The capacitor is so critical that I always look for it in the product description. If they don’t mention the kind of capacitor, I assume that they don’t know what they are doing.
We know the MFJ mag loop tuners use butterfly capacitors because you can buy the capacitors from MFJ. The W4OP loop describes avoiding wipers in the variable capacitor. I’ve considered getting an MFJ loop tuner and some flexible copper tubing, the stuff used to hook up refrigerators. But I tune around too much. wunder K6WRU Walter Underwood CM87wj http://observer.wunderwood.org/ (my blog) > On Jun 4, 2019, at 4:24 PM, Mark Petiford via Elecraft > <elecraft@mailman.qth.net> wrote: > > Gene, > > I don't usually comment on products like antennas. I cannot, however, resist > this one: > > I cannot comment on the current Alpha Antenna Mag Loop, because I don't have > one. My Alpha loop is 4 or 5 years old. It was on sale and heavily > discounted, so I purchased it without looking into its construction. After I > bought it I learned that is wasn't worth much of anything. I won't tell you > how much I paid for it; even on sale I am embarrassed. > > > Keep in mind that one of the contributors to inefficiencies in a mag loop is > the resistance of the loop (which is the inductor) and parallel capacitor. > The first contributor of resistance was the overlapping anodized aluminum > sections that were bolted together. Each overlap had a few milliohms or more > of resistance. With 8 of these joints, the resistance added up, and a few > milliohms is a lot in a tank circulation circuit. > > > The second bad feature was the capacitor. Again, the resistance must be kept > to a minimum. The lossy part of a variable capacitor is the wipers that > transfer current from the rotor (which moves) to a terminal that doesn't so > you can pick the current up there. The way to eliminate that resistance is > to use a butterfly capacitor (which are had to find), or use a dual section > capacitor. Current goes in one stator, to the rotor, and out through the > other stator. You end up with 2 caps in series, and no wipers in the current > path. > > I never made a contact with the antenna except very local ones. I have had > much better success with the 26ft. wire and 16 ft. counterpose Walter (K6WRU) > mentioned (exact dimensions are NOT important), and I would have almost $300 > back in my pocket. Oops! I almost let the price slip. > > That was the first antenna I had purchased in 40 years. It left such a bad > taste in my mouth that I went out to Palomar Engineers website and bought > Kurt N. Sterba's book, "Kurt Speaks Out" and read it from cover to cover. I > feel better now. The antenna sits in the back of my garage waiting to be > broken down for parts. It has a great tripod, and a nifty little RF > indicator in the form of a neon bulb. I will probably give the cap to some > young ham to use in a transmitter project. > > > Do what Jim (K9YC) said! Get educated first, then do your research on how > the thing is made and works before you buy. If Alpha has improved the > antenna, I would love to hear about it. > > I just checked their website, and it appears they have switched to coax for > the loop. Much better. I cannot see what capacitor they use. > > Sorry, Eric. Way too long! > > > Mark > KE6BB > > KX3, AX1, and some wire. > > > > On Tuesday, June 4, 2019, 8:19:50 AM PDT, Gene Moore via Elecraft > <elecraft@mailman.qth.net> wrote: > > I have just submitted my order for my KX3. As part of the next step, I am > continuing my search for an antenna. At this point, I am leaning towards an > Alpha Antenna mag loop. > > ______________________________________________________________ > 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 > Message delivered to wun...@wunderwood.org ______________________________________________________________ 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 Message delivered to arch...@mail-archive.com
