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Mar 8, 2013

Matched Listening Antennas - Why?

Antenna
I had a revelation the other day regarding listening antennas.

As a ham I am very well aware of the importance of SWR and matching antennas to transmitters. Well enough versed in fact to never have worried about it too much. An Elmer I had in the 1970s told me, whatever power is reflected back from your antenna to your transmitter will meet a mismatch there as well, and will just get reflected back to your antenna system, and will be radiated. Don't worry about it he said, unless you're running VHF or higher, or your transmission line is extremely lossy.

I have seen this wisdom repeated by knowledgeable folks here and there and trusted this advice for decades. It has served me well.

Here's the revelation part. Whenever is has come to receiving antennas, I have never given much thought to resonance, matching or anything of the like. I've always had good results from attaching a random length piece of wire to my shortwave receiver and enjoyed good results. Or so I thought. I even wondered to myself why SWLs even bothered to worry about resonance or feed-lines? It only mattered when transmitting in my book. I think I was wrong now.

RF coming into your shack via some feed-line will meet your radio at whatever impedance it presents. Yes, if there is a mis-match, RF energy will be reflected back up the feed-line. Here's the very important point- Once reflected back to the antenna system, the majority of the energy will be radiated out of the antenna!

I never considered that before. I figured, I'm receiving only, nothing is getting transmitted here. Bouncing RF will end up in my radio. That, is a wrong assumption. We might only be talking about micro-volts, but your antenna is transmitting reflected energy! It suddenly makes sense. Now I know why "those guys" worry about this stuff!
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