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.

In these articles we will discuss cables and talk about the different antenna's found on typical aircraft and see where, how and why they are used.

Antenna cables

A special cable is used to connect the aircraft radio to the antenna. This circular COAX (coaxial) cable has certain characteristics for the frequencies used and must match the radio and antenna (electrically and mechanically). It must not attenuate the signals too much and radiate in the aircraft or pickup signals like ignition pulses or the strobes.

The cable normally used for this is coax RG-58A, although it is not the best in the world for our purposes. This cable is a thin one (easy handling and cheap) but the signal characteristics at 100 MHz and higher are not the best in the world. If you do need to use such a cable please select the RG-58C/U mill spec or the Aircell 5. The cable impedance is also 50 ohm but it has less loss compared to the RG-58A. Use it only for very short runs. No more than a couple of feet. RG-58A is only usable to 30 MHz or so and for low power transmissions only.

Coaxial cable is characterized by impedance and cable signal loss at specified frequencies and lengths. For normal coaxial cable used at aviation frequencies, the characteristic impedance depends on the dimensions of both conductors and cable loss is determined by conductor dimension compared with the type of dielectric used (air, foam).

Good Antenna Cable
Good communication over long distances, either receiving or transmitting, usually means not saving money on the cable connecting your radio and antenna. Do invest in a good cable. The types we would recommend are LMR-400, AIRCOM plus or the CLF-400. RG-213/U would be in between these cables and a RG-58C/U mill spec or Aircell 5, it is a reasonable priced alternative.
These cables are about 10mm thick (RG-58 and Aircell 5 are 5mm) have a sturdy connector, they do not bend as easily as the thin types but they are the best, have almost no loss in signal on VHF/UHF frequencies and can be used for your transponder with equal performance on UHF (950-1050 MHz) too. Try that with your RG-58A. Did you even wonder why ATC sometimes says no radar contact?

When installing the antenna cables in the aircraft, keep them away from other cabling. This reduces the possibility of having RF power radiated in other aircraft cables and receiving strobe pulses in the radio's. Also, do not kink the cable, or fit tightly around corners in the aircraft structure. These kinks in the cable can lead to splits in the outer jacket and braided shield, which will lead to a downgrade in performance over time, check the specifications for minimum bend radius.

Balun
As a dipole antenna (VOR) is a symmetrical antenna and the coax cable is asymmetrical. Connecting these two will make sure that the coax cable becomes part of the antenna, i.e. it will receive or radiate signals. Inside and from the aircraft too. To overcome this glitch a transformer must be used and the term is 'balun'. Short for balanced unbalanced.
This balun is sometimes included with the antenna, but if you buy it without the balun you can save some 60 odd dollars and buy good coax RG 58 C/U mill spec cable and build your own balun with it and connect it to the antenna. And still have money left to buy a beer.

If you need to know how to build a good and simple 1:1 balun follow those instructions. The antenna show is not bend (thus 73 ohms) but in our case you will need to use 50 ohm cable to construct the balun and you are good to go with our aviation VOR antenna (which is bend and thus 50 ohms).

The calculated length of the balun with a RG-58 C/U cable and tuned for 113 MHz (center of the NAV band) is 45 cm or 17.7 inches. See formula for more details. To make this balun durable you will need to isolate it from the weather with insulation tape, heat shrink tubing and some sealant.
If you use a cable with a different velocity factor make sure to change this in the formula. For example: Aircell 5 has a VF of .82 so replace the .68 by .82 in the formula and you are good to go.

COMmunications

Aircraft radio's use frequencies between 118 MHz and 136 MHz to communicate and as the location of the receiving end varies the signal is send in all directions (omni) with a vertical polarization (upright, vertical antenna). These VHF frequencies have a line of sight capability. This means VHF range is from where you stand or fly to the horizon. These signals do not follow the curvature of the earth. Ok, well just a little..about 20% The actual range depends also from 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 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 groundstation antenna in feet.
The range is a theoretical optimum, actual range is less due to transmitter power, receiver sensistivity, 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.

With composite aircraft this can be solved by installing a sheet of aluminum of at least 30" by 30" to act as ground (anything less and the installation will not radiate its full power) and install the antenna in the middle of the sheet. Some antenna manufacturers sell a dipole, which is a symmetrical antenna and solves the ground problem altogether but this type must be mounted vertically. 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.

Long Range Communications
As mentioned previously, VHF has a limited 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 over long distances). For reasonable predictable long range communications the HF band (3 - 30 MHz) is used. 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.