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VHF characteristics and radio operation
Rev. 11a — page content was last changed October 28, 2011
VHF provides a simple, reliable and high-quality communications system. It is essentially short-range, as reception is limited to a line-of-sight direct path between the transmitter and the receiver. Modern VHF aircraft transmitter/receiver systems are versatile and easy to operate, if properly installed, but the noisy cockpit environment does cause some reception/transmission difficulties.
Electromagnetic waves travel in straight lines, but the transmission process is modified by interaction with the Earth's surface and by reflection, refraction and diffraction occurring within the atmosphere. The major source of modification of the paths of radio waves is the radiation-related layers within the ionosphere. The process by which the signal (the fixed carrier frequency plus the information) is conveyed between the transmitter and the receiver is propagation. Radio signal energy loss (attenuation) increases with distance travelled through the atmosphere or other materials.
Propagation of radio waves within the high frequency [HF] band (the 'short wave' bands between 3 MHz and 30 MHz, with 12 aeronautical sub-bands in the domestic and international HF networks between 2850 and 22 000 kHz) is significantly modified by reflection/refraction within the ionospheric layers — a 'skipping' process that facilitates transmission over very long distances while using low power and small antennas.
Propagation in the VHF band (30 MHz to 300 MHz), when using low power and small antennas, is chiefly in the form of a direct path. It is relatively unaffected by reflection, refraction and diffraction within the atmosphere; but is heavily attenuated by the Earth's surface and readily blocked, diffracted or reflected by terrain or structures — as experienced with VHF-band TV reception. Therefore for good reception of a VHF transmission there must be a direct line-of-sight [LOS] path between the transmitter antenna and the receiver antenna. The transmitter radio frequency [RF] output energy must be sufficient that the signal is not overly attenuated over that LOS distance.
The rule-of-thumb is: the maximum direct path distance (the distance to the horizon) between an aircraft and a ground station, in nautical miles, is equal to the square root of the aircraft height, in feet, above the underlying (flat) terrain. Actually it is 1.06 times the square root of the height, but for our purposes that can be ignored.
Estimating the square root: mental calculation is easier if you ignore the two least significant digits of the height, then estimate the square root of the remaining one or two digits and multiply by 10. For example; height 3200 feet, the square root of 32 is between 5 and 6 — say 5.5 — and multiply by 10 = 55 nm LOS distance. Another example; height 700 feet, ignore 00, the square root of 7 is between 2 and three — say 2.6 — multiply by 10 = 26 nm LOS distance.
For air-to-air communications the LOS distance is the sum of two 'distance to horizon' calculations; i.e. with one aircraft at 5000 feet the other at 10 000 feet, the maximum LOS distance will be 70 + 100 = 170 nm. It may be a bit more than that because of wave diffraction at the intervening horizon. Intervening mountain terrain may reduce the distance.
Be aware that the LOS distance is the theoretical maximum range for direct-path VHF transmission/reception. The actual distance is likely to be a lot less depending on the transmitter/receiver system, the type and placement of the antenna, the quality of the receiver/headset system, and quite a few other considerations. The effective range may be as low as 5 nm or as much as the full LOS distance — but an effective range of 50 nm is probable for a good low-power installation.
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When the transceiver is powered up and the pilot speaks into the microphone while depressing a 'press-to-talk' [PTT] button, the transmitter circuits amplify and broadcast, via the antenna system, the selected output frequency — 126.7 MHz for example — modulated with the audio frequencies from the microphone. This may also include the cockpit background noise. The low-fidelity R/T audio frequencies added are in the range 50 Hz to 5000 Hz; much the same as the domestic AM radio broadcast or the public telephone system.
The transmission power of handheld transceivers is usually around 1 to 1.5 watts carrier wave. Fixed-installation transceivers are around 4 to 8 watts carrier wave.
Some hand-held transceiver suppliers quote the peak envelope power [PEP] output which, for ordinary speech, is probably around three times the carrier wave value. The peak envelope power of an AM signal occurs at the highest crest of the modulated wave.
ReceptionAn aircraft antenna continually collects all passing RF energy in the band for which it is designed, which at any time will normally consist of many transmissions. The receiver tunes out all transmissions on all frequencies except one — the selected, or active, frequency. Signals on this frequency are demodulated to isolate the voice information from the carrier, amplify it and pass to the speaker system to convert to the sound waves heard in the earphones or speaker.
Thus, normal procedure prior to take-off is to set the airfield frequency as the active and the flight information area [FIA] frequency as the standby. When departing the airfield area, pressing the flip-flop will make the FIA frequency active for the required listening watch. On return to the airfield area pressing the flip-flop again restores the airfield frequency to active.
Generally when selecting, keying or dialling another frequency during flight the new frequency changes the stand-by frequency.
Some transceivers have 'dual-monitoring' capability – the ability to listen-in on more than one frequency (e.g. the FIA frequency and an airfield frequency) – but transmit on one frequency only.
Features common to most transceivers
Headsets serve a dual purpose in providing hearing protection whilst improving communications. The basic headset consists of two earphones with some physical sound sealing capability plus a directional microphone mounted on an adjustable boom, so that it can be positioned within 1–3 cm in front of — and square on to — the pilot's lips when transmitting. The headset cables are jacked into the transceiver input/output sockets or patched via a cockpit intercom unit. Standard headsets may not be able to be used with hand-held transceivers without an adapter device.
Additional facilities — such as individual volume control on each earphone with an electronic noise reduction system and cockpit noise cancelling microphones — are available. You can get headsets specifically designed for two-stroke engine noise reduction.
Normal headsets rely solely on passive noise reduction — creating a physical barrier around the ear to attenuate noise — which usually works quite well for middle to high-frequency sound but doesn't block low-frequency engine noise and background rumble.
Active noise reduction technology uses electronics to determine the amount of low-frequency (50–600 Hz) engine and other noise entering the system and then generating out-of-phase noise, in the same frequency range; this counters the background noise and leaves a soft 'white' noise in the headphones. But the technology doesn't significantly affect the higher-frequency noise.
The 'squelch' or 'gain' or 'RF gain' or 'sensitivity' control is an adjustable filtering device which, for operator comfort, can be set just to filter out the hash but still allow any strong signals to be switched through. The squelch control should only be switched on and adjusted when contact with the active frequency has been established, volume set and headset connection checked. Otherwise, when the signal is weak, there is a high risk of also filtering out the active frequency transmissions which, in effect, turns the receiver off.
Some transceivers have an automatic gain control. In which case, pressing the test facility will override the squelch, allowing the background hash to be heard.
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electromagnetic spectrum section that the frequency in MHz = 300/wavelength in metres — or restated, the wavelength in metres = 300/MHz.
Thus the wavelengths involved in the aviation VHF COMMS band, 118.00 to 136.975 MHz, are from 2.54 metres to 2.19 metres and the mid-point is about 2.37 metres. The Multicom frequency — 126.7 MHz — has a wavelength of 300/126.7 = 2.37 metres. Wavelength is important as the efficiency of the antenna (a passive electrical conductor that radiates the signal energy when transmitting, or collects signal energy when receiving) partly depends on its length relative to the frequency wavelength. Most ineffective radio installations are because of ineffective antenna installations and/or RF interference generated by the engine ignition system or the aircraft's electrical components.
The two halves of a COMMS dipole antenna can be end-to-end vertically mounted with a centre feedline and built into the fin of a fibre-reinforced composite aircraft — but not if it is carbon fibre. Similarly a half-wave dipole antenna might be used on a trike where the longer length can be mounted vertically end-to-end and strapped to the king-post.
The telescopic 'rabbit's ears' antennas used with the old black and white TVs were dipoles — as channels (frequencies) were changed the length was adjusted to maintain the half-wavelength dimension.
The photo shows the ground plane, in Leo Powning's Jodel project, mounted under the ply turtle deck (looking aft). The centre plate and four 25 mm wide radials are cut from light gauge aluminium sheet sold in hardware stores. Total dimension from the antenna socket to the end of each radial is 57 cm — about the mid-point of the COMMS band. The sloped radials provide an antenna impedance of approximately 50 ohms. The 50 ohms coax connecting the antenna is attached to the turtle deck formers with plastic P clips. ATIS, AWIS or AERIS location, then circle while listening to the signal strength. A few turns should be sufficient to plot the directions, relative to the aircraft's longitudinal axis, from which signal strength weakens and/or reduces to nil.
Because the attitude of the aircraft also affects transmission/reception, it is advisable to first fly non-banked turns to ascertain the normal pattern then fly banked turns to check the consequent effects.
The RF performance of the antenna system is expressed in terms of the voltage standing wave ratio [SWR or VSWR]. A perfect (but most unlikely) antenna system would have a SWR of 1:1 but generally a SWR less than 2:1 results in quite acceptable performance and limits transceiver overheating. The Microair 760 — described in the next module — requires a SWR between 1.3:1 and 1.5:1. If the transmission performance is okay then the reception performance should also be okay.
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Groundschool – VHF Radiocommunications Guide
| Guide content | Abbreviations and acronyms |
| 1. Transmitter licensing | 2. R/T phrasing | [3. VHF characteristics and radio operation] |
| 4. Microair 760 transceiver | 5. R/T procedures | 6. Safety and emergency procedures |
| 7. Aviation Distress Beacons | 8. Understanding SAR services |
|The next section of the VHF radiocommunications guide introduces you to the controls and operation of an Australian designed COMMS transceiver|
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