A Magnetic Loop for 6m.

Started by gil, January 03, 2013, 02:50:49 am

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Since my big loop works so well, I decided to build one for 6m. I have a couple working Gonsets (Communicator III & G-50) which are mostly used on 50.4MHz. I haven't heard anyone yet, but i am hoping a 90% efficiency loop will help. Great thing is, it is only 1.4ft. in diameter!

Be warned that magnetic loops are dangerous! Lethal voltages are present on the exposed parts while transmitting, and 6m is the worst wavelength as far as RF exposure goes. DO NOT stay near the loop while transmitting, and DO NOT let anyone get near it!

I started with a 3/8" soft copper coil. It came in a 25' long coil, and I have enough left for two 10ft. perimeter loops (more on that later). The length of the copper is 56 inches. My capacitor is a 3-37pf air variable (Hammarlund) available here: http://www.surplussales.com/Variables/AirVariables/AirVar1.html. The support tube is a PVC 1" thick-wall type. I also got an end cap and a "T" coupler. The screws used to secure the cut ends of the copper in the PVC tube are 1/4" brass. The toroid used for coupling is an FT-140-43.

First step was to calculate a maximum efficiency loop on 50.4MHz without getting to low on the capacitor. The program used is RJELOOP1.exe:

7pf is too low. It turns out that RJELOOP1 overestimates the capacitor value a bit. The same thing happened with my 20/30/40m loop, which ended up being a 30/40m loop due to my 9-110pf capacitor being too high, or the loop being slightly too long. I made the same mistake here with the 6m loop. 4ft. would have been better. But back to the construction:

Here is the top cap and tube drilled using a drill press and 5/8 drill bit:

Below is the bottom where the loop will be fed, thus creating a horizontal polarization:

I had to open up another rectangle window below the round holes to allow for the wire to go through the toroid a few times. Here is the result:

The capacitor is bolted on the PVC tube, through the end cap and the tube. A few washers on the bottom keep it level. I made slightly larger holes on the other side to get a screwdriver through.. I was not very happy about not being able to test the conductivity of the brass screws to copper tube connection (impossible to solder inside the PVC), so I used an external copper wire, which I soldered to the copper tube, then to the brass screws and the capacitor terminals.

Here is the completed loop with a 12" ruler for size:

I still need to put an SO-259 plug on the T coupler. Right now I used a binding post. At first I was able to thread five turns of wire through the toroid. As soon as I plugged it in, I new it worked! Though the capacitor plates barely overlap for peak band noise, which means my loop is a tad too long. It is fine, because the highest frequency it will be used on is 50.4MHz AM or below on SSB. SWR looked like 2.5:1, which is of course too high. I used a smaller feeding wire and managed to thread 7 turns through (every time the wire goes through the toroid counts as a turn..). The SWR got down to 2.1:1. I need to experiment some more. The SWR should go down to at least 1.5:1. My big loop gets 1.3:1... Maybe adding a couple turns will do the trick..

The loop is of course not meant to be carried by hand, though some people do that. I think it's crazy and way too dangerous. Look at the top image, it says 2750V with 27W in! I plug in another piece of PVC tubing into the T to get it high off the ground and away from anyone's reach or head... At first I thought I would paint it, but I like the copper color and might just spray a coat of clear acrylic on it..

Here is the latest addition to my stables, and the reason for building the 6m magnetic loop:

Now if only someone would answer my calls!

Ya'll have a happy new year!