The idea of using loop antennas to minimize interference and to help dig weak stations from beneath stronger ones on the same frequency may be old hat to many, but the idea evolved for me. Like many other things in my younger days of DX-ing, without a local mentor or access to a lot of printed material, it was a learn as you go process.
The upside of all of that was while learning, I also figured out how to get the effects of a loop without spending a lot of money. Many today may look longingly at some of the commercially built loops for sale or at the projects in publications or on line and find the cost of the loops or the materials puts the idea out of reach. However if one finds it acceptable to make a few compromises that reduce performance only a little or provides a product with something a little less polished in appearance, the cost can be reduced to almost nothing.
My first inkling of the directional affects of loops came when I was using one of my very earliest receivers, my old trusty Watterson five-tube broadcast set. It had the requisite loop on the back and I noticed that when it was placed on the side of my desk to my left, some stations were not as strong as when it was on the desk in front of me, or rotated 90 degrees.
At first this was an annoyance more than anything, because it meant that to hear my favorite Chicago station at night ( of course in the early sixties it was WLS!!) the radio had to be to my left, making use of the desk for homework and other things a little awkward. There was still no clue planted to use the effect to find other stations. Then when I began using an outside antenna connected to the radio, the effect was overridden and more or less forgotten about.
It was when my younger sister got her first portable transistor radio that the impact of a rotatable loop began to be driven home. She received a high quality eight-transistor radio for Christmas one year that was not only highly sensitive, but selective as well. It had an RF stage and two IF's and was small enough to be portable, but large enough to have a five inch long ferrite loop in the top of its little cabinet.
Somehow I managed to get her to let me use it to listen one night...probably with the promise of a couple set of batteries since it went through them like water. Sitting at the the desk, it could pick up stations that the Watterson without the outside antenna could not separate. I was a little annoyed at first that there was no way to connect my outside wire antenna to it and the thought of opening it up to find a way to attach it to the circuit was not an option with my sister watching very closely what I was doing with her prized radio.
The effects of the internal loop on the little radio were amazing to me. First of all, the little radio was picking up by itself most of what I could hear on the Watterson with the outside antenna connected. Rotating the radio did not produce much of a peak in signals, but the nulls were amazingly deep and I could almost make local stations disappear. It was easy to hear KLOU form Lake Charles, Louisiana on 1580 khz despite the presence of a local station, KBGO-AM on 1580. For the first time I found that I could pick out stations on some of the crowded regional channels, in some cases logging two stations and in one case three stations on the same frequency.
I only had the little radio for an hour or so before my sister demanded it back, but a lesson was learned. At some point, a loop antenna would be an asset.
It was a few years later that the next lesson was learned when I was a college student at the University of Houston. My roommate had a Hammurland Super Pro receiver that he had been working on but we didn't have an antenna to connect it to. There was no way to string a wire outside the dorm as I had done while at Texas A&M ( a whole 'nother story!!!) and he wanted to listen to some broadcast DX. I remembered lessons in a radio-tv correspondence course my dad had taken that described tuned loops, but the details were a bit hazy. I remembered that they involved a resonant tuned circuit for the loop and a link coil to provide a low impedance connection to the receiver.
We had some insulated wire and scrounged a broadcast band variable capacitor from an old radio and went to work. But what to make the loop out of was the question. We both spotted a yellow plastic trash can at the same time. Why not? We started out with 15 turns of wire lengthwise over the top and bottom of the trash can and attached the variable cap to make it a parallel tuned circuit. The turns were spaced about a quarter inch apart with the ends of the windings anchored by punching holes in the plastic near the top of the can with an icepick. The coupling coil was spaced over a couple inches and had two turns. We ran a short piece of RG 58 coax we had from the link to the receiver.
The first try near the top of the band resulted in signals, but not too strong. Rotating the capacitor did not result in a peak. Obviously our wild guess on number of turns was off. Checking at the bottom of the band resulted in a peak with the capacitor almost completely open. Obviously too much inductance was the problem. We started removing turns one at a time and when we got down to eleven turns, we could get a peak at the top of the band but not quite at the bottom. The capacitor was of the two gang variety, so a clip lead to parallel the two gangs resulted in a successful resonating at the bottom of the band, and the top could be reached by unclipping the jumper.
The whole project took about thirty minutes. A pretty good null could be obtained and the sensitivity was pretty good. All the “ usual suspects” could be heard...the Chicago stations WLS, WBBM. WGN, WMAQ all were in good...as was WSM Nashville, WWL New Orleans, WLW and WCKY Cincinnati. As a bonus, it appeared that a good bit of the flourescent light noise was reduced.
This was the beginning of my own love affair with the improvised loop. While I have made some “ serious “ loops, including a wood framed three foot wide beauty my dad helped me build years later, to loops inside plastic pipe in a round shape with a neat tuning box at the bottom, there have been many times I have put together impromptu tuned loops almost anywhere. Many times while on engineering trips I have put one together in a hotel room to do some DX-ing or to get rid of noise. Many times these were wound around cardboard boxes of various sizes, a few times around trash cans, and even once around a door ( opening and closing the door had the effect of “rotating” the loop!!)
While some very fine, pretty and well performing loops can be purchased if you want to spend as much as $300, some pretty good performance can be had for next to nothing. The size is not critical, though less than a foot across is about the smallest I have gotten really good DX results with. I can't provide you with exact numbers of turns for various sizes, that can be determined experimentally, thought I would imagine there are formulas that could be used to determine. However the time and effort to do that are greater than just winding one and either adding or taking off turns to get it to resonate.
These impromptu loops don't require exotic materials other than a broadcast variable capacitor. I will admit these are getting harder to come by in recent years, but anyone playing with radios for any amount of time probably has an old tube junker around that could be cannibalized. One could be ( Gasp!) bought through a mail order parts house and kept for DX “emergencies”. Some of these are probably good to have around for various things in a well filled DX-ers junk box anyway. I usually start with ten or eleven turns and just hunting for the resonant spot. An old fashioned grid dip meter could be used to find resonance very quickly. I have never tried one of the fancy new ham antenna analyzers or a loop and am not sure what one of those would really tell you anyway. Maybe someone who knows or who has tried it might add a comment to this blog with their experience.
Impromptu loops are inexpensive and can “grow” anywhere. They are particularly good when traveling precludes carrying a lot of stuff on an airplane ( or doing so would raise eyebrows in the security check!!!).
Coupling to the loop can be done either with feedline or inductive coupling by placing a portable receiver with an internal loop next to the larger loop's windings. I actually did just that using the table top clock radio in hotel room on a trip to the Bahamas once! The loop was wound on a plastic trash can and set on top of a small table in the room and the loop and radio moved around the table for rotation.
While the homemade impromptu loop might not give quite the full performance of a commercially made device or the more complex big ferrite core loops in some ways, its practicality and availability for use outweigh those issues. After all, the idea is “ DX With What You Have” and “ DX Anywhere, Anytime”
By the way,. By adding more turns, these loops do an excellent job on long wave frequencies and work very well for NDB DX-ing, and with fewer turns do the same in the 160 meter amateur band and the lower tropical bands ( 120 and 90 meters) They don't seem to do much better than a random wire up higher.