Re: [Elecraft] Magnetic Loop antenna...off Elecraft topic
Hello Ron Thank you for your comments. I had little choice in antennas or stay off the air until the roof was accessible since the walls are so thick, apartment so small and the metal clad roof combine to render a wire antenna useless. Even on receive the wire antenna has worked little better than a dummy load. The losses as you mention in the turner, either the T or L form is what I am able to avoid by having a good match directly from the 50ohm unbalanced feedline. I had used the MFJ tuner's watt meter alone in tuner-bypass mode. I run the K2 with the KAT2 bypassed also. The match at resonance is 1:1 on the loop, and the feedline is short, less than 15 feet. I built a two gang piston capacitor yesterday using cooper pipe with Teflon tape as a thin dielectric layer between the inner and outer pipes and the Q improved quite a bit, and resulted in much narrower bandwidths. On the air tests with a station about 20 miles away on 40 indicated a 1.5 S unit difference in signal strength between the loop and indoor dipole connected with the two halves of the 20meter dipole open wire balanced feedline shorted together working against ground through the tuner. On 20, with the dipole connected as a resonant conventionally balanced-line fed dipole the signal was weaker for both but the other station reported 1 S unit difference, both cases the loop was stronger. I was seeing his signal stronger by about the same on the loop but the lower noise of the loop made it seem much stronger. Using the piston caps the tuning covers from about 5.5mhz to 22Mhz, and no additional fixed caps in parallel. I don't think I will bother trying to push it down to 80m. My girlfriend's family as a dacha outside the city and we are planning a party for our friends there next weekend so that will be a good test of the loop, in the clear compared to a wire dipole suspended between trees in the woods. I would expect the loop to lose that comparison test. The antenna problems could be all moot if I move in with my GF if I get way. Her apartment is in a taller building where she has access to the roof with no other antennas installed. That way I could put up a shortened yagi or quad on a short tower. Besides her apartment is beautifuljust as she is;) I am going back to California for a couple weeks at the end of this month and will be able to bring back some parts or more test gear unless I blow all my money on a K3 or new lenses for my camera. I need the lenses more than a K3 since the K2 does all I really need, particularly if I add the DSP filter. I like running the 10-14 watts of the K2 but I also have a TS50s here running 100watts if more power is needed. The K2 has a better receiver and has a lot more features so I only use the TS50s for its AM short wave broadcast capability. I did try the 100 watts with the loop and the capacitor did not arc or heat so if I got the 100watt upgrade kit for the K2 I would probably be safe. Thanks for the comments Stan -Original Message- From: Ron D'Eau Claire [mailto:r...@cobi.biz] Sent: Monday, August 03, 2009 7:38 PM To: 'Stan Jacox'; Elecraft@mailman.qth.net Subject: RE: [Elecraft] Magnetic Loop antenna...off Elecraft topic I'll certainly second David's, G3UNA, comments about using a wire for transmitting. Loops can be very effective for receiving though, as you noted. Any decent receiver (like the K2) has plenty of excess gain to make up for the losses in the loop itself, and they do tend to pick up less noise. With a receiving antenna, signal-to-noise ratio is everything while in a transmitting antenna efficiency becomes very important. You'll likely see more articles about small transmitting loops during the next sunspot peak when extreme low power will work the world and the very poor efficiency of a small loop is not so apparent. If you have access to the attic space in your building you might consider a single wire or doublet directly under the roofing material if it's not a metal roof. You can fabricate open wire line with some nominal sized wire and makeshift spacers. The spacing isn't important nor does it have to be entirely consistent. Such feeders can pass through tiny holes in most ceilings no larger than a small nail and which are easily patched when you leave. A bit of spackle or even the apartment dweller's friend (tooth paste) will plug the little holes when you're done. Depending upon the composition of those bricks (some clay has much more metal ore in it than others), you may not see as much attenuation as you expect if you're limited to a wire inside your unit. You wrote: 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. I wouldn't assume that is true unless you are talking about transmission line losses between the loop and the K2. You don't mention
Re: [Elecraft] Magnetic Loop antenna...off Elecraft topic
Stan Jacox wrote: 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? Never tried one. It sounds like you've come up with something that will at least improve your reception. You should do some systematic measurements to see if transmission is better or worse than with the dipole. There are many magnetic loop success stories, but my impression is that even more hams find them inefficient for transmission. Your 80 kHz 2:1 SWR tuning range on 20m is a warning sign to me. I believe it should be narrower for an efficient magnetic loop. But building a better capacitor should improve that. Good luck Jon LA4RT __ 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
Re: [Elecraft] Magnetic Loop antenna...off Elecraft topic
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
Re: [Elecraft] Magnetic Loop antenna...off Elecraft topic
I'll certainly second David's, G3UNA, comments about using a wire for transmitting. Loops can be very effective for receiving though, as you noted. Any decent receiver (like the K2) has plenty of excess gain to make up for the losses in the loop itself, and they do tend to pick up less noise. With a receiving antenna, signal-to-noise ratio is everything while in a transmitting antenna efficiency becomes very important. You'll likely see more articles about small transmitting loops during the next sunspot peak when extreme low power will work the world and the very poor efficiency of a small loop is not so apparent. If you have access to the attic space in your building you might consider a single wire or doublet directly under the roofing material if it's not a metal roof. You can fabricate open wire line with some nominal sized wire and makeshift spacers. The spacing isn't important nor does it have to be entirely consistent. Such feeders can pass through tiny holes in most ceilings no larger than a small nail and which are easily patched when you leave. A bit of spackle or even the apartment dweller's friend (tooth paste) will plug the little holes when you're done. Depending upon the composition of those bricks (some clay has much more metal ore in it than others), you may not see as much attenuation as you expect if you're limited to a wire inside your unit. You wrote: 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. I wouldn't assume that is true unless you are talking about transmission line losses between the loop and the K2. You don't mention how far apart they are, but indoors it's usually a very short distance. In either case you are resonating the system with lumped values of inductance or capacitance. Whether they are at the antenna or at the rig in the KAT2 should make no difference except, as you noted, it's much easier to tune a high-Q antenna at the rig. If your MFJ tuner is one of their most common 300 watt (or lower) units, it's a T-network. While they can be very good matching networks, a T-network is notoriously inefficient when matching to a very low impedance load like a small loop, so I wouldn't expect it to do as well as the L-network in your KAT2. Looking at some scenarios in an on-line T-network simulator (http://www.ve3sqb.com/hamaerials/w9cf/), an antenna presenting a non-reactive feed point impedance to the T-network of 100 ohms at 7 MHz will see 0.1 dB loss while an antenna presenting a non-reactive impedance to the tuner of 0.5 ohms (not unusual for a small loop) at 7 MHz will show a loss of over 5 dB. Like the small transmitting loop, those loses are resistive losses in the inductor in the tuner and go up as the inductance required goes up at lower frequencies. The losses just about double, for example, on 80 meters. The bottom line is to get as much wire out there as possible to raise the impedance at the feed point. That reduces circulating currents which are the greatest source of loss, whether they are in the antenna as in a small loop, in the a transmission line with high SWR, or in the matching network, either at the antenna or at the rig. Ron AC7AC __ 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
Re: [Elecraft] Magnetic Loop antenna...off Elecraft topic
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:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
Re: [Elecraft] Magnetic Loop antenna...off Elecraft topic
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:Elecraft@mailman.qth.net 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:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
