The "Four-Foot Vertical" 20m Magnetic Wire Loop

Stealth antenna? Can you spot the wire? Neither can we. The wires are almost invisible in this picture. The support post on the left side with the crossarms is about all you (or the neighbors) can see.


I discovered this antenna written up in The ARRL Antenna Compendium Vol. 2, p 39, by Russell Prack, K5RP. Figure 2 is a drawing from that article that tgives an idea of what the antenna looks like. Mine is built for 20m, not 40m, so the dimensions are different. My vertical wires are 4 feet, and the horizontals are 32 feet.

I was amazed at how well it works. It is not a high gain antenna. A yagi wins in that race. This antenna excels in the signal-to-noise competition. Operating this antenna reminds me of being camping in the mountains where the noise level is so low you can copy even the weakest station. It made me want to find out more about the theory.

Part way into my research I concluded it needed a new name to be more descriptive and help to explain it. I believe a clearer name for it would be "Folded Skeleton Slot Antenna". Let's examine that name word-by-word beginning with the word, "Slot".

Fig. 13-2 is from Antennas, Second Edition, by John D.Kraus,W8JK, p 626.

Kraus is describing a slot antenna which is primarily designed for microwave appliations. A true slot antenna is fed along it's ends. At microwave wavelengths that is probably practical but it isn't at HF. The feed points would be 32 feet apart (for a 20m loop), making it very difficult physically to connect a coax or twin lead feed line. Another issue is the feed point resistance is 550 ohms when fed at the ends.

As shown in Figure 13-2 in one attempt at an improvement, Kraus moved the feedpoint close to one side which lowered the feedpoint distance--but the connections are still 4 feet apart at 20m.

Alas, someone tried a simply elegant solution, and that is to fold the whole antenna back onto itself, close to but not touching. When the antenna is folded we accomplish two things. First, the feed points are physically close to each other, solving the problem for the feed line. And second, it cuts the feed point resistance by a factor of 4, to 135 ohms, making it easier to match. Voila!

Now a look at the next word in our new name, Skeleton.

A slot antenna like shown in Fig 13-2 would normally be used at microwave frequencies and be constructed from sheet metal. At HF we'd need a LOT of sheet metal. As is commonly accepted, electrons at RF flow only on the surface, and in the case of a slot antenna, rf current only flows along the inside edges of the slot. What a break for us because the edges can be simulated electrically by wires. This means we don't really need all that sheet metal. We can build it from wires, instead. An antenna that is constructed from wires has been termed a "Skeleton". Voila, again.

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