Time for inspection: Antenna system winterised?

Winter is coming and brings many a nasty surprise for radio amateurs. There is still time (and bearable weather) to think about preventive maintenance.


Yes, coaxial cables with a black sheath are UV-resistant. But this only means that they resist UV radiation from the sun longer than other materials, not forever. Because there is no 100% protection here either. The outer sheath of most coaxial cables offered today is made of PVC, which in itself offers good UV resistance. Adding black dyes (carbon black) improves this resistance even more. Nevertheless, cables used outdoors suffer, among other things, from acids such as those found in the exhaust air of chimneys. In addition, there is the constant movement around the rotor and UV radiation. Despite all protective measures, coaxial cables can become cracked after many years of outdoor use, allowing water to penetrate and permanently damage the cable at the latest with the next frost. But even without frost damage, water in the cable can severely alter the electrical properties such as the impedance in places. In the worst case, corrosion occurs on the connector, for example at the transition points from the copper conductor to the nickel-plated connector housing or to the gold-plated inner conductor. The result is HF shock points, which can lead to a poor standing wave ratio (SWR) and high reflections.

 


Another little-known effect is the "breathing" of a cable. One part lies inside under the roof and has largely always the same temperature. The other part is outside, exposed to the weather. In summer, such a cable experiences temperature differences of 20 degrees and more. This strong difference in temperature causes a pressure difference in the cable, which in turn causes humid air from outside to flow into the cable. The water then condenses at the latest when the cable cools down again. It is not without reason that many coaxial cables in professional installations are filled with nitrogen and operated at a slight overpressure. This prevents the condensation of humidity.

 

Therefore, after 10 years at the latest, you should take a look at your mast and check the cables. This includes control cables, but especially coaxial cables. Here, the cable should be inspected over its entire length for cracks, porosity or even rodent traces. Particularly susceptible are the transition points such as those found on lightning protection systems or on the plug, but also the area of the cable that is moved when the rotor is turned. And while you're at it, you should also check the popular cable ties. They don't last forever either, and poor UV resistance becomes apparent very quickly. And then suddenly the whole cable with its weight is only hanging on the connector...


If a coaxial cable is damaged, the cable should be replaced completely. Yes, that sounds like a good argument to make more sales. But practice shows that "patching" HF cables is mostly disadvantageous. New joints are created and the cable has more losses. In individual cases this can be accepted, but for high demands every dB counts.

 

Most of the coaxial cables with black outer sheaths offered by WiMo are reinforced in their UV resistance. When choosing, you should pay attention to the attenuation; a VHF omnidirectional transmitter for relay operation may not need the very thickest cable, but a UHF system for DX operation does. Cables that are to be routed around a rotor should have a flexible inner conductor (stranded wire). Or - if it has to be a cable with a solid inner conductor for reasons of lower attenuation - it should be routed in a very wide arc around the rotating point.
All plugs should be disassembled during the inspection and checked for corrosion on the inside as well. When reassembling, each plug should then be protected at least with heat shrink tubing, but better with self-welding adhesive tape or sealed with rubber liquids such as "Rubbaseal"

 

This way, with timely maintenance before winter, you will have peace of mind again and can sit in the warm shack and enjoy the hobby without disturbances.