Monday, April 19, 2010

Check those cables

The performance of my 435 MHz AA2RX lindenblad has not been quite as good as the simple vertical that it replaced, leading me to take the assembly down and debug it. I don't have an antenna analyzer, so I relied on a good ole' 70cm SWR and power meter with a two-carbon-resistor 5w dummy load.

My suspicion rested on the 4m cable running from the antenna to the preamp. Sure enough. To test it, I set the TS-2000 to 5w output, put a 1.5m patch cord of LMR-240 with N-connectors between the radio and the meter, and terminated the meter with the dummy load. Sure enough, the measured power was just shy of 5w, maybe about 4.8w. A longer run of LMR-400 gave the same result; however, the cable that had been out of doors gave me only 3w of power, meaning that 2 of the 5w in were gobbled up in the cable. This is about a 2dB loss, and absolute murder for an omni antenna setup because we have no gain from the antenna pattern. Moreover, all losses before the preamp go straight into the noise figure. No wonder the vertical was winning out.

Trying other cables around the shack led to other alarming facts. I had a similar run of LMR-400-like cable that was producing something like 8 dB loss! I'd love to know why these failed.

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