A Few More Words about Magnetic Antennae I found this article I wrote a couple of years ago. There is good information in it. Click HERE to download the pictures of my Two Turn Magnetic Antenna. I have experimented with a couple of Magnetic Antennae after reading about them in an old CQ or 73 magazine. My first one was 5 feet in diameter with a sliding plate capacitor. It worked well. My first contact was a fellow in Moscow. We were on 20 meters SSB. I received 57 58 on that QSO. I thought that this might be a very good way to go since it lends itself quite well to being used as an attic antenna. The properties I liked was that fact if designed well, it would not get "into" things like Stereo's etc. As we all know radiated energy consists of both electrostatic and magnetic components. Electrostatic will get into things where magnetic will not. The object here was to surpress the elctrostatic component as much as possible. The antenna must radiate only the magnetic component theoretically. In fact, both are being radiated but the electrostatic component is greatly reduced. As the radiated energy reaches about 2 or 3 wavelength away from the magnetic antenna, the electrostatic component is beginning to be built back up to achieve the ratio of 377:1, electrostatic to magnetic (the normal ratio so I've read somewhere). I need you to refer to a document that's on the internet. If you would please click HERE and print out this entire web site. It will probably take 11 pages. There are some very nice pictures, formulae and plans for a good tuning capacitor. The secret to supressing the electrostatic component is in the feeding and the tuning capacitor designs. Let's take the feeding first. If you would refer to the web document pages 2 and 6. Page 2 shows a couple of loops, one upright and one down horizontal (I don't know why he has it horizontal - that invites problems). Look at the horizontal loop and imagine it upright with the small feed loop at the bottom (center point of the antenna). The feed loop is 1/5 the circumference of the big loop. You'll notice the tuning capacitor is at the top. On page 4 you'll find the formulae for determining the dimensions of the loop. Now refer to page 6 where there is a feed loop represented. Still view the big loop upright with the feed loop at the bottom. His feed loop picture is upside-down. The point where he solders outer on outer is actually the bottom. The point where you can see the inner conductor is the top. This feed loop is a modified Farady Shield Loop. The RG-58 coax is formed into a loop that is 1/5th the diameter of the big loop. I suppose you could use RG-8 or RG-213 as well. At the end of the coax, solder the shield to the inner conductor, then solder that end to the coax shield where he labels it "Solder Outer to Outer". That point must also be electrically connected to the very center point of the big loop. Connecting it the the big loop prevents capacitive coupling to the loop. We want it to connect solidly without any reactive component there. At the top of the feed loop, bare the inner conductor like shown in the picture. Ensure that the inner and shield are well insulated from each other on the Tx side of the bare inner conductor. The other side, however, solder the inner to it's respective shield (where he labels "Solder Outer on Inner"). Form the feed loop into a perfect circle. The tuning capacitor is between the two ends at the top of the big loop. The author of the document suggests using a butterfly capacitor. He does show a drawing of a Piston or Trombone type capacitor. I chose to use the latter for my last Magnetic Antenna. What I did was to feed the feed coax with a Dip Meter to determine the Mag Loop's self-resonant frequency. Then I tested with different values of capacitors connected between the ends where the butterfly capacitor will finally be connected. Once I determined what values of capacitors are required to operate the loop in the desired bands I set forth to developing the final capacitor arrangement to do the job. I suggest the Butterfly type like he suggests. It's less bulky and easier to weather proof. It's also easier to adjust with a motor/gear box. If you refer to page 8 where he shows a drawing of a trombone type capacitor. At the top is the two ends of the loop (which would be at the top of the loop). He uses coax for the inner portions (which is a very good idea). I would shrink on an additional layer of heat shrink over those coax inners ensuring that the open ends of the coax are fully covered (to prevent arcing). The way he show it, by turing the shaft 4, the coax pistons move in or out of the copper tubes. I used a copper pipe in place of the PCB at 3 on the drawing. The reason for the double capacitors is to prevent the use of any sliding contacts like you would encounter in any single type variable capacitor. The currents in a Magnetic Loops Antenna are just too high for any sliding contact. My final design: - I modified the single loop antenna (because my attic is not very high) by forming the single loop into a double loop - two turns. By doing that, both the center point of the antenna and the two ends of the antenna are located at the bottom. I separated the turns by about 2 inches. This space must be held quite solidly since the interwinding capacity now affect the resonant frequency of the antenna. One thing this two turn loop did was to lower the self resonant frequency. My little 29 inch diameter two turn loop self resonates at about 14.5 Mhz. I added two trombone type capacitors at the ends in parallel. One was constructed with 3/4 inch copper pipe for the outer and 1/2 inch copper pipe for the inner (piston). The other capacitor was 1/2 inch copper pipe for the outer and either 1/4 or 3/8 inch copper pipe for the inner. I shrunk on 3 layers of heat shrink on all inners ensuring that the ends were well covered. I used Line Printer Ribbon Motors (and gear box) to drive the inners out and in similar to the document drawing on page 8. The Motor assy's were no longer than about 4 inches and ran OK on 12 volts. I got them at a used parts place for 10 bucks each. The trombone capacitor made of the larger material was used to pull the antenna down from 20 meters to 30 and to 40 meters. The small trombone cpacitor was used to tune inside the bands. I had difficulty at the beginning because of the feed loop configuration. The circumference of the Feed Loop is still 1/5 of the total length of pipe in the 2 turn loop. The problem I encountered was SWR. It was too high. I found that by deforming the feed loop to be elongated up and down improved the SWR. I was able to achieve near 1:1. Don't use a tuner with this antenna. Just adjust the tuning capacitor(s) (however you may chose to do that with whatever design you use) and you'll have no difficulties. When I used just the single turn loop I had no SWR problems at all. If you mount a Magnetic Loop in the vertical plane (upright) it need not be any hieght above the ground to work well. You can be just a couple of feet for example. If, however you decide to mount it in the horizontal plane you will most likely have to mount it high above the ground. I find that using my loop, noise is much less. The signal strengths seem to be about the same as compared to my Telex 8 band vertical. Some signal strengths are less on the mag loop. I contribute that to difference in radiation angles in the two antennae. I may be all wrong on that, I don't know. I hope this is helpful to you.