Toroid Coupler for Half Wave Antennas

In the IHAB-2 Class A 20M beacon transmitter I used a half wave wire antenna and a toroid coupler. A diplole dangling below the payload container would have been difficult to feed, but an end fed wire was easy.

A toroid transformer is an unusual device in hamdon to transfer power from the transmitter to the antenna, and I thought I'd share it here with you.. I call it "Nick's Coupler" because I first saw it on Nick Kennedy, WA5BDU's web site. John Stevens, K5JS mentioned that Jerry Haigwood, W5JH, has also used this type of coupler but in an auto transformer configuration, with the primary tap up 4 or so turns from ground.

Nick's uses one of Diz's FT-114-43 toroids, and is a step up transformer. In my version the transmitter side winding has 4 turns and the antenna side has 32 turns. The rationale for the 1:8 turns ratio is that in practice the impedance at the end of an end fed half wavelength antenna is on the order of 1800 to 5000 ohms (from Steve Yates, AA5TB's web site So I chose a middle value of 3200 ohms to match, and the impedance ratio of 1:64 does that.

I'm sure that the SWR isn't a perfect 1:1, but it's close enough. As Nick says on his web page "Some hams think SWR is a measure of how well an antenna "works". In other words, an antenna with a 1:1 SWR works better than one with a 1.6:1 SWR. That's not necessarily so. But if you have a 1:1 SWR fetish, this system might not be for you."

Future plans for this design are to do some SWR experiments with it, and possibly use it for portable ops.

Revised 2 Dec 2012
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