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I currently have a linked dipole up and running at 40,20,15 and 10M. While I have looked at trapped dipoles the losses discourage me as I wish to use as little power as possible.

So I now I'm wanting to put in latching relays instead of the banana links, but I'm unsure of what specifications I need. As they will be at the end of each dipole, where the voltage is high and current is low I have guessed that high voltage might be a necessity. Hope to get some insights on this, and if it's doable.

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  • $\begingroup$ Interesting question — I haven't heard of anyone doing this. Another thing you will need to think about (not the question you asked) is how to run your relay control wires $\endgroup$ – Kevin Reid AG6YO Sep 11 '16 at 14:41
  • $\begingroup$ Trap losses can be pretty low. w8ji.com/traps.htm $\endgroup$ – Phil Frost - W8II Sep 12 '16 at 1:49
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    $\begingroup$ If you don't want to deal with the fuss of constructing traps, you might also find it's easier to construct a fan dipole, or have a fixed length dipole with a number of matching networks at the feedpoint selectable by relay. There is also a trick with an off-center feed which is a bit fussy to get right, but can work pretty well on several bands. $\endgroup$ – Phil Frost - W8II Sep 12 '16 at 15:51
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Interesting idea. Here's a start: What is the peak voltage at the tips of a dipole antenna?

The conclusion there is for a 100W transmitter and a "normal" dipole, you can expect a peak of about +/-900V relative to ground at the dipole ends. You'll want to pad that with a healthy safety margin. Also keep in mind that power is proportional to the square of voltage, so if you have more than 100 watts that voltage can get very high, very fast.

As the relays are closed and they find themselves somewhere in the middle of the dipole, they'll have to deal with the current. Current is highest at the feedpoint, so in the worst case, for a 100W transmitter the current is $ \sqrt{100\:\mathrm(W) / 72\:\Omega} = 1.8\:\mathrm A$. In practice lower, since the relay is not at the feedpoint. This is an upper bound.

You'll want to be sure the relays don't switch while transmitting. I'd imagine there'd be some arcing and the contacts would wear out quickly.

I suspect the bigger challenge will be controlling the relays, since being on a wire with 100 watts of RF is a little tricky. Not only do you need to get the wires to them to control them, but once you've got the wires there you'll need to devise a way to keep the RF out of the control wires and the relay coils.

I've never done this, and I can't find descriptions by anyone who has, but I might try using twin-lead as the antenna element. The common-mode current radiates as a dipole normally would, while you can use the differential mode to control a DPDT relay.

The trouble then is the mechanism to control both common and differential modes from the shack. You don't want to put common-mode RF on the feedline: that would make it an antenna. If only you had a 3rd wire...

If you use a folded dipole, you can make one. At the center of what would normally be the undriven element is "ground". You can just attach a wire there. And if "ground" is actually DC +12V, you can use that to control the relay coils.

schematic

simulate this circuit – Schematic created using CircuitLab

Of course you'll need to impedance match the folded dipole somehow, like a 4:1 balun.

You could also avoid running the 3rd wire to the shack if instead you use a DC bias on the coax and separate that from the RF at the feedpoint with a capacitor.

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  • $\begingroup$ The voltages would be higher if there were any sort of SWR mismatch, right? So it seems to me that the relays should be designed for an even higher voltage, in case the operator accidentally transmits on a band other than the one the antenna is set for. $\endgroup$ – rclocher3 Sep 12 '16 at 15:35
  • $\begingroup$ @rclocher3 To the extent that you have an amplifier that can transmit more than it's rated power into a mismatch, I suppose so. I think the thing to do is determine the peak power the transmitter can generate under any circumstances, and design for that. $\endgroup$ – Phil Frost - W8II Sep 12 '16 at 15:46
  • $\begingroup$ I know most modern transmitters "fold back" when they discover an SWR mismatch, but I wonder if the relays would have time to arc over before the SWR measurement circuit discovers the mismatch. $\endgroup$ – rclocher3 Sep 12 '16 at 15:51
  • $\begingroup$ Sounds like a valid concern. I wonder if you might get by with some TVS diodes in series. I'm also not entirely sure how to calculate the peak voltage when the dipole is not self-resonant. Intuition tells me it might be lower for the same power, but I'm not sure. $\endgroup$ – Phil Frost - W8II Sep 12 '16 at 16:04

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