[Elecraft] Magnetic Loop antenna...off Elecraft topic

[Doug Person]

All I can say is: WOW.

Doug -- K0DXV

Augie Hansen wrote:
> 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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