Aircraft Communications Antenna
An aircraft uses a range of radio frequencies to navigate and communicate with air traffic control. To do this successfully the radio equipment uses different types and sizes of antenna.
Each of these antenna have their own characteristics regarding frequency and thus location on the aircraft. Even the connection between the antenna and avionics has its own set of specifications.
Aircraft short range communication uses the VHF band between 118 MHz and 136 MHz to talk with air traffic control. And as the location of the receiving is not always the same, the signal must be send in all directions (omni) with a vertical polarization (upright, vertical antenna).
COMmunications
These VHF frequencies have a line of sight capability. This means VHF range is from where you stand or fly to the visible horizon. These signals normally do not follow the curvature of the earth. Ok, well just a little.. about 20%.
The actual range depends also on the height of your antenna. Thus, an aircraft at FL400 has more VHF range than one at 1500 ft. It also depends on how much power your transmitter has (some have 7, 10 or even 16 watts). Atmospheric conditions can either help increase range or make some frequencies unusable, this effect is sometimes noticeable on VHF but even more so on HF (3 MHz - 30 MHz) and LF (300 KHz - 3 MHz).
VHF Radio Range
To calculate VHF range use this formula: VHF Range = 1.33 × (√H-aircraft + √H-gs).
Where: VHF Range in statute miles, H-aircraft is the altitude of the aircraft in feet and H-gs is the height of the ground station antenna in feet.
The range is a theoretical optimum, actual range is less due to transmitter power, receiver sensitivity, antenna cable losses and efficiency of antenna's. To compensate use a multiplication factor of 1.2 for a more realistic range.
COM Antenna
A vertical, slightly bend backwards quarter wave rod placed on top or on the bottom of the aircraft. Usually white or stainless steel. Sometimes blade antenna are used, they have a greater bandwith (less selective) but a drawback is a somewhat worse SWR (standing wave ratio, a factor indicating how good the antenna performs. This SWR should ideally be 1:1 meaning no loss of signal.
Or as someone once said: "The Standing Wave Ratio is a measure of how much power goes out of the antenna and how much is 'reflected' back towards the transmitter"). This antenna needs a good 'ground'. With a metal aircraft this is no problem, there is enough good 'ground' available.
Composite Aircraft
With composite aircraft this can be solved by installing a sheet of aluminum of at least 20" by 30" to act as ground (anything less and the installation will not radiate its full power). Make sure to install the antenna around third from the edge of the sheet so that it can 'see' its reflection. For bend wip antenna a sufficiently large oval should do the trick.
Some antenna manufacturers sell a dipole, which is a symmetrical antenna and solves the ground problem altogether but this type must be mounted vertically and is then omnidirectional.
A number of composite aircraft are made of conductive (carbon fibre) composites. This acts as a shield (Cage of Faraday) and it will reduce radio reception and transmission range. In this case you will need external antenna's and a sheet of metal as carbon fiber is not a perfect ground conductor.
Long Range Communications
As mentioned previously, VHF has a limited line of sight range and is not really suitable for direct communications over very long distances (although sometimes ionospheric conditions can open up the band and reflect even VHF beyond the curvature of the earth). For reasonable predictable long range communications the HF band (3 - 30 MHz) is used.
Vertical Fin
For long range communications the aircraft is equipped with separate radio's and a vertical antenna in the vertical fin (or a wire from wing tip to tail) dedicated for the HF band. For example: transatlantic flights use HF to communicate with either Gander in Canada or Shanwick in Ireland when they fly out of VHF range. It can also be used when onboard situations arise needing consulting with the home base.
