An aircraft uses a range of radio frequencies to navigate to its destination and communicate with air traffic control and or other aircraft. To do this successfully, the onboard radio equipment uses different types and sizes of antenna's, each designed for their own frequency band.
Each of these antenna's have their own characteristics regarding frequency and application and thus location on the aircraft. Even the connection between the antenna and avionics has its own set of specifications.
The connection usually consists of a number of connectors and a round coaxial cable. It is wise to invest in a good quality cable so that cable losses are kept to a minimum and communications are clear. Which contributes to safety.
In these series of articles we will discuss cables and talk about the different antenna's found on typical general aviation aircraft and see where, how and why they are used.
We will start with antenna cables as these are used to connect the radio to the antenna and are very important for good and clear communications over long distances.
A special cable is used to connect the aircraft radio to the antenna. This circular coaxial (coax) cable has special 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 from the engine(s) or strobes.
The cable normally used for this is coax RG-58A, although it is not top of the line 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 even better the Aircell 5. The cable impedance is also 50 ohm but it has much less loss compared to the RG-58A. If you decide to go for RG-58 type use it only for very short runs. No more than a couple of feet. RG-58A is only practible usable below 30 MHz or so and for low power transmissions only.
Coaxial cable is characterized by its impedance and cable signal loss at specified frequencies and lengths. For normal coaxial cable used at aviation frequencies (VHF and UHF), 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 (dielectric is the material isolating the two conductors inside the cable, which can be air or some kind of foam).
Perfect communication over long distances, either receiving or transmitting, usually means not saving money on the cable connecting your radio and antenna. Please do invest in a good quality 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 10 mm thick (RG-58 and Aircell 5 are 5 mm diameter) have a sturdy connector, they do not bend as easily as the thin types but they are the best, having almost no signal loss at 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 wiring and receiving strobe pulses in the intercom system. Also, do not kink the cable, or fit too tightly around corners in the fuselage 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. Remember to check the specifications for minimum bend radius for the selected cable of your choice.
As the dipole antenna (VOR) is a symmetrical antenna and the coax cable is asymmetrical, this will not result in a perfect match. Connecting these two together will make sure that the coax cable becomes part of the antenna, i.e. it will receive or radiate signals in stead of shielding them. Inside and from the aircraft too. To overcome this problem, a transformer must be used and they are called 'balun'. It is short for balanced-unbalanced.
This balun is sometimes included with the antenna. But if you buy one without the balun, you can save some 60 odd dollars. For that money you can get a good coax like the Aircell 5 specifications cable and build your own balun with it. Use the same cable to connect antenna and balun to the VOR receiver. And you will have money left to buy a beer!
If you need to know how to build a good and simple 1:1 balun then follow the instructions in the image. The antenna shown is not bend (thus its impedance is 73 ohms) but in our case you will need to use 50 ohm RF coax cable to construct the balun and you are good to go with our aviation VOR antenna (which is bend and thus has around 50 ohm impedance).
The calculated length of the balun with a RG-58C/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 weather and UV proof 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 velocity factor of .82 so replace the .68 by .82 in the formula and you are good to go.