Magnetic Loop Analysis

[wa7son]

I am in the process of building a magnetic loop, frequencies to be decided.

I am inquiring if anybody has done any AIM analysis on this particular type of antenna?

Since this is a project of the near future, I am open for comments/suggestions.

Thanks,

Bob/WA7SON

[kdavis]

I have spent considerable time measuring my mag-loop.
My 3 ft diameter loop tunes from 13 MHz to 50 MHz.
I found that the optimum position for the feed loop is different for each band.
I also explored using different types of toroid core feeds.
I tried 3 or 4 different ferrite types and different numbers of turns on the core.
I found that the optimum number of turns on the toroid is different for each band.
Basically I learned that it is possible to create an excellent match for one band but multiple band operation is always a compromise in one way or another.
With the AIM 4170 you will be able to see the bandwidth curve change as your feed arrangement gets better or worse.
If I understand everything I see in my measurements, a narrower bandwidth curve indicates a higher "Q" and higher efficiency.
If you try a loop feed and a toroid feed you will see different shape phase curves for each type.
The loop feed has a "V" shaped curve similar to the SWR curve.
The toroid feed has the sideways "S" curve
If someone can expain this for me, I would appreciate it.

Have fun!!
kdavis

[wa7son]

Thanks, I appreciate it.

BTW, I was planning on either starting off with the toroid feed, and I was going to devise a way to switch toroids, one way or another, depending on the bands I plan on using.

The other method involved a gamma match, but I haven't done anything with mag loops yet, just starting off. I plan on 40, 30, 20 meters, but it does appear by utilizing the online calculator that I could easily get 2 bands, but it would be a compromise to get 3 bands, (of those in particular).

It will be interesting to say the least, and this will be a good project for the AIM.

The wire antennas were a little easier to throw together, hence, no problems.

Thanks for your input.

[ka3yan]

So there's nothing like reviving an old thread, right? Anyhow, I've been working on a small magnetic loop of my own and thought I'd share some comments. I've only just started any kind of analysis of it, but basically I chose the following building materials:

- 10 feet of 1/2" soft copper tubing from local home improvement store for main loop
- 2 feet of 1/4" soft copper tubing from local home improvement store for the feed loop
- Random (short) length of RG-8X coaxial cable with a UHF (PL-259) connector on one end and unterminated on the other
- 4 feet of 3/4" schedule 40 PVC pipe to hold the loop
- Oren Elliot model 73-1-45-45 450pF air variable capacitor
- 8" of 10 AWG copper wire to connect the capacitor to the main loop
- Heavy duty soldering iron
- Solder
- Blow torch
- Zip ties to hold things in place

Here's what I've learned so far...it isn't as easy as some of these sites make it look. My results are very similar to kdavis' results. Placement of the feed loop seems to be band specific. Moving the feed loop up and down the PVC pipe changes the tuning range significantly. I really wasn't expecting that. Most places show the feed loop practically touching the main loop. I wanted mine to tune mainly for 20 meters, so the feed loop is 1 5/8 inches from the main loop.

It's very difficult to make adjustments to the loop without drastically affecting the tuning. This of course is due to its very high Q. The bandwidth of my loop is approximately 20kHz. In case anyone reading this isn't aware...that's pretty darn narrow. If I could find a 100:1 reduction gear for air variable capacitor, I'm not sure it would be overkill. A simple temperature change (e.g. the sun shines on the capacitor) throws off the tuning of the loop. It's really much more sensitive than I imagined. My intent when I went for the larger capacitor was to be able to tune multiple bands, but after my first couple limited experiments, I'm not sure that's going to be realistically possible. I'm thinking about changing the large range variable capacitor to a much smaller range vacuum variable. I think I really only need a range of 50-100 pF for the whole 20 meter band.

I'm using a RigExpert AA-600 analyzer along with Dan, AC6LA's Zplots plug-in for Excel. I really don't see how someone could build and tune one of these loops effectively without a decent analyzer. Mine is in full time workout mode when I'm playing around with the mag loop. I've spent way more time fiddling with the loop and analyzer than the with my radio. Hi hi hi!!

73,
Brad KA3YAN

[wa7son]

I enjoyed reading your responses with regard to the magloop, and your experimentation. I have been out of town for awhile, and havent done too much with the antenna yet. Its built, but I have to do the fine tuning on it. I've been working on a Softrock RX/TX kit, so trying to get the horse before the cart.

I do like the idea of using a vacuum variable capacitor, and I may do likewise. Any info is appreciated. I am glad I have my AIM to help with the experimentation, it is quite the device.

Bob/WA7SON