Compact Loop Antenna Notes
This application note refers to my article: "You Can Build A Compact Loop Antenna for 30 Through 12 Meters", QST, May 1994, p33. The amount of interest in the miniature loop has resulted in an overwhelming amount of correspondence. What I have done with this document, is to answer all of the questions that have come up since the article was written. You can contact the ARRL for a reprint of the article.
The article has one error. On page 34, column 1, it gives two methods for attaching the capacitor to the loop. Disregard the strap method (it raises SWR), and use the first direct mount method with holes. Trust me, it will work better and give you a lower SWR. Here are the other answers to everyone's questions:
* The gear reduction device that I used is a common vernier drive (like the tuning dial of a QRP rig). These parts are commonly found from electrical supply companies like Mouser. Parts suppliers are listed below.
* Some people have written to me asking about a shaft coupler. Unfortunately, I do not have a source, because I found mine out of a scrap time-clock assembly. But I do have two methods for you to connect it without a coupler. Drill a hole in the capacitor plastic tuning shaft. The diameter for the hole should exactly equal the diameter of the shaft on your motor. Now, with a fine hacksaw, saw a slit through the center axis of the plastic capacitor shaft. This slit will pass through the hole that you just drilled. Insert the motor axis into the capacitor hole. Tighten the shats together with a small plumbing hose clamp.
Alternative method: Drill a hole in the capacitor plastic tuning shaft. The diameter for the hole should exactly equal the diameter of the shaft on your motor. Now, insert the motor axis into the capacitor hole. Take a very small screw, and screw it into the side of the plastic capacitor shaft to act as a set screw. Before you screw in the set-screw, gently drill a starter hole into the side of the capacitor shaft where you intend to drill the set screw. Make sure the set screw meets the flat side of the motor shaft. Do not overtighten the set screw, as this will strip the threads.
* Page 34, column 2, of the article gives information on building a 40 meter loop. The ARRL Antenna Book has data for other loop sizes. It provides both loop size and capacitor value.
* My loop is very similar to Loop 6 in the ARRL Antenna Book. You'll notice, that to cover 10 meters you need a capacitor that goes down to 5.4 pF, so you'll need to find a capacitor that basically goes down to zero pF to cover 10 meters. Another way to help cover 10 might be to shrink the antenna 25 %, but I have never done this, and cannot assure you that it will work. If you try it, simply multiply the circumference of both the big loop and the small loop by 0.75. If anyone tries this, please write to let me know how it works.
* A small diameter copper loop or a 10-guage length of insulated solid copper wire will work fine for the small loop. But I think the coax is easier to work with.
* The space between both sides of the loop at the top is not critical, but keep it as small as possible, less than 1.0 inch. My gap is about 0.5 inches. One person wrote to me, and told me he used a gap of several inches, and his antenna did not work. After he closed the gap to 0.5 inch, it worked.
* Remember, the article is for a QRP loop. If you plan to use more than 7 watts, you will need a bigger capacitor. I have no personal experience with this, so I can't advise you. Franz Freller in Germany reports a plate spacing on the capacitor of 3mm is need for 100 watts, and I can't really add any info.
* Two excellent sources for parts, including variable capacitors, are Ocean State Electronics (401) 596-3080 or (800) 866-6626; and Mouser Electronics (800) 346-6873. You can call them, and I'm sure that they would be happy to send you a catalog.
I hope that this information is helpful. Be sure to contact the ARRL for a reprint of the article, if you want complete building instructions.
Photos:
The Original Compact Loop Antenna