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 > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[email protected] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
