HFU HF Underground
Technical Topics => Equipment => Topic started by: jackson_r on March 06, 2016, 2011 UTC
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After a couple of months screwing around (and following some tweaks recommended by some fine folks here), my magnetic loop antenna project is complete. The photos are here:
https://drive.google.com/folderview?id=0B1Sx8UoqolwleE5MdFFUT3d2akk&usp=sharing
It's constructed of four parallel copper loops at approximately 3.25' diameter each. Loops were constructed from 3/4" copper tubing. That weird looking thing on the capacitor shaft is a proximity sensor to allow me to measure how many rotations of the shaft I've made. I went this route because I couldn't find a suitable way to use limit switches. The capacitor shaft length only varies 2 mm between full left (shaft unscrews) and full right (dead stop). I didn't take any photos after I completed painting it with a sort of home brew camouflage pattern.
Finally, yes, I'm aware of how close to ground it is. Part of that is because I ran out of PVC tubing, originally intending to mount it about 5' off the ground. But I've read several sources that claim that when oriented vertically, mag loop antennas can be used mounted right on the ground just fine. I guess we'll find out. Additionally, it will be nice to have it closer to ground level if the damned thing bursts in to flames and I need to take a fire extinguisher to it. ;D
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Wow, you added 2 more loops! Looks like it was a lot of work. Congratulations on finishing the thing!
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Wow, you added 2 more loops! Looks like it was a lot of work. Congratulations on finishing the thing!
Yes, I was reading a website from a guy who is somewhat of an expert on the multi-loop antennas. His tests have showed that going from one loop to four loops can increase efficiency by something like 5 db. The proof is in the pudding, as they say, and I'll have to wait a few more days to see how well it works (if at all). Just waiting on a couple of coax connectors and a new soldering gun that arrives today. Might be able to fire it up this weekend and see if get out any distance, or if I start a fire in the backyard. ;D
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First test was not successful. No fires or explosions, but I couldn't get the SWR below 3 no matter what I tried. Somebody here recommended at the start of this project that I start with just one loop and go from there. Clearly what my father used to say about me is true: I'm one of those people who sometimes just has to learn a lesson the hard way. ;D I built this thing so damned complex from the start, I don't know where the problem could be. I'm going to knock off the three extra loops and go back to one and see if I can get that working first.
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Your antenna is beautiful, I hope you can get the bugs flushed out and use it with all the loops as planned.
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Back to the basics has been a success! Cut the 4 loop antenna back to one loop. Confirmed receipt of broadcast via SDR in New Brunswick, from the Pacific Northwest of the US....and on 20 watts AM!!! Very scratchy and barely audible, but it was definitely me. If you read a story about a strange looking man in Oregon dancing a jig out in his front yard in the rain...that's me. ;D ;D ;D ;D
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Dang, New Brunswick is pretty far from your region.
With the 4 loop configuration, what were you doing to change the SWR?
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Dang, New Brunswick is pretty far from your region.
With the 4 loop configuration, what were you doing to change the SWR?
Just using a reversible motor circuit that is hooked up to the vacuum variable capacitor. Even with my one loop configuration, I'm still not pleased with my SWR though. One of the reasons I couldn't crank it over 20 watts was SWR climbs over 2 if I do. Perhaps something wrong or needs to be tweaked with my inner loop shape or location. This test was done with the loop literally mounted 6" off the ground too. I can get it up at least 3 or 4 feet off the ground for proper use.
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You're up early this morning :D
As I understand it, you can get a good SWR from manipulating the area of the pickup loop.
I just ran your loop through the computer program this guy, Glen Gardner made: http://gridtoys.com/glen/loop/loop3.html
And it says your pickup should be 0.0306 square meters. But I guessed on the parameter "resistivity of loop material". I think its measured with an ohm meter.
On that web page, he tells how to troubleshoot your loop using this program.
Here's the results I got:
LOOP TYPE (CIRCLE or SQUARE): circle
LOOP CIRCUMFERENCE(METERS): 3.14159
CONDUCTOR DIAMETER(METERS): .022225
NUMBER OF LOOP CONDUCTORS: 4
RESISTIVITY OF LOOP MATERIAL (OHM-METER): .000000037
RELATIVE PERMEABILITY OF LOOP MATERIAL: .999994
LOOP COEFFICIENT OF COUPLING (K>=1/N,K<=1): .4
TRANSMITTER POWER (WATTS): 20
LOW FREQUENCY LIMIT (MHz): 6.925
HIGH FREQUENCY LIMIT (MHz): 6.95
FREQUENCY STEP (MHz): 1
Frequency: 6.925 MHz Rr: 0.0219 ohm
Rhf2: 0.0540 ohm Efficiency: 0.2882
L: 1.5260 uH Xl: 66.3983 ohm C: 346.2 pF
Z: 0.2753 ohm Q: 438.1103
Bandwidth: 15.8066 KHz. 2:1 SWR Bandwidth: 5.3953 KHz
Dynamic Resistance: 29089.78 Ohm. Potential across the Capacitor: 762.76 Volt
Pickup loop area: 0.0306 square meters
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Thanks Antennae! I see you figured out how to use that calculator. I spent half a day trying to research how to compile code and decided it was beyond my brain's capability. :D If you get time, any chance you could rerun the calculator with a loop diameter of 3.2' (closer to the actual size) and two conductors? I got the single loop tuned in pretty good with an SWR of 1.5. I'm looking at adding a second loop back in now and it would be nice not to have to futz around with the pickup loop.
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Yah, sure, I will private message you.
I had trouble with the compiling with windows, but it was fairly easy to do on Linux. I got lucky I found the right incantation to make it work. I'll send you a private message.