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Author Topic: Rectangular magloop: which part to stick out the window?  (Read 1238 times)

Posts: 313


« on: April 09, 2013, 10:14:11 PM »

I have a question about a optimizing the placement of a magloop on a concrete balcony (on the 2nd floor).

The "window" in the subject line isn't exactly accurate, but it conveys the general idea. Here's the situation: the antenna site will be an almost completely concrete-enclosed balcony. Essentially, it's surrounded by concrete (left, right, below, above, in front, and behind) except for a small window onto the open air, dimensioned 3m wide by 1.3m high. Fortunately the sky is visible from the balcony.

I've had some small luck with a copper-strip magloop (1m x 1m) located completely inside the concrete confines of this balcony. Now I am building a new 7 MHz magloop from 12.7mm copper pipe, 2.5m x 1.3m, to be oriented vertically. I plan to stick part of the rectangular magloop over the edge of the front balcony wall so that the antenna is partially in the clear.

My question: I believe the best part of the magloop to stick out in the clear will be the capacitor section. Is that right? In other words, placing the capacitor section inside of or nearby to the concrete walls will be more lossy than sticking the capacitor out in the clear - right?

The mechanical design would thus be that the capacitor (probably a homebrew two-plate affair) would be located on the left edge of the rectangular loop in the center of the left edge. The right edge would be a coupling loop for feeding the antenna. I understand that the coupling loop has the best chance for preserving loop balance and preventing common mode currents, even though maintaining balance will be difficult because the top and bottom edges of the loop (along the width of the rectangle) will be running very close to concrete (the bottom edge of the loop will be resting on the concrete balcony wall).

Another option would be to make a much larger antenna, say 2.5m x 2.5m, but that would make it impossible to stick any part of the antenna beyond the confines of the concrete walls. My intuition is that a smaller magloop, with part of it in the clear, would be better than a larger magloop that is totally surrounded by concrete except for the 3m x 1.3m "window" to the outside. Does that sound right?

I know this is a very tough antenna situation, but I know it's not impossible since I did make one brief contact with my previous loop. I'd appreciate any advice.

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