Re: VF Varies With Frequency

Posted by Vic Rosenthal on Apr 02, 2020; 6:00am
URL: http://elecraft.85.s1.nabble.com/75ohm-Impedance-question-K3S-and-Antenna-side-tp7659364p7659441.html

I put a shorted 1/4 wave stub — about a foot long on 144 MHz — on the coax to my 2m beam. The beam is right under my HF antenna, and thanks to the stub I can run 1.3 kW on 40-10m with absolutely no interference to the 2m rig, and no effect on 2m SWR. Before I had the stub, hitting the key on the HF rig caused the 2m transceiver to reboot.

The stub is near the radio, although I don’t know if that matters. I just used the published VF for the coax to figure the length.

To be fair, I should mention that the 2m beam is vertically polarized and the HF antenna is horizontal. But they are just a few feet apart.

Victor 4X6GP

> On 1 Apr 2020, at 20:38, Jim Brown <[hidden email]> wrote:
>
> On 4/1/2020 7:58 AM, Don Wilhelm wrote:
>> When dealing with a length of transmission line, the use of the '468' factor should not be used - compute the actual wavelength and then apply the velocity factor.
>
> There is yet another variable -- VF varies with frequency. At low frequencies, it is lower (slower), increasing until it converges to the published value at VHF. For this reason, matching sections and stubs must be measured at or near the operating frequency with an analyzer or as a stub placed in line with a generator and receiver. They should be cut long, then trimmed so that the null in that generator/receiver circuit is heard, or the analzyer reads a short or open.
>
> How much is this variation? For typical transmission lines, it's on the order of 1% from 80M, a bit more for 160M as compared to the published value. If what you're building is a stub to kill harmonics, it's the difference between the CW and phone bands on 80M.
>
> 73, Jim K9YC
>
>
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