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Author Topic: Vertical antenna SWR diagnosis  (Read 175 times)

Offline i_hear_you

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Vertical antenna SWR diagnosis
« on: June 14, 2019, 2112 UTC »
I just hung and tested a new 20m minimal vertical with peak about 22', feed point just over 6', and two raised radials sloping down to about 2' at the ends. Coax terminates to a UHF connector, which is hooked to a choke made of twin power cable wrapped through two stacked 31-material ferrite toroids for 13 turns. I am using 25' of rg8x for current testing.

I originally cut all three wires for about 17'. Initial SWR measurements showed a nice dip to 1.5 around 13 mhz, so I "shortened" each wire by about 6" by bending them back on themselves at the end insulators. SWR was now dipping at around 13.250 mhz, so i bent back another 6". Another check showed the same SWR dip at 13.250, so I didnt bother to shorten it yet again.

Any ideas what might keep the lowest SWR where it was after the second shortening? The wire is solid bare 18 AWG, so it makes electrical contact where it loops back and wraps itself. Could the toroid choke be causing it? Or my length of coax?

Offline Josh

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Re: Vertical antenna SWR diagnosis
« Reply #1 on: June 14, 2019, 2127 UTC »
I suspect the toroid isn't going to influence the resonance point, you'll have to trim it some more. I'd trim it with the chokes out of circuit with just plain coax going to the antenna for starters. Also yes coax length can influence issues like swr, however if I have more coax than needed and don't want to slice it, I make a ugly choke/balun with the excess and secure it in place.
Conveniently located near Vincennes Indiana.

Offline redhat

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Re: Vertical antenna SWR diagnosis
« Reply #2 on: June 14, 2019, 2232 UTC »
Depending on how good (or bad) your radial and ground system is, I've seen very noticeable shifts in resonant frequency when 100' or so of coax is connected to the feedpoint.  The antenna tries to use the coax as a radial and this will detune the radial/driven element system.  The fewer radials you have, the more the shift and also the lower the system efficiency.

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Offline i_hear_you

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Re: Vertical antenna SWR diagnosis
« Reply #3 on: June 14, 2019, 2251 UTC »
I'll try shortening it some more.

The choke should be preventing the coax shield from acting like a radial, but who knows what's going on in this thing right now 😎

Offline Josh

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Re: Vertical antenna SWR diagnosis
« Reply #4 on: June 14, 2019, 2309 UTC »
Try some ferrets at the ends of the coax instead of the baluns and see what you get.
Conveniently located near Vincennes Indiana.

Offline i_hear_you

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Re: Vertical antenna SWR diagnosis
« Reply #5 on: June 15, 2019, 0133 UTC »
I shortened it even more and the dip moved further to the right. I had about 3:1 on the frequencies I operated on 20m, made a handful of contacts on 15 watts. Then I plugged it into my pl880 and was amazed at hearing broadcast stations my skywire seems to miss. So far I'm happy with it.