Hi Glenn, thanks for the comments. I re-read your posting and hadn't caught the comment about the 8X before. I've been in the radio racket for about 40 years, and hold a FCC commercial with radar endorsement. Coax has been my friend (and enemy) for a long time!
I snooped a bit more, as I thought others on the list might benefit from the research and findings. I don't advocate the use of anything but teflon (plenum rated) cable for airplanes, and there aren't always equivalents, so please bear with me as I babble a bit. RG-142 is an excellent compromise for what us KR types are doing, and it is commonplace in certificated aircraft. At 122MHz, a 20 foot run will only cost about 0.8dB, not horrible, not great. With 5 watts, that gives us 4.1 watts at the antenna, or roughly a 20% loss, as you suggested. This is probably worst case for KR people, as 20 feet of coax is a LONG haul inside a KR! (My longest coax run is 12 feet.) I made a judgment call and went with this cable as a compromise between safety, performance and weight. For the transponder, RG-142 starts to become a problem. The attenuation is over 13dB per 100 feet, and that could become a significant problem. RG-393 is about 7.5dB loss per 100 feet, very significantly better, but at the cost of about 3X the weight. What is nice about the 393 is that one can use standard LMR-400 (RG-8) size connectors. I used RG-8X for my transponder run the first time around. The specs. were decent, but not excellent. Then I switched to LMR-240, which is significantly better, but still isn't plenum rated. LMR-240 is about 1/3 again better than 8X for performance, but neither are really suitable from a safety perspective. The run from my transponder to the antenna is 8 feet, and this is the sole piece of non-teflon cable remaining in the airplane. I knew what we needed, and I knew it was out there somewhere, so the search continued, and I found it. We really need something in the LMR-240 size range (like RG-59 diameter and weight), but teflon (or equiv.) A good possibility is LMR-240-LLPL. The performance is excellent, the insulation is what we're after, and the weight per foot is excellent. It is, however, very expensive, averaging about $3.60 per foot in bulk. This makes it impractical for most of us that only need 10 feet or so. If a group member were to buy a 1000 foot roll and then sell it out to the rest of us, life would be even better! FYI, the within spec. performance life of most coax is about 3 years. As a final thought, my Warrior is a 1981. I tore everything apart last summer and was horrified to discover that the original coax cables were intact. I was looking at 30 year old coax. Nothing had ever been addressed via the annuals, and if a signal got from point A to point B, it was OK. Not true. Coax is cheap, failed radios are not! It comes as no small surprise that owners of certificated aircraft are happy if they can call a tower that is 10-20 miles away, while KR type people often report communications of 100-150 miles. Dave.
