

Frequencies: 54 – 216 MHz

62. Describe the directional characteristics of the following types of antennas:

a. Horizontal Hertz Antenna

ANSWER: The pattern is circular because radiation is uniform in all directions perpendicular

to the antenna.

b. Vertical Hertz Antenna

ANSWER: Two major lobes radiate in opposite directions that are at right angles to the

antenna

c. Vertical Loop antenna.

ANSWER: Vertically polarized.

d. Horizontal loop antenna.

ANSWER: Horizontally polarized.

e. Vertical Marconi Antenna.

ANSWER: The conductors are spread out in a straight line to a total length of one-quarter

wavelength.

63. What is the directional reception pattern of a loop antenna?

ANSWER: Small vertically polarized loops are very often used as direction – finding

antennas. The direction of the received signal can be found by orienting the loop until a null

or zero value is found.

64. A loop antenna used for DF purposes detects a null from a signal with the loop rotates

35° CCW from a line of latitude. The antenna is moved 3 mi west along the same line of

latitude and detects a null from the same signal source when rotated 45° CW from the

line of latitude. On a sketch, show the two points when readings were taken and the

exact location of the signal source with respect to the two points.

ANSWER:

3Miles

45° CW 35° CCW

65. What is a ferrite loop antenna? Explain its application and advantages.

ANSWER: Ferrite loop antenna is an extension of the basic loop antenna. The effect of using

a large number of loops wound about a highly magnetic core serves to greatly increase the

effective diameter in loops. This forms a highly efficient receiving antenna considering its

small physical size compared to the hundreds of feet required to obtain a quarter wavelengths

for the broadcast AM band.

66. What is the radiation resistance of a standard folded dipole? What are its advantages

over a standard dipole? Why is it usually used as the driven element for Yagi-Uda

antennas instead of the Hertz antenna?

ANSWER: The folded dipole antenna offers the same radiation pattern as the standard Hertz

antenna but has an input impedance of 288 ohms and offers relatively broad –band operation.

For Yagi – Uda antennas, Folded dipoles helps to maintain a reasonably high input

impedance since the addition of each director lowers this array‟s input impedance.

67. Describe the operation of a slot antenna and its application with aircraft in a driven

array format.

ANSWER: The individual drive to each slot is controlled by phase – shifting networks.

Proper phasing allows production of a directive radiation pattern that can be swept through a

wide angle without physically moving the antenna. This allows a convenient mobile scanning

radar system without mechanical complexities.

68. An antenna has a maximum forward gain of 14 dB at its 108-MHz center frequency. Its

reverse gain is -8dB. Its beamwidth is 36°and bandwidth extends from 55 to 185 MHz.

Calculate

a. Gain at 18° from maximum forward gain. (11 dB)

SOLUTION: 𝐺𝑎𝑖𝑛 = 14𝑑𝐵 + (−3𝑑𝐵)

𝑮𝒂𝒊𝒏 = 𝟏𝟏𝒅𝑩

b. Bandwidth. (130 MHz)

SOLUTION: 𝐵 = 185𝑀𝐻𝑧 − 55𝑀𝑕𝑧

𝑩 = 𝟏𝟑𝟎𝑴𝒉𝒛

c. F/B ratio. (22 dB)

SOLUTION: 𝐹

𝐵𝑟𝑎𝑡𝑖𝑜 = 10 log

185+55

2

𝑭

𝑩𝒓𝒂𝒕𝒊𝒐 = 𝟐𝟐 𝒅𝑩

d. Maximum gain at 185 MHz. (11 dB)

SOLUTION: 𝐴𝑀𝐴𝑋 = 10 log(185 − 108) − 8𝑑𝐵

𝑨𝑴𝑨𝑿 = 𝟏𝟎.𝟖𝟔 𝒅𝑩 = 𝟏𝟏𝒅𝑩

69. Explain the difference between antenna beamwidth and bandwidth.

ANSWER: Antenna beamwidth is simply the angular separation between two half – power (-

3dB) points on the major lobe of an antenna „s plane radiation pattern, usually taken in one

of the “principal” planes. Antenna bandwidth is vaguely defined as the frequency range over

which antenna operation is “satisfactory” and is taken as the difference between the half-

power frequencies but sometimes refers to variations in the antenna‟s input impedance.