Tuning up Your Magnetic Loop
|Hang in there, it isn't that hard to carry a tune:
We've come a long way. We've soldered, cut coax, seperated braid, clamped on radiator clamps, and what you have before you should look something like what's pictured on this page. I think it's appropriate at this point to make a strong suggestion: tuning loops can be made easy if you have an antenna analyzer. Without an antenna analyzer, well, let's just say you are going to be a frustrated ham. You see, you need the ability to see where the loop is resonant, and that may be outside the ham band area (before it's tuned). So go out, and borrow one -- do anything you have to in order to get your hands on one of these babys. It will make your like easy peasy and you'll get where you want to be a hell of a lot faster. Having said that, let's go ahead and make some other important issues clear. You are going to have to tune this loop up wherever you plan on using it. If you tune it in place A, and then move it inside your house, you'll need to tune it again. It's like ANY antenna. Surrounding objects couple to the antenna and change impedances. However, on the bright side, once you understand the basics of loop tuning, re-tuning it is as easy as using a coffee maker.
Here we go:
Placement: Mount the loop on something non-conductive. I think between two supported PVC pipes will do fine. You can also hang the loop from each corner (from a tree) or whatever clever way you want to mount it. BUT, please, no metal masts, no conductive materials. If you want to tape PVC as a mast, that should be fine. Even if it's indoors, try to keep it away from metal. However, get it in the clear as much as possible. Make sure you have the coaxial capacitors mounted on the loop.
Tuning the Feed First: There are two parts of loop that relate to tuning, the feed on the bottom and the coaxial capacitor. First we are going to focus on the Feed. The feed is very important. Here is the general feed principal, memorize it if you have to: the more efficient the loop is, the smaller the distance will be between the ground tap and the center conductor feed tap. In other words, that thick copper wire, or thin copper pipe will be closer to the center if the loop is efficient. On the flipside, if the loop is incurring losses the loop feed will be larger, and the tap point will be farther away from the ground tap. For example: if you tune the loop inside the house where there is usually a lot of metal, the loop will be less efficient than if it is high off the ground and away from coupling objects. So, if you tune the loop in the house, the tap will probably be farther away from the center ground. If you take the loop outside and get it off the ground, the tap point will be close in. Experiment with this and see for yourself, it seems to be the case every time.
With this understanding, losen the radiator clamp just a little so it moves a bit if pushed on the feed where the center conductor pipe connects to the loop near the edge of one of the sides. Turn on your analyzer, and sweep through the frequencies near the band you want to operate and then keep going up and down on the analyzer in frequency, eventually you will find a dip in the SWR. Sometimes the dip can be as low as 7 MHZ, do don't be afraid of checking most of the HF frequencies. This dip in SWR, however, will probably be about 5:1 or so -- but you will see a dip. It will be obvious.
Now, slowly move the the center conductor thick copper wire or thin pipe right and left and see if the SWR starts to go down at all. It should start to drop as you lengthen or shorten the distance between the two feed points. If you start to go up a vertical side of the loop and that's where you find resonance, so be it, clamp it there. The important thing is to get the SWR as low as possible. When you find that low SWR, even if it's not on your desired frequency, tighten the radiator clamp. Step 1 to tuning is complete!
Finally, Tuning the Coaxial Capacitor: This is the coda folks. Once your feed is matched, even if it's on 10.225 and you want to work 20 meters, that's OK, because our caoxial capacitor is now at bat. Take a good coax cutter. Grab your antena analyzer and check to see where the loop is resonant. Let's say it's 1.1 on 10.225 -- we'll need to trim that coxial capacitor to the desired frequency choice. Let's say that we want to work 20 meters on 14.225 MHZ...we'll here's how. Each piece of RG-8U or RG-213 equals about 50 Pf per foot of capacitace, which means every inch of coax is worth about 100-200 KHZ. So if we are down on 10 MHZ and want to go up to 14 MHZ, we have some room to cut. Go ahead and make a one inch cut in the coax, just snip off the end. OK, what does the analyzer say? Probably about 10.665 or there about. The resonant frequency went up. Cut another inch off, and check the meter. Keep cutting untill you get to about 13.800 or so. Don't worry if you end up going to high. It takes practice and we can use the shortend stub for 17 meters later. Folks, this is where we have to be careful, when we are so close to 20 meters our ham noses can smell it (and the analyzer confirms it), stop. Take a deep breath, and then take the end of the coaxial cap and cut lightly about one inch up from the end and remove the rubber coating. You should end up with a nice piece of braid on the coax. Check your meter again. Your frequency will probably have gone up again. Let's say at about 14.000. Now, we want the phone bands, so push the braid away from the end a little, that's right, bunch it up slightly. Boom, the frequency went up again, maybe to 14.100. Bunch it up a little more, now we have 14.250. And there you have it...a tuned magnetic loop. If you've gone too far, pull the braid back down onto the center conductor dielectric. Once you have your loop just where you want it, tape up the end of the center dielectric and it's conductor. This will keep the coax from arcing and catching fire.
There you have it, a completed Magnetic Loop, be proud!
Let's talk about Operating Your Magnetic Loop -->>