I get a lot of questions about my homebrew MagLoop antenna, so I decided to attempt a page on it. I did a fair amount of research before deciding to build one and, like most things, there are a ton of opinions out there on how to go about it. I think most of them will work just fine and its more a question of how much time and effort you have to put into it, and how anal retentive you are about RF design. ;) I'm not very interested in the most exacting design - just one that works and can be implemented in short order. I don't have all day, every day like a lot of (retired) hams - so we have to fit it all in between the whistles. So with that being said, what I did is best bang for the buck and surprisingly effective.
Things I've learned about these beasties:
- They are really easy to build. Really. And you can make them out of lots of things, but there are a couple of key things to keep in mind:
Surface area = better performance. Bigger the diameter of the tube and the better conductor, the better it works. So 1/2" or better copper pipe is a good thing.
They really do hate things around them. Treat it like a dipole antenna and keep things at least 5+ feet away from it.
They really are tough to tune. So unless you want to spend a lot of time designing a sophisticated tuner, know you will probably sit on a frequency for a very long time. This makes them great for digital modes.
They are really really efficient. You can get long way out there with very little power.
An Antenna Analyzer is a must....
You really should buy a air gap capacitor. If you must, a vacuum tube capacitor is really nice, but a good ol basic capacitor will work fine. Providing its rated for 5+ Kilovolts... ;)
It probably cannot be stressed enough that the voltages present at the capacitor are in the Kilovolt ranges. That means that coming into contact with it during operation could suck the paint off your house and give you a permanent orange afro. (Translation: it will ruin your whole day and that of any pet, child or relative that should come into contact with it either.) RF Safety is always the operator's responsibility.
With those things in mind, here's what I did (Click on any image for a larger version):
Links to resources:
Really I could paste in a ton of reference pages, but this is the best. Plus he has already nicely linked in a ton of other pages (many very good) for your education. His calculations for loop size are good.
Parts (capacitor in particular):
Making Coax Capacitors
Then I started buying (and scrounging) parts....
I went to Home Depot and bought a roll of 3/8" soft copper pipe - twice as much as a needed as it turned out, A 10 foot piece of 1" PVC Pipe (you want this to be very sturdy pipe. If its not, the Loop is really top heavy and bendy pipe will just p*ss you off). A 25 # bag of play sand (yes play sand) and a number of wormdrive clamps Then I ordered a air gap capacitor from RFParts after I got REALLY REALLY frustrated with trying to make my own coaxial capacitor. The one I chose is about 70pf-40pf. Not optimal, but it was only $40. And it was rated at 6 Kv. I dug around the house and found a six gallon bucket with a lid and a fist full of #14 solid wire. Oh and stop by Radio Shack and get a SO-239 connector...
Construction (pictures coming):
I am not a metal head, nor a plumber, so my bending skills suck. To put it mildly, so my loop is not perfect. But it doesn't seem to matter a lot, or at least in ways I can notice. My loop will no doubt make your average OCD/Anal Retentive Ham gnash his teeth. Too bad. It works and I had time for a beer afterward. Priorities.
- Cut Copper pipe at 10 feet in length and form into a rough circle about 3 feet in diameter. I used a garbage can as a guide and bent (gently!) across my thigh where needed. Measured the diameter a lot as I went and it was close. And a bit lumpy. Oh well.
- Cut PVC Pipe at ....oh whatever works for you. You might use the whole 10'. I though that was awkward and cut it at 6 feet.
- Place Copper (rough) loop by PVC and mark for 3 holes (bottom and two top). Drill with 3/4" bit all the same way through the PVC (as you will feed the pipe through this that's important).
- Thread copper pipe through the PVC, ending with the open ends at the top, running through the top two holes. Put wormdrive clamps on the ends of the top two ends. That way they won't flip out. And p*ss you off.
- Take a 1 1/4" hole saw and cut a hole in the bucket lid (on the bucket works well). Slide PVC pipe and loop through the hole and pour play sand around the PVC....aha! That's what the sand is for! Make you a nice, solid, portable, non-conductive stand. Easy.
At this point, you are ready to prep a couple of minor electrical items:
- Get out that SO-239 connector and solder a short piece of the #14 wire to the shield (mount bracket) and a 16-20" piece of FULLY STRIPPED #14 solid to the center connector. Set this aside. Its your gamma match.
- Solder connectors to the air gap capacitor (about 6" pieces of #14 solid) . Set this aside.
Ok lets finish this:
- Strap air gap capacitor to PVC Pole at the top (Velcro, whatever as long as its non-conductive)
- Connect the capacitor to the two worm drive clamps that you ran through the top two holes (one wire to each). Rough up the ends of the pipe with sandpaper and slide a stripped end under each clamp and tighten.
- Strap the shield connector of your SO-239 connector to the bottom of the loop directly opposite the top (burnish with sandpaper first).
- Form the stripped wire connected to the center tap of the SO-239 connector so that it follows the curve of the loop, about 2-3" off it. Strap it about 15" up the pipe (again burnishing with sandpaper first).
Your loop is basically constructed. (Yes really).
Tuning this beast....
This is the hardest part of building the Loop. It made me craz(ier)y ... Your own body will affect the readings you will get from your antenna analyzer. Its a pain. This is the point where I tell you that (for me) using a gamma match was a whole lot easier to tune than a faraday loop as the coupling device to the loop. It seems to be at least.
- Set the loop away from as many things as you can.
- Hook up your analyzer to the SO-239 with a fairly long jumper cable (10+ feet would be nice). That will allow you to set things, back away and see if they hold (remember your body will influence the swr readings).
- Set the Capactor to roughly half open
- Pick a frequency you want to tune to (I used 14.070) and move the long end of the center tap from the feed up and down the pipe until you get a nice low SWR (around 1.5 - 2 is likely). Lock that in place on the pipe with the worm drive, back away and see if it holds.
- If #4 is good, then you have tuned the feed. If not, you'll have to fuss with it. The loop couplers you have to deform in order to tune the feed. Uck. I'll stick to this for now.
- Now that the feed is tuned, slowly, painfully slowly, rotate the plates of the capacitor until you get a good SWR for your chosen frequency.
Done!
This antenna will work surprisingly well indoors or outdoors at low altitude (think 3 and 6 feet off the ground). It is directional and can be weatherproofed etc. That extra pipe bought? Yeah, its becoming a 15m digital mode antenna with a fixed capacitor. I'm going to use the air gap capacitor from the original to tune the feed of the new one, then I can experiment with coax capacitors...removing one variable. Without that feed tuned....ugh a mess.
