THE HOOP LOOP By John Hoopes, AB4MS (Lowdown, Dec. 1992) I've been experimenting with LF antennas for four years now, and have tried numerous antenna designs in my quest to hear the elusive LowFER stations. Most have met with limited success. It wasn't until Todd Roberts (WD4NGG) demonstrated a ferrite loopstick at his house did I become a firm believer in the attributes of this wonderful antenna. During the visit with Todd, we tested several antennas in order to try and hear my LF beacon "H," then located in Warner Robins, Georgia. Todd lives on Hilton Head Island, South Carolina, about 160 air miles from me. I first tried to copy my beacon on a home brewed active whip antenna, but to no avail. Todd then set up a loop- stick antenna that he constructed from a kit that was offered by Jim Hagan (WA4GHK). After turning the antenna to the general direction of my beacon and peaking the tuning capacitor, we could hear my beacon with no problem. It was a solid Q-5 copy. Needless to say, I was impressed. After a lengthy discussion with Todd about antennas, I decided if I were ever going to do any serious LowFER listening, a loop would have to be part of my LF listening arsenal. When I finally made up my mind to build a loop, I decided to construct an air core or box loop instead of the 42 inch ferrite loop like Todd was using. This was more of an economic factor than performance, due to the high cost of ferrite rods. After designing the loop on paper, a trip to the local hardware store was next. I decided to use two 4-foot long pieces of 4" X 3/4" pine for the frame crossmembers. This would give me a loop diameter of 34 inches, which I thought would be a good compromise between antenna aperture and portability. Once I completed the frame, I stained it and put several coats of polyure- thane on the wood to enhance its appearance and give it a protective finish. Several articles on air core loops suggested I would need somewhere between 30 and 40 turns of wire to resonate my antenna in the 150 - 500 kHz range. Not knowing exactly, I decided to take the experimental approach and wind as many turns as I could, and then check for resonance and adjust the turns accordingly. I started by winding the frame with 18 gauge stranded copper wire with a thick plastic insulation. The insulation helps maintain a one wire diameter spacing between turns, which will give a higher Q, not to mention keeping the turns from shorting. I figured this would be easier than cutting grooves into the wood to space the turns. When I was finished, I ended up with 31 turns. Checking the self-resonant frequency with a dip meter indicated resonance occured around 450 kHz; not bad for a first time effort. I added a standard 365 pf air variable capacitor in series with the coil and measured the available tuning range. As it turned out, the loop will tune from 160 - 440 kHz, just perfect for my range of listening. The next order of business was to couple the RF energy from the loop coil to the preamp. A low noise preamp with 20 to 30 db of gain is necessary for this type of antenna as the natural gain of a loop antenna is very low. I had several ideas in mind. One would be to use a high impedance preamp and directly couple its input to the loop, which would create an unbalanced condition, since one side of the loop would have to be grounded. Not exactly what I wanted. The other was to use a balun to feed the unbalanced input of the preamp. Not a good idea, since I hate winding toroids. I had a Burhans H-90A low impedance preamp lying around the shack that wasn't being used, but how was I to couple several thousand ohms' loop imped- ance to a low impedance amplifier? The a brain storm, sort of . . . link coupling! Now, I know link coupling is nothing new and has been around as long as antennas. But the fact remained it was a novel approach, and what the heck, it worked well to couple the final of my LF transmitter to its antenna. Using an impedance bridge, I measured one complete turn of wire suspended one inch on the inside of the main loop coil. The one full turn measured 200 ohms, not the 50 ohms I needed. Well, if a full turn is equal to 200 ohms, then it stands to reason one fourth of a turn must equal 50 ohms. Another quick check on the impedance bridge proved my theory correct. A near perfect match to the amplifier was obtained by running the link wire parallel and one inch below the top main winding, and running the remainder of the wire down the middle of the cross members and directly to the input transformer of the preamp. Several tests using one full turn and one quarter turn proved that the extra effort to optimize the amplifier feed impedance was worth the trouble. During testing, I noted about 10 db of gain and sharper tuning with the quarter turn. Now for the smoke test. In the shack I was able to obtain S meter readings slightly less than what I could get using a L-400B active E-probe antenna up about 50 feet. Tuning was sharp, indicating a relatively high Q, which is desirable in this type of antenna. I was planning to use the antenna for portable listening, so retuning every few kilohertz was not a problem. I noticed I could null a local NDB from an S9+20 db to inaudible on the receiver, indicating good loop balance. Boy, was I happy! There was still too much localized noise pollution at the shack to do any serious LowFER listening, so the next step was to go portable. After loading the loop and receiver into the van, I headed to a favorite listening spot. Now for the real test. I set the antenna on the ground, pointed it east toward Todd's QTH, turned on the receiver and--lo and behold, music to my ears. I heard "ABC ABC ABC QSL WD4NGG" followed by a long dash. Although not a Q-5 signal, it was clearly identifieable. I couldn't believe it! I'd been trying to hear Todd's beacon for over four years, and finally, success! I made a recording for posterity and also to send Todd a copy. After an hour of listening and comparing signals to my whip I decided the loop is definitely the way to go. Signals barely detectable on my home brew whip (which is a good performer in its own right) were clearly audible on the loop.... Anyone having problems with their active whips would do well to give this loop or any loop a try. -------------------- (SYSOP NOTE: The Burhans preamp and RC-86 power coupler mentioned in the article were from Burhans Electronics, 141 Grosvenor St., Athens, OH 45701. However, since then Ralph Burhans has reportedly ended his association with Burhans Electronics. I do not know if the amplifier is available from other sources, but similar units may be available from LF Engineering and/or Curry Communications.)