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 beautiful....just 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:[email protected]] Sent: Monday, August 03, 2009 7:38 PM To: 'Stan Jacox'; [email protected] 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 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:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
