Feed Line Loss - Reciprocal? - QRP Forum at eHam.net

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Feed Line Loss - Reciprocal?	Reply
by K8QLW on September 5, 2009	Mail this to a friend!
I was checking coax feed line loss for 30M QRP using http://ocarc.ca/coax.htm. Just to keep it simple I plugged in the power as 100 watts and the SWR as 1:1 and the cable length as 100 feet. Tandy RG-58 showed a loss of 24 watts (24%) while Beldon RG-8 showed a loss of 12.009 watts or about 12% for the same length. Question: Are those losses reciprocal? In other words would a received signal be attenuated by the same percentage? Always wondered about that. Thanks, Kent (K8QLW) (Posted in CW and Tower Talk Forums as well.)

RE: Feed Line Loss - Reciprocal?	Reply
by W5FYI on September 5, 2009	Mail this to a friend!
The loss is in unit length. It doesn't matter if the signal is going or coming; it's 25% loss (or 12%) if the signal is being transmitted or listened to. That's why you sometimes see preamplifiers on some VHF/UHF lines at the antenna; it's to boost the signal strength to make up for losses in the coax going to the receiver.

RE: Feed Line Loss - Reciprocal?	Reply
by KB1KGA on September 6, 2009	Mail this to a friend!
Don't forget two things. On receive, your receiver input is the load so the SWR is not the same as what you measure on transmit. The SWR would depend on how well matched the feedline is to the receiver or antenna tuner. Let's say you were using 600 ohm open wire line into a doublet or something and because of it's length, it presented a 1500 ohm impedance at the shack on transmit which you matched with a tuner. What's the SWR on receive? I don't know the answer but I think it depends on the mismatch between 600 ohm and whatever it sees looking into the "output" of the tuner. I guess the loss on receive depends on that SWR. In practice it probably doesn't matter at all with any sort of modern receiver on noisy HF bands. The other thing not to forget is that on receive, signal to noise ratio is what really matters. When you consider the whole system, ie antenna, feedline, tuner if you have one and receiver then so long as the general background noise is high enough and the receiver's own front end noise is low enough then your ability to hear someone or not is determined by the signal to noise radio at the antenna end of the feedline. Losses between there and the receiver might change S meter readings but it won't make or break your reception.

RE: Feed Line Loss - Reciprocal?	Reply
by K8QLW on September 7, 2009	Mail this to a friend!
Thanks much gentlemen for the input. (No Pun) kent

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