RADIO THEORY

Air Waves in Aviation

DEFINITIONS

• Radio transmission is an electromagnetic wave with the same characteristics as light or heat.

• Wavelength is the linear measurement of the wave.

• Cycle is the interval in which the wave rises and falls between its crest and trough.

• Frequency is the number of cycles/second.• Amplitude is the strength of the signal.

RADIO BANDS

• Low and Medium Frequency: Non Directional Beacons and Marker Beacons transmit signals on the LF/MF bands of 200 to 415KHz and 510 to 535KHz.

• High Frequency: used for air/ground communication in remote northern areas and on transoceanic flights. (longer range than VHF) 2,500KHz to 30,000KHz

• Very High Frequency: the most common frequency range in aviation, used for voice communication, VOR, and ILS. 30MHz to 300MHz

• Ultra High Frequency: allocated for military use, DME, and glide slope. 300MHz to 3,000MHZ

3 kHz to 30 kHz Very Low Frequencies (VLF)30 kHz to 300 kHz Low Frequencies (LF)

300 kHz to 3,000 kHz Medium Frequencies (MF)3,000 kHz to 30,000 kHz High Frequencies (HF)

30,000 kHz to 300,000 kHz Very High Frequencies (VHF)300,000 kHz to 3,000,000 kHz Ultra High Frequencies (UHF)

PROPERTIES OF RADIO WAVES• Reflection: change of direction occurring at a

surface separating two different media.• Refraction: bending of wave as it passes from

one medium to another.• Diffraction: bending of wave as it passes

beside a non-conductive object.• Attenuation: weakening of wave as it travels

through a medium.

PROPAGATION

• LF, MF, and HF transmit in two ways.• Ground Waves: follow the surface of the earth

as an effect of diffraction and surface attenuation.

• Sky Waves: travel into the atmosphere and are reflected back to earth by the ionosphere.

• A skip zone exists between the points where the ground waves end, and the sky waves strike the earth. (erratic or non-existent signals)

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• VHF waves are not affected by the same propagation characteristics.

• These waves do not reflect off the ionosphere but continue into space.

• They do not follow the curvature of the earth.• This line-of-sight (space waves) characteristic

means reception is dependant on altitude. 1.23√AGL=range(nm)

ATMOSPHERIC CONDITIONS• LF, MF, HF are affected by the changing height of the

ionosphere from day to night.• At night sky waves travel are reflected by the

ionosphere which allows for reception at greater distances.

• Sunspots and electromagnetic disturbances affect the reflectivity of the ionosphere causing signal fade out.

• Electrical fields associated with clouds (notably thunderstorms) create precipitation static.

• VHF is virtually free from atmospheric and precipitation static.

The ionosphere is a collection of ionized particles and electrons in the earths atmosphere. It is responsible for reflecting LF, MF, and HF radio waves. The D-layer which develops shortly after sunrise and disappears shortly after sunset absorbs this radio energy minimizing the reflection of sky waves. When the D-layer fades at night the radio waves are reflected back to earth as sky waves by the E and F-layers.

HF COMMUNICATION

• Because of HF radios propagation characteristics it is very useful for long range communications.

• It is used in remote areas such as Northern Canada and during transoceanic flights.

• The higher range of HF travels farther in the day while the lower HF signals travel farther at night due to the changing ionosphere.

– Sun up, frequency up– Sun down, frequency down

ANTENNAS

• Radio antennas operate on the principle of resonance: when a tuning fork is struck it will vibrate at its natural frequency; when two tuning forks are tuned to the same frequency the first tuning fork will induce vibration in the second.

• Radio antennas are tuned to the frequency to be received.

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• The ideal antenna length is one wavelength long. That means a standard VHF antenna would be 6-30 feet long. Antennas this length are impractical, so sub-multiples are used.

• LF,MF,HF: long wave=long antenna (wire or Pod)

• VHF: short wave=short antenna (mast)• UHF: very short wave=very short antenna

(blade)

DATA LINK COMMUNICATIONS

• VHF data link: used for limited data transfer on the ground such as ATIS, taxi instructions, clearances, and graphical weather services. The information is displayed on a display unit on the flight deck.

• Satellite Communications (SATCOM): used for communications in oceanic and remote environments. Allows virtual global coverage and high quality communications. Not subject to typical radio reception problems.