Ham Radio Forum

Hello All,

Anyone ever heard of someone’s trying to build a two element beam from a couple of magnetic loops?

I got the idea for a 30 meter version the other morning, and try as I do to forget it, it won’t go away! No doubt the problem with such a lash-up for most bands would be having to do synchronized remote tuning of the capacitors for the reflector and driven element.

Virtually all the 30 meter CW activity that I hear is in the lowest 25khz of the 50khz wide band. It might be possible to tune the two loops for minimum swr at 10.112 mhz, and make no attempt to re-tune the antenna for small frequency changes.

The point of the exercise would be to have a relatively quiet directional antenna with some gain and F/B ratio at a manageable size. If it could be made to work, it might even be practical for 40m.

I would appreciate any input on the idea, especially to have someone shoot it down so that I can stop thinking about it (;->)…

Cheers,

Brian Burns

That is an interesting idea. It can be made to work, but there will be some issues.

It will be necessary to have the right phase relationship between the RF currents flowing in the two magnetic loops to obtain the desired antenna directivity. Relatively-efficient resonant loops that are small compared to a wavelength have very high Q's. Very small tuning changes cause big phase shifts in high-Q resonant circuits, so the tunings of both loops will be very critical. Because of that, it will be important to protect the loops as much as possible from environmental changes due to rain, snow, blowing tree branches, and anything else that might change in the near vicinity.

Even though the two loops can be individually small, the spacing between them will have to be a significant fraction of a wavelength to obtain useful directivity. That may not be a problem, especially if the antenna does not need to be rotatable, but if it must be rotatable the relatively-large spacing required between the loops compromises the initial attractiveness of the design somewhat. However, except for critical tuning and resulting narrow bandwidth, a highly-directive, fixed, multi-loop antenna could be constructed quite easily.

Let us know how the antenna works if you decide to proceed with your idea.

Skywave

I am very interested in this idea and thinking about giving it a try myself.

Radiation resistance is another issue I didn't think about when responding above. The radiation resistance of loops that are small compared to a wavelength is low. Adding a parasitic loop element will make the driven loop resistance even lower, so it will be necessary to use large conductor sizes and highly-conductive loop material to keep the resistive losses from offsetting parasitic element gain.

Skywave