Why a long lossy feed line masks true SWR

Locally members of our ham community have long though that long feed lines of indifferent/generic/almost-like feed lines are the bee’s knees.

During Field Day there has been times where really long feedlines were deployed as the trusty old MFJ SWR meter reported a good match. Yet the setup was nearly mute and while it appeared to hear better it was more deaf than expected.

Why?

Because the long lossy feed line does do stuff, including somewhat masking the true situation.

The perceived RX sound like it is suddenly “good stuff” But what is happening is the perception of receiving well is more about the noise drop with the signal attenuation the long lossy feedline creates.

And then your struggle to get a useful signal out, because the TX is crippled by the feedline loss.

Do the math!

Symptoms are the station thinks it hears well because noise is attenuated and then is never heard when transmitting

The fix is better feedline.

Better feedline is only way you will really make up for the feed line’s length issues when it is a lossy line at your operating frequency.

Here are people much more expert spelling the situation out:  https://www.physicsforums.com/threads/what-causes-swr-to-change-along-a-feed-line.961327/

To Paraphrase:

Loss characteristics affect the measured SWR by attenuating both the transmitted signal and then taking a second bite out of the reflected signal, making the SWR measurement closest to the operator look better than it really is. 

The substandard and lengthy feed line “sucked up” the energy that your SWR meter would read.

The measured SWR on a feed line differs for every point on the feedline because of this phenomena, with the difference being greater with feedlines that are lossy.

If your measured SWR is greatly different when taken at the radio and when taken near the antenna, the only real fix is better quality and lower loss feedline, which will cost you money.

For most ham station setups there is a point where spending more money on feedline won’t get you enough improvement to be worth it, especially if your feedline length is short.

I’ll cover the ROI (Return on Investment) math in a later post.

73

Steve
K9ZW

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