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I'm looking at a 40MHz short transmitting loop (STL) with 6 rectangular loops: I need to fit within some design parameters and this gives the good radiation that I need in the near field.

If I want to feed with a twisted gamma match then should the loops of the gamma go around all of the antenna current loops or just one?

I'm also confused about how many loops.... The modelling code says about 0.1 ohm radiation resistance and 0.1 ohm "ohmic" resistance, which I can live with..

So if I want a twisted gamma with 50 ohm coax feed, does that mean that I need 250 twists to make the gammma-ray match?!

The devices that I see discussed on the internet seem to have 5-10 twists.

I'm confused.

Thanks, T.

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  • $\begingroup$ Oops forgot square root, so sqrt(50/.2) = 16 turns. $\endgroup$
    – Tunneller
    Nov 30, 2023 at 2:48
  • $\begingroup$ Ah.... I understand slightly more... As far as the incoming coax is concerned, the extension of its core is not really twisted. It thinks that the coax ends with a new "virtual" transmission line that has the length of the unwound twist, and current return on a conductive plane. That virtual configuration ends in a short, so the coax load impedance can be set to 50 ohm (or whatever) just by varying to total length of the wire. Now, how this efficiently couples to the antenna 0.1 ohm is the next question. The antenna probably doesnt know that the twist is even there. $\endgroup$
    – Tunneller
    Nov 30, 2023 at 18:59
  • $\begingroup$ Are the six loops in parallel like a cage, a way of making the loop nice and broad? Or something else, multiple turns, individual capacitors etc? $\endgroup$
    – tomnexus
    Dec 1, 2023 at 16:32

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It seems to me a "twisted gamma match" is a way of getting a small magnetic coupling to the primary loop, with some extra resistive losses.

There's no harm in trying it out, that's the beauty of building antennas yourself. But know that it's not an exact, highly tuned thing like the length of a yagi element. With good software it can probably be simulated fairly accurately, but it's probably quicker to experiment.


The magnetic fields from the twisted wire will couple nicely with the main loop. The effective secondary loop area is determined by the insulation thickness and the wire length. (Well, the portion that's parallel to the main loop, the perpendicular component doesn't couple...). As the area is small and the wire is thin there will be more resistance which may or may not be desirable.

In general, the performance of the small transmitting loop is fixed by the parameters of the large loop and its capacitor. Anything placed inside or near the primary loop will excite it; a small loop is one choice that is mechanically strong and stable.

My main concern with the twisted gamma match is that the coupling will be very sensitive to any slack in the cable and could be detuned by water.

I would expect the number of turns is not very important, it's the length that matters. It would work with "no turns" just with the wire taped tightly to the loop. Choose a turn pitch that keeps the wire nice and tight, maybe at a 30 degree angle? Adding turns makes the wire longer and its resistance higher as you noted, they may also be some distributed capacitance effects.

There is no relationship between the resistive loss of the wire and the 50 ohm coax feed. The 50 ohms you see when it is fully matched comes from the coupling to the primary loop and its radiation and resistive losses.

One last note - if the wire insulation is too thin, you may not get enough coupling area even as the tip of the wire gets to the capacitor at the top. At this point you have no choice about to find something with thicker insulation. The insulated inner core from some coax might be appropriate.

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