[Elecraft] Magnetic Loops

[Doug Person]

Just for fun once, I decided to make the smallest possible antenna and 
see if I could make any contacts. I wound about 30 week of #12 solid 
insulated wire around a 3/4" PVC pipe. Parallel to it and electrically 
in series, I created a tunable tubular capacitor using copper pipe and a 
threaded brass rod.  You  screw the rod in to increase C and out to 
decrease.

Using these strictly guesstimations, I found I had good SWR across the 
entire 20 meter band with about 1.7:1 on the edges. I mounted it on an 
8' aluminum mast which was also connected to the shield side of the coax.

I got on with my little Yaesu FT-840 and in 5 minutes I had a pileup of 
stations wanting an explanation for my report of an 18" vertical. I 
guess the efficiency must have been only a couple of percent.  But, 
there it was, the smallest possible vertical for 20 meters!  I worked 
about 15 countries that afternoon with it.

Doug -- K0DXV

Ron D'Eau Claire wrote:
> I'd like to be clear when I wrote earlier that a small transmitting loop is
> very inefficient. 
>
> I didn't mean they don't work, only that they only radiate a small part of
> the RF applied to them compared to a larger antenna. 
>
> Consider a typical mobile "whip" antenna. They're terribly inefficient too
> but people get out with them, sometimes working DX when band conditions are
> good. 
>
> Of course mobiles generally run more than the 10 watts or so a K2 produces,
> but contacts are made with QRP power and such antennas all the time. 
>
> One way to make a given antenna "larger" is to use a higher frequency band.
> It's a matter of the antenna's size in wavelengths or fractions thereof that
> is important. That's why small antennas do so much better when the sunspots
> are active and the higher frequency bands are open. 
>
> Ron AC7AC
>
>
>
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