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 <[email protected]> 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 > > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[email protected] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > Message delivered to [email protected] ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [email protected]
