In my experience magnetic loops and QRP are incompatible UNLESS we are at a high sunspot situation. Lots of articles have been written over the years and a good one was in QST using a trombone-shaped tubing for the tuning capacitor with PTFE between the parts. Use minimum 22mm copper tube and very few or no junctions.
The bandwidth is very narrow indicating very high Q and very high voltages across the capacitor and very high currents in the tubing. A very slow motor control is needed. They are quiet due to mainly being sensitive only to the magnetic portion of the wave, ie discriminating against electic field noise. Conclusion I would make is: good for reception where the low gain is easy to make up in the rx poor for transmission, I've heard 20dB down on a dipole. So, keep your dipole for transmitting and use the loop for rx. Watch out for feedback from transmitter antenna back into rx: use a relay changeover. You can turn the loop to reduce interference from particularly bad sources. David G3UNA ---- Stan Jacox <s...@nevanet.net> wrote: > Hello Elecraft'ers > I have been trying to figure out a decent antenna that can be used indoors > with my K2(until I can get permission to install an antenna on the roof) to > use on the 3rd story of a 5 story 1828 apartment house in downtown St > Petersburg Russia. Being such an old brick building the walls are 1 meter > thick at their thinnest. The leakage from cable TV, DSL and electric trams > cause a high noise level and the poor antenna options make for weak signals. > I went to the woods this weekend and tried a new 40m dipole and heard a lot > more with the low noise level out there. > When I returned to the city I was more determined than ever to get something > up that was more effective than the 20m inside dipole I have been using. > > 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. None of it is permanent yet but > after only working on it for 30 minutes total, I have been amazed how well > it works with the low power version K2. Comparing the loop sitting vertical > in my living room the noise level is 10db lower than the indoor dipole and > signals are steadier and much easier to copy. The difference in fading depth > is dramatically improved. 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). > So back to the plumbing shop tomorrow for parts to make some piston caps. > I'll build the caps with 5kv or higher so if I get the 100watt K3 I'll be > ready. A big plus is being able to match the antenna directly bypassing the > KAT2 for higher efficiency. My built-in K2 tuner is more efficient than my > MFJ tuner even though the MFJ has some usefulness for use with balanced > lines and built-in dummy load. > > > Stan > KM6XZ > St Petersburg Russia > > ___________________________ ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
