Let me add to what Ric has said, that there are very high voltages and
currents developed in these small loops, 5000 v or more even at 100 watts.
According to one calculator, a 5' diameter loop at 40M and 500 watts would
develop nearly 10,000 volts. It is essential to keep living things away
from these loops while transmitting is possible, and not a good idea at all
to be close to them while transmitting.
I'm in the process of gathering the materials to build a helically loaded
magnetic loop for 20-10M, with a vacuum variable tuning capacitor, using
copper 3" wide wound on a flex PVC loop about 40" dia.
73 de W6OGC Jim Allen
73 de W6OGC Jim Allen
On Sun, Apr 27, 2014 at 1:43 AM, Jim Brown <k9yc@audiosystemsgroup.com>wrote:
> My neighbor, W6GJB, has built a large TX loop Here's how described it to
> another ham. The "army mast" he's talking about is a bunch of 4 ft sections
> of 2-in o.d. Al tubing that fit together, and fit into a triangulsr base.
> They've been sold at Dayton and other flea markets for several decades, and
> are quite useful. With a tripod section made from two sections per leg,
> it's easy for one guy to push them up, then slip another section under it
> at the bottom, and so on They will, for example, support a small tribander
> like a C3 at 30 ft if guyed properly. We've also used them to build a
> 2-element vertical array for 40M.
>
> = = = = = = =
>
> The mag loop is homebrew, made of 8 segments of Army mast bent (very
> carefully) in a pipe bender. The whole thing can break down and go into
> the big green bags that go with that tubing. The air-variable capacitor
> should be able to handle 500W, but I have not tried that yet. Running 100W
> at this point with a KX3+KXPA100.
>
> With 8 segments it is close to 1/4 wave on 40, which should be over 90%
> efficient with some of it going into low angle radiation. Should be better
> than a low dipole. I can add fixed capacitance to tune it on 80, or add
> another 4 tubes and it should be better than 80% efficient on 80.
>
> = = = = =
>
> So to clarify, his loop consists of four sections of mast that are each
> bent twice at 45 degrees. They go in the four corners, then a straight
> section goes between each of the four sides. That's the 40M loop. So what
> he's saying is that to build it for 80M, you simply add another straight
> section on each side.
>
> This is a current project for him, and he's in the process of on-air
> testing using RBN to get comparative signal reports with his other antennas.
>
> This is fairly inexpensive to build (I'd guess around $500), but you need
> a good pipe bender, some practice, the sections, and suitable HV caps. I've
> given him several big air variables from my junk box to play with.
>
> An objective is for something he can throw in the back of his truck and
> set up quickly for FD or CQP.
>
> 73, Jim K9YC
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