SWR variance on end fed antenna
I have an end-fed antenna from Emergency Amateur Radio Club ( http://www.earchi.org/ ) in Hawaii. It is labeled 6-40 meters. For the first time I have put an antenna analyzer on it. On 40 meters the SWR is just over 2. On 20 meters SWR is over 20! Any idea why the dramatic difference?
I assume your build was off this paper? http://www.earchi.org/92011endfedfiles/Endfed6_40.pdf
I played with end-fed "random wire" antennas a good bit a couple years ago. Without picking too much on that article, there are a couple glaring problems. First, 9:1 random wire antennas aren't really random at all. To work, the length must be very specifically picked to not be resonant on any frequency you wish to operate. I'll bet the length of yours is a bit too close to a half wave of 20m (33-35ft). Second, contrary to claims at the end of that piece, a counterpoise is absolutely necessary. Yes it's true the coax shield can behave as the counterpoise but it must be the proper length and it must be lying on the ground. Probably better to find the optimal counterpoise length for your antenna and use that ground wingnut - then you're not worried as much about exact coax length or positioning to your radio. One point that article makes that's true however - an antenna tuner will be needed to get the most out of a random wire end-fed. Here's a good article on the ins and outs of these antennas: https://www.kb6nu.com/playing-end-fed-wire-antennas-91-ununs
A close cousin to the random wire end-fed is the half-wave end-fed. I've been a big fan of these lately, especially for QRP field ops. Toss the wire up in a tree, run the coax on the ground (it also uses the coax shield as a counterpoise but isn't nearly as picky about length) and get on the air. No tuner required. I built mine to 40m with clips at 30 and 20m for changing bands. SWR under 2 on all three bands, again no tuner needed. Here's an article for more info on how half-wave end-feds work: http://www.aa5tb.com/efha.html
AB0L (Formerly N0KAI)
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Thanks Jeff. I played with shortening the antenna and got the 40M down very low. 20M is still about 7, which is better than 20. Will keep experimenting.