Stan Jacox wrote:
> ...
> I found a small plumbing shop open late and bought 4 meters of 1/2in copper
> tubing and made a loop this evening. I had no high voltage capacitors so cut
> various lengths of RG-58A to use as coax caps and kluged a Faraday shielded
> loop coupling system to drive the main loop.
Coax capacitors made from RG-213 can handle up to 150 watts or so before
arcing over if the open end has good separation between the shield and
the center conductor. RG-58A probably won't do that well, but can easily
handle QRP levels as you have reported.
> ... The bandwidth for 2:1 SWR on 20 is 80khz without
> retuning the center of which is 1:1. I only set it up for 20 and 40 but
> using fixed lengths of coax as the tuning capacitor but during my experiment
> I found the loop worked on 80m also but with higher SWR. Obviously I need a
> real variable cap which the electronics parts stores here don't have(all the
> experimenters it seems were born in the digital age).
With a good quality capacitor and a 0.5 inch copper loop conductor about
13 feet long, the instantaneous bandwidth on 20 meters should be about
24 kHz with 4.3 kV across the capacitor at 100 watts. Efficiency would
be around 66% provided all connections are solid and the capacitor has
very low series resistance. That means either a properly welded
butterfly or split stator cap, or better yet, a vacuum variable
capacitor. Your 80 kHz value shows that losses are high in your
temporary configuration, but that is to be expected.
On 40 meters with a good quality capacitor a loop this size exhibits a 7
kHz instantaneous bandwidth and about 18% efficiency at best. On 80
meters bandwidth drops to 4 kHz and efficiency to less than 2%.
> Anyone else build small loops for use with their QRP rigs for a while? The
> only down side I've seen is the need for more complex remote tuning and
> narrow range before needing to retune. What has been your experience? What
> am I missing, since the magnetic loop seems to solve so many problems for
> antenna restricted stations why are they not talked about more often?
>
Small loops that are efficient can be made for QRP and QRO use. It's
just that the QRO version has to be able to handle very high loop
current and capacitor voltage simultaneously. Fatter conductors, such as
the copper outer of 7/8 inch hard line, raise the loop Q resulting in
higher efficiency but narrower bandwidth.
For what it's worth I wrote an article about a two-turn small
transmitting/receiving loop (STL), a.k.a, magnetic loop for antenneX,
the on-line antenna magazine, for their May 2009 issue. That article is
available only to subscribers, but I put two pictures on the web site
that anyone can view. They show 1) the loop hanging about eight feet off
ground, and 2) the remotely controlled vacuum variable capacitor (VVC)
unit that tunes it. This antenna covers 40-80 meters (actually 3-9.5
MHz) at up to 1KW. The VVC is rated at 15 KV at 65 amps. The pattern is
a tight figure eight with the nulls perpendicular to the plane of the
loop (i.e., along the axis). Here are the URLs for the pictures:
http://download.antennex.com/listarch/files/2T_STL.jpg
http://download.antennex.com/listarch/files/2T_STL_vvc.jpg
By using two turns, the loop has been reduced to a 4.5 foot diameter
size. A comparable single-turn loop would be a bit over nine feet in
diameter.
I wish you great success with your magnetic loop efforts.
73,
Gus Hansen
KB0YH
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