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VHF radiocommunications

VHF characteristics and radio operation


Rev. 11a — page content was last changed October 28, 2011
  
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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.

3.1 VHF radio wave propagation

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.
LOS distance
LOS distance between a ground station and an aircraft station, or between two aircraft stations, is limited by the curvature of the Earth's surface, and dependent on the elevation/height of the two stations and the elevation of intervening terrain.

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.

Theoretical LOS distance to horizon
Aircraft height (feet) Maximum LOS distance (nm)
103.2
10010
100032
500070
10 000100


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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3.2 Transceiver operation

The apparatus that comprises an aircraft station is:
  • an antenna system and feedline coaxial cable
  • a radio transmitter/receiver unit or transceiver with modulating, transmitting, receiving, demodulating and power amplification circuits, plus mounting for the operator controls and displays
  • a speaker/earphones and circuits to convert electromagnetic waves to sound waves
  • a microphone and circuits to convert sound waves to electromagnetic waves
  • the necessary interconnection cables, connectors and matching devices.
All the system components must be correctly matched (electrically) to each other and to any separate cockpit intercommunication unit installed in a two-seat aircraft.
Transmission
Amplitude modulation [AM] of the fixed RF carrier wave, rather than frequency modulation [FM], is used in the aviation band to impress the voice information on the carrier wave generated by the transceiver. AM occupies less bandwidth than FM, consequently the AM channel spacing in the aviation COMMS band is only 25 kHz.

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.
Reception
An 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.
Setting and changing frequencies
The frequencies required are usually entered into a VHF transceiver via an electronic keyboard, concentric rotatable knobs, toggle buttons or a set of thumbwheels. There may be a switch to set channel steps at either 25 kHz or 50 kHz. Most transceivers allow the user to set one frequency into the unit as the active frequency and to set a second frequency as the standby frequency. All transmission and reception is done on the active frequency. Pressing a flip-flop, or similar switch, causes the standby frequency to become the active, and the active to become the standby.

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
  • a number of memory positions (5–50) allows storage of frequently used airfield/FIA and other frequencies
  • an associated fast-scanning function of those stored frequencies
  • instant access to the emergency/distress frequency of 121.5 MHz
  • high and low transmit power settings for hand-held transceivers, giving a choice of minimum battery drain or maximum range
  • hand-held transceivers are usually supplied with adapter(s) to connect the unit to the aircraft's COMMS (and NAV) antenna(s)
  • hand-helds usually have key locking facilities to prevent inadvertent frequency changes or transmissions
  • Hand-helds may also provide access to the 200 channels in the NAV band between 108.00 and 117.975 MHz, which gives a limited VOR capability if the transceiver can be adapted to a NAV dipole antenna. The main advantage provided by this facility is access to any ATIS or AWIS frequencies between 112.1 and 117.975 MHz.
Headsets
The cockpits of powered recreational and sport aircraft are notoriously noisy and those close to a high rpm two-stroke engine are the worst. Propeller tip speeds may approach mach 0.8 and generate noise at fairly high frequencies while the engine produces noise in the low to middle frequencies. External airflow noise may or may not be significant depending on the existence and effectiveness of cockpit sealing. In all, the cockpit noise level may approach 100 dB and long-term exposure to noise above 90 dB will damage hearing. Also, noise and vibration add to pilot fatigue and the low-frequency engine noises below 300 Hz are particularly fatiguing. Consequently all pilots must wear some form of hearing protection — which may be incorporated within a good quality protective helmet.

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.
Using the squelch control
All transceivers have some form of ON/OFF/TEST/VOLUME control. As aircraft cockpits are very noisy, the output volume control must be set fairly high. This of course amplifies the weak atmospheric background radio frequency noise — the hash — which is always there when no strong transmissions are being heard on the active frequency; this hash can be quite annoying.

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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3.3 Wave length and antennas

It is stated in the 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.
Dipole antennas
Aircraft COMMS antennas are usually dipoles or monopoles. A dipole is an antenna that is divided into two halves insulated from each other. Each half is connected to a feedline (coaxial cable and RF BNC series bayonet connectors) at the inner end, which routes the RF energy between the antenna and the transceiver. The length of each half is about 5% less than the mid-point quarter-wave — usually about 56 cm, or 22 inches. (The mid-point quarter-wave is 2.37/4 =59 cm.) Rather than being set out end-to-end horizontally, each half is canted up about 22.5° to form an internal angle of around 135°, which prevents a deep "null" zone off both ends. NAV or COMMS dipoles may be mounted within the fuselage if the aircraft is not metal-skinned or metal-framed. A NAV antenna must be horizontally polarised; i.e. mounted horizontally.

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.
Monopole or whip antennas
The most common recreational aircraft COMMS antenna — the monopole — is just one half of a dipole; i.e. quarter-wavelength. (To calculate antenna quarter-wavelength in centimetres, divide 7130 by the frequency; i.e. 7130/126.7 = 56 cm.) Thus the monopole is usually about 56 cm long, mounted vertically (vertically polarised) — normally on the top of the fuselage (away from the undercarriage legs) — with the feedline conductor to/from the transceiver connected to the bottom end of the antenna. The 56 cm length should provide very good mid-frequency reception and reasonable reception at the lower and upper ends of the COMMS band and, usually, increasing the thickness of the antenna element increases its effectiveness. The antenna element may be enclosed within a streamlined fibreglass fairing to add structural strength. To replace the other half of the dipole a conductor system is placed just below the antenna to serve as an earth ground — a ground plane, ground screen or at least four ground radial strips or rods, connected to the coax cable shielding. The radius of the ground equals the length of the antenna; i.e. 56 cm. In a metal-skinned aircraft the fuselage acts as a ground plane, which is electrically insulated from the antenna by a very small gap.

Ground plane 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.
Transmission/reception pattern
Because of antenna characteristics and airframe shielding, the radiation/reception pattern of the antenna will be weaker in some directions and may even exhibit null zones. The easiest way to check this is to tune in the continuous broadcast — at a reasonable (say 30 nm) distance — from a known 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.
Impedance matching
All VHF transceivers are designed for a standard load (impedance) of 50 ohms. Ideally the coaxial cable, BNC connectors and antenna match that 50 ohm impedance all the way; then all the transmission power sent to the antenna will be radiated as RF energy. However, the resonant frequency of any antenna will match only one frequency, and the COMMS operational frequencies range over 19 MHz. So for most transmission frequencies the antenna will exhibit positive or negative reactance (or impedance), which results in the phenomenon known as 'stationary' or 'standing' waves in the feed line and reduces the output of the antenna. Also the incoming signals will be weaker.

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 |


Next – Microair 760 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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