Tuesday, April 12, 2011
It's amazing how much UHF power can be consumed in sub-optimal connectors! A week ago I measured the 70cm power output at the Az/El antenna system, and found only 2 watts was reaching the antenna! The FT-817ND was putting out almost exactly 5 watts, so this means 60% of the power was being lost in the feedline, connectors, and coax.
I did some quick measurements with the transmitter and a dummy load, and determined that most of the loss was actually coming from the SO-239 to BNC adapters I was using to connect my PL-259 terminated cable with my BNC-connector 70cm preamplifier. The actual loss in the RG8/U cable (Tandy, almost 20 years old) was fairly minimimal in comparison.
Based on this, I decided to get rid of the adapters entirely and replace the whole UHF feedline system with 40 feet of LMR-400, terminated in N-connectors at the preamp, and to use a pre-amp with stock N-connectors. This reduces the total number of connectors to a minimum: a PL-259 at the radio, N-connectors in and out of the preamp, and another PL-259 at the antenna.
I haven't measured the direct losses yet, but calculations show negligable losses from the connectors, less than 0.5 dB from the preamp itself, and around 1.1 dB for the LMR feedline. That should result in something more like 4 watts reaching the antenna, instead of the 2 watts I had last week. Woohoo!
Receive audio seems much better, and downlinks from AO-07 and VO-52 (both 70cm uplinks) seem strong as well.