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Coax loss, common mode or antenna


Yeah its turned off, touch it
May 9, 2011

This is a 440mhz sweep of a comet 146mhz mono band antenna using rg8 coax. Yes I’ll be changing to lmr400 I’ve just been lazy. The sharp sawtooth pattern of purple and the fairly sharp swr pattern is what I’m curious about. Being new to really fooling with the higher frequencies I have to wonder why it looks like that.

Is it the antenna having very narrow usable harmonics?

Is it the coax having a bunch of loss?

Is it common mode currents?

Is it picking up interference in the system?

Did you calibrate out the coax before taking these measurements? Doing so would eliminate the coax from your possibilities. To do so you need to put your calibration loads on the far end of the coax and recalibrate the device with the coax attached. It may take one or two additional adapters. Also, if this wasn't done, then the readings your getting are affected by said coax, and not what is happening at the antenna itself.

The DB
Whenever you have a mismatch, a length of coax will rotate that mismatch around the smith chart once for each half wavelength of coax. Each time that yellow trace crosses the horizontal line in the smith chart, the purple trace crosses it’s 0° line (may need to select that trace to see that), so having the purple trace on is redundant and doesn’t offer much else here.

Not sure what dielectric that coax has, but assuming a VF about .7, that comes out to about 20’ of coax based on those 13 trips around the chart that I could count. So there is about 20’ of common-mode path, and this time, the VF of that path is not as low because there is not a second conductor, so lets call it .95. The lowest frequency where that common-mode path is a quarter wavelength (to better accept the current), is 6.3MHz, and it also accepts power on odd multiples of it. So the common-mode path also accepts power on 422, 435, 447, 460, 472 etc. Now, this may not correlate perfectly with your SWR trace, but I made some serious assumptions and estimations here to illustrate the point.

For that reason, I would also add a clip-on ferrite to the coax at the antenna end to prevent the coax shield from being part of the antenna. Or use a balun of some type.
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One way you can easily tell if all those dips are from common-mode currents is to add a piece of coax and see if the separation between the dips increases.

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