Going Magnetic Loop

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w3jn:
OK, I've been there and done that.  Living in an apt here in Greece I felt this might be a nice antenna to try.  I built one out of 2 10' sticks of 1" copper pipe, silver soldered at the top.  I used a motorized vacuum variable out of a Collins 180L aircraft antenner tuner to tune the loop.  It worked, but...

One problem is, if you use a sufficiently small loop, copper, and robust connections the Q is really high.  So, the bandwidth is extremely narrow; my 3:1 SWR points were about 10 KHz on 40.

Which brings up the second problem.  Because the Q is so high and the BW so narrow, the tuning was REALLY touchy.  The gear ratio on the 180L capacitor was way too fast, and reducing the DC voltage on the motor didn't help much either.  The thing would heat up a bit and shift its tuning, or the gear would slip a bit due to backlash, and I had to shuttle back and forth trying to find the sweet spot again.

Feeding it was another PITA.  Most recommend a smaller loop, connected to the transmitter and mounted inside the diameter of the antenna for a feed.  I dicked with that thing for the better part of a day with the network analyzer trying to find the best coupling, and then the feed loop would spring back or something and it would go to hell again.

Finally the thing was pretty top-heavy and kept tipping over.  I finally gave up on it.

I'm not sure aluminum would work all that well, Clark.  The resistivity of Al is quite a bit more than Cu and the difference might be a substantial portion of the total resistance so you'll end up giving up quite a bit in efficiency.  If you do build one out of Al, make it as as big as you can, given your target frequency.  If you make it too big, however, you won't be able to tune it.

flintstone mop:
Quote from: W1AEX on October 20, 2011, 09:34:30 AM

I have worked Ted - K1QAR a number of times on 160 meters while he was using his TX/RX loop. His signal was comparable to other stations running a dipole. It's really amazing when you think about it.

Fred, I have been looking at that Pixel loop and wondering how much signal it captures on 160m and 75m. In spite of the improvement in SNR it often seems that small loops don't grab enough signal to establish much quieting with our mode unless the station you are receiving is quite strapping. Still, it might be fun to mess around with one on a rotator installed away from the house. I've played around with several homebrew tuned loops, but the preamplified and broadband nature of the Pixel makes it very interesting. Any chance you could make a video showing it in action as you tune a few bands?

Rob W1AEX

Good suggestion Rob,
I got our little still digital camera and it makes very nice youtube videos, the audio will be a trick with its built-in mic. I'll post something soon. Thanks
Fred

W1AEX:
Quote from: flintstone mop on October 20, 2011, 12:22:25 PM

Good suggestion Rob,
I got our little still digital camera and it makes very nice youtube videos, the audio will be a trick with its built-in mic. I'll post something soon. Thanks
Fred


That would be excellent Fred! I wouldn't worry too much about the audio as speaker to microphone will still give a very good indication of how it works. Thanks for taking the time!

Rob

W0BTU:
Quote from: KL7OF on October 20, 2011, 10:44:23 AM

... silversoldering (silfos) copper tubing.....the way that these antennas use skin effect would seem to mandate low resistance joints...


Absolutely they need low resistance joints! :-)

However, I'm not convinced that silver solder would make any noticeable difference vs. ordinary tin/lead solder. Reason being, the difference in resistance between the best silver solder and regular solder is only about 15%. The very best silver solder I found had a resistance in the 20% range (copper being 100% and pure silver being ~102%).

I think the main thing is to have as little solder exposed at the joints as possible (think skin effect). If you get a lot on the outside of the copper pipe, remove it with emery cloth or a small abrasive wheel made specifically for removing solder.

WB2EMS:
Quote

Is aluminum Good enough?

It's not as good as copper, but it can be usable. There are various calculators for small magnetic loop antennas and some of them allow you to select copper or aluminum to see the difference in predicted performance. As I recall, aluminum ends up being about 40% as good as copper - mainly because of it's higher resistive losses in a circuit where milliohms count a lot. You can make up for the resistivity by using a larger size of aluminum or make the loop bigger.

For a legal limit antenna on the lower bands, probably not the best choice - but it may be a lot more available than copper. It may be that you can build a bigger aluminum loop than you can afford with copper and improve the efficiency that way. I've got a 30 foot chunk of aluminum 75 ohm hard line that is earmarked for a fixed 75 meter loop for a qrp setup at a remote location. The hardline was free, and long enough, and should be reasonably efficient because it's a larger loop. If I had to buy copper, I'd probably make it smaller to keep the cost down and lose efficiency that way.

There are a number of smaller loops (1 meter) used for portable use on the higher bands that are made from aluminum and apparently have respectable results. There are also some larger loops used on the lower frequencies, see http://www.standpipe.com/w2bri/80meter.htm

If you have some aluminum material available to experiment with, I'd say go ahead and get your feet wet. Then look into upgrading material later if needed.

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