Do antennas wear out? What kind of coax is best for Nav-Comm radio leads to the comm and nav antennas? Should it ever get replaced?
Response credit to Jerry Kaidor, with added content by Mike Rellihan:
Antennas can and do deteriorate with time. Any time new radios (Nav or Comm or both) are installed, new antenna cable and antennas should be made part of the project, unless they have already been very recently replaced. If the existing antennas were wisely installed, there will be doubler plates and anchor nuts in the airframe. If you can find a replacement antenna with the same “footprint”, replacement can be very straightforward. You unscrew the mounting screws, pull out the old antenna and cable connector, properly prepare the surface area (according to the new antenna’s instructions), and screw on the new antenna. Most experienced installers do not use any gasketing that comes with a new Comm-type antenna. They install it against bare skin, and then seal the perimeter with white or clear polyurethane or silicone caulk. This is in the interests of gaining a better high-frequency ground plane connection to the aircraft skin, rather than relying on the mounting screws alone.
More often, the screws will have nuts and washers on the inside, which makes it a more difficult job. Sometimes the nuts will be accessible by removing the overhead console and the aft bulkhead trim panel. In other cases the headliner or floorboards will have to come out. Sometimes the coax will be clamped down inside near the antenna base, and can’t be pulled clear of the hole in order to transfer it to the new antenna; this situation will mandate internal access, just like having nuts on screws rather than having anchor nuts. Sometimes you will find that there is no structural member or doubler, reinforcing the skin at an antenna mounting area.
If not already present, a doubler should be riveted in before installing any new antenna. Otherwise the antenna will “buzz” in the wind, shortening the life of its finish, and leading to cracks in the aircraft skin. It can even cause breaks in the conductor inside the antenna. The fact that the old antenna didn’t cause any skin cracks doesn’t mean that the new one won’t. Anything that changes flexibility or resonance can lead to very different effects on the mounting area.
Whenever possible, if you have an older antenna exposed or are installing a new one, you should install new antenna cables. Coax is a disposable good that ages, even on the shelf. The higher the frequency, the more it matters. Comm and Nav frequencies are high enough to make it matter. At some point it takes a set, and the plastic deteriorates, so that when uncoiled and installed it no longer retains a good impedance match. After installation, it is often subjected to vibration, crimping, bends that are too tight, contamination, and corrosion. And as the dielectric ages, it becomes an absorber of RF. More and more of the RF that you put into the coax is spent just heating the coax. In fact, at a sufficiently high frequency, using sufficiently aged coax of sufficient length, you can use the coax as a perfectly good dummy load. Transmit into it, and nothing comes out the other end; and to the input it looks like a nice low-inductance resistor. Most of our original antenna cables are now past the 30-year point.
Use RG-400U coax, and new connectors. You could probably go to an avionics shop with the old coax lead, and just have them fabricate a new one. Insist on the RG-400U – they might try to convince you to take RG-58U. Most “everyday recommendations” will be for the 58U, but it is not as robust, and will have higher inherent losses.