Magnetic Loop AntennasBy Jose B Rivera - N2LRB
April 10, 2016 - For the post few weekends I have been teaching myself and creating Magnetic Loop Antennas. I started thinking about magloops as they are called when looking for an alternative to tall, lengthy verticals for both indoor and mostly outdoor use. I figure that a magloop might be a less imposing an antenna, given it’s short vertical height, than a quarter wave vertical. Especially when out in public spaces, like Central Park.
The first thing I did was look up the needed dimensions of the antenna. I went to the Magnetic Loop Calculator website at: http://www.66pacific.com/calculators/small_tx_loop_calc.aspx. I wanted to create a magnetic loop for 20 meters. The calculator called for a length of wire/coax/pipe from 8.4 to 16.8 feet. I went for the longer length because as the length of the main loop increases, so does the efficiency of the antenna. From this information I have gathered that most commercial magloops are small, have to little in terms of length of the main loop to be anywhere near efficient.
The next thing I learned and looked into was the mechanical construction of the loop. Something did not make sense. One of the loop designs calls for having the inner loop (the second smaller loop) connect to itself. Meaning, having the copper core of the coax (if using coax) soldered to the shield of the very same coax. I thought would that be like creating a short? Looking it over a few times I realize, yeap, that is what is called for unless I wanted to make some complicated and little understood other mechanism. By the way the length of the inner loop has to be one fifth of the length of the main (outter) loop.
I ordered the MFJ-23 Butterfly Capacitor and waited to get an e-mail from MFJ about the tracking information. When two days passed and nothing appeared in my inbox I wrote them. It turns out that they don’t have enough of those and I would have to wait a month to get one. I then ordered one on ebay. The one from ebay was large like the MFJ one. While waiting on the second variable capacitor I ordered some LMR-600 coax with PL-259 on each end. I used some S0-239 and attached them to a piece of wood facing each other. I wired the SO-239 to the variable capacitor, created the small loop and put it all on my 31 foot orange telescoping mast.
On March 5th I attached the main loop to the mast, then added the small loop. I then attached my coax to the small loop. The other end of the coax went into my Elecraft KX3. The whole contraption was hard to hold up straight as I did not have a stand on which to keep it from tilting over. I turned the capacitor and listened for peaks and nulls on 20 meters. I did not hear any as I turned the capacitor knob. I switched over to 40 meters and heard peaks and nulls on that band. Thus ended that test. I was able to hear some stations on 40 meters but no contacts where made with that first test.
I spotted a smaller variable capacitor on the RF Parts website and ordered it. When I tested that loop I was able to hear peaks and nulls on 20 meters and made my first loop contact on March 12, 2016 with W9RG, Richard Grant from Monticello, IL. not bad and it was done on low wattage from inside my bedroom!
I have subsequently tried different things with the capacitor and various other types of coax, RG-213 and RG-8. I also purchased an MFJ 9232 Loop Tuner and used 10 ga wire and was able to make a contact with N303 in Maryland on 40 meters! Yesterday, I made some contacts with the MFJ-9232 into GA from my bedroom and Maryland up on the roof of this building. April 9, 2016, I made contact with WA6KHK in Menifee, CA on 20 meters from inside my bedroom. That was over 2,403 miles away!
I am hoping to finalized the structure of the loop for portable use later this Spring and Summer.