Going Magnetic Loop

[The Slab Bacon]

My lot is about 100' (E/W) X 70' (N/S). No trees that'd do any good and the roof of the 24' X 36' house has the Hustler 5-BTV in the geometric center for 80-10. The homebrew coil-loaded 160 dipole is about 20 feet off the deck along the south fence. It has lousy bandwidth, but I made S-9 into IA with it a while back on 100 watts SSB. I'm planning on getting an amp and I need something that'll take the power and give some decent bandwidth and coverage from a fairly low elevation.
ldb

Try this! ! !

[W1AEX]

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

[W3GMS]

Hi Joe
this was the PIXEL 3 foot diameter receiving loop with pre-amp and mount and control that will shut down the pre-amp and prevent your TX RF from coupling into your receiver. Their price jumped up from last year, to $400.........but very good sturdy construction.
http://www.pixelsatradio.com/product/shortwave-magnetic-loop-antenna/
http://www.hlmagneticloopantennas.com/
Fred

Thanks Fred for the response.  I was kind of guessing that is what you were using.  Like Rob, W1AEX said in a later posting, it would be good to hear some more of your analysis on its performance especially on 160 and 80M.  Maybe a separate thread is in order.  The video was pretty impressive but the signal in the video was very strong.  It would be nice to find a weak signal on the "wire antenna" and then switch to the loop and see what the overall perceived signal to noise ratio improvement was

I have never messed with an un-tuned receiving loop.  I guess for the lack of resonance he is just making it up with sheer gain from the low noise preamp.
 
It seems like as time goes on my receiving conditions just worsen.  I can remember moving in some 35 years ago and the S meter would be around an S-1 or so.  That certainly is not the case anymore and one could make a full time job of finding the noise sources.  We are out in the county and one would think that would help but not the case anymore.  Our little country general store installed some modern computer gear to scan prices as you checkout along with new computer terminals.  Ever since then, I get "buzzies" on 75 meters about every 15 KHz or so.  That issue alone even with diplomacy dial in could take major time and effort to correct.

I have established a good working relationship with our utility company.  I find the noise and let the guys whose job it is where the spots are.   

Joe, W3GMS       

[KL7OF]

I would think that resistive and or corrosive losses in the joints of a loop built of copper or aluminum tubing could be eliminated by welding aluminum tubing at the joints and silversoldering (silfos) copper tubing.....the way that these antennas use skin effect would seem to mandate low resistance joints...

[ke7trp]

Is aluminum Good enough?  I found aluminum loops.  I could have the fabricator at work cut one down and Tig weld fittings for me.  This would be lightweight, Cheap and easy to mount.

C

[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]

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]

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]

... 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]

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.

[W0BTU]

OK, I've been there and done that. ...  I built one out of 2 10' sticks of 1" copper pipe, silver soldered at the top.  I used a motorized vacuum variable ...  It worked, but...

Sounds good. That's exactly what I have in mind. Once I see how that works, I may build one that works on 80 and 160.

If your bandwidth was that narrow, that means you built it right, with very little ohmic resistance. I have a 50:1 gear motor that really slows down when you reduce the voltage; hopefully, I won't run into the problem you did. EDIT: If you look at K8NDS's YouTube videos, you can see that he did it right.

Anyway, how high did you have your loop, and were you able to compare it with another antenna?

[w3jn]

The gear motor in the 180L tuner will go really slow too, but if you reduce the voltage enough there's not enough scrote to overcome the tension in the capacitor bellows.  Were I to do this again I'd put a relatively large fixed vacuum cap in there and use a smaller value vacuum.  Or HB a trombone cap, or a book cap, or something.

The loop is wholly unsuitable to tuning around the band, because you need to tune the antenna to receive anything at all if you move more than a few KHz.

I made a few contacts on it, but never compared it side-by-side to anything else.  It was mounted on my balcony, about 3' from the wall, on the 4th (top) floor of a cast-concrete apt building.

However my working condx aren't everyone else's, and this is a really simple and potentially quite effective antenna.  I certainly don't want to discourage anyone from trying one; just want to give everyone the benefit of my experiences.

[WB3JOK]

This guy has already done it.

http://amfone.net/Amforum/index.php?topic=12451.msg91347#msg91347


So has this one  
http://amfone.net/Amforum/index.php?topic=4601.msg35203#msg35203
Don't know what happened to my pictures (from 2005) though, will try and re-load them.

[WB3JOK]

Found 'em 

Wall-mounted 8' loop for 40m/75m:




This is the little (4') loop for 20m... the coax stub capacitor is fine at 100w but when I put the SB-200 to it, the dielectric caught fire 

[W0BTU]

Found 'em    Wall-mounted 8' loop for 40m/75m ...

Holy smoke! You ran an SB-200 into that thing inside the house? I don't think that's a best idea in the world.

Having said that, I once ran an SB-220 into one not any farther away from my operating position than you have there. :-)

[N8UH]

This is all very interesting!

I was at Lowes yesterday, and found a roll of copper roof flashing. 3oz stuff. I was thinking that I could maybe make a loop out of PVC or even spa-flex, and wrap it up with a continuous piece of the flashing. The stuff isn't cheap at around $30 for a 10"X20' roll, but it's a heck of a lot more affordable than copper pipe around here. A 10' section of 1.5" copper pipe was over $100... 

[Steve - K4HX]

LOL!

Holy smoke!

[flintstone mop]

Found 'em 

Wall-mounted 8' loop for 40m/75m:




This is the little (4') loop for 20m... the coax stub capacitor is fine at 100w but when I put the SB-200 to it, the dielectric caught fire 

Magnetic loop Transmitting in the house.........Like Jerry said, not goodness. RF exposure is a LOT more than other antennas we typically use.
One of the links mentioned here said that he could operate hjs vacuum cleaner 200 feet from his loop...legal limit.
And Tim thanks for the heads up on the flashing....I'm looking more for a 20-10M loop now. My Yagi is a mess.
Fred

[WB3JOK]

Magnetic loop Transmitting in the house.........Like Jerry said, not goodness. RF exposure is a LOT more than other antennas we typically use.

As discussed in the 2005 thread, there is a fairly deep null (20-30 db IIRC) perpendicular to the plane of the loop and I'm looking right through it... didn't seem to hurt me any, what were we talking about, oh look a chicken! 

Why would exposure be more? It has less gain than a dipole, and the radiation pattern is NOT a narrow beam with concentration of the energy in any given small area (such as a microwave horn or parabolic dish antenna). 

One of the links mentioned here said that he could operate hjs vacuum cleaner 200 feet from his loop...legal limit.

I call BS on that one. 

[WD5JKO]

Don't try to jump through an energized loop! Remember the "Guardian of Forever"?

[WU2D]

Dirty Copper!

I am having lunch today with kW2S Spence, who has his own antenna design company AntennaSYS. http://www.antennasys.com/design-gallery/

He apparently has a lot of experience on HF loop technology - I will squeeze him for info. Hee Hee.

That UHF Kraus helical design to the top right was one that I had him do for my company. Very nice.

Mike WU2D

[WA1GFZ]

I run loaded delta loops for RX 30 foot base 15 foot apex about 6 feet above ground. Load is 1K, feed side a 4:1 BB transformer. I have 1 going East and the other West. They give a good front to back up to about 15MHz so much wider frequency range than a K9AY. Each one needs a feed line so just have a selector switch in the shack.
TX loops are high Q so a pita to QSY. Also the field is not safe at high power levels. Much better off with a larger antenna if possible. 

[AB2EZ]

"Why would exposure be more? It has less gain than a dipole, and the radiation pattern is NOT a narrow beam with concentration of the energy in any given small area (such as a microwave horn or parabolic dish antenna)."

An antenna produces a "near field ", that is non radiating, and which decays exponentially with distance from the antenna. Of course, it also (hopefully) produces a "far field" which propagates away from the antenna... and whose propagating E and H fields decay as 1/r (where r is the distance from the antenna).

In an electrically short antenna, the ratio of near field to far field is much higher than it is in a dipole.

Therefore, for a given radiated power level, the nearby fields (both E and H) produced by any electrically short antenna are much larger than the nearby fields of a dipole.

The reason it is hard to make an efficient, electrically short antenna, is that the relatively high near field levels produce much more heating of the surroundings (ground, trees, buildings, etc.) than a dipole does. Therefore a much larger fraction of the total power that leaves the antenna is converted to heat in the nearby surrounding objects.

Stu

[flintstone mop]

Hi Stu
This copy/paste was from a link Steve provided from one of our AM guys who, built and is using his loop on 160M. This was his thoughts and technical response from the ARRL about magnetic loop radiation. And his actual experience. Running legal limit carrier.
back to the original link from Steve

http://www.bulldogtrust.com/index12.htm

DEATHRAY

Work with the ARRL's engineer in charge of public RF exposure revealed that this particular magnetic loop's danger zone is 30 times larger than a dipole's, or about 30 feet at legal limit power. A wrought iron table 60 feet from the antenna will pick up enough energy to make a visible arc to ground; At 200 feet, field strength spontaneously starts and runs our vacuum cleaner at a speed proportional to the carrier power. You don't want to even think about what it would do to a pacemaker.

[WD5JKO]

A wrought iron table 60 feet from the antenna will pick up enough energy to make a visible arc to ground; At 200 feet, field strength spontaneously starts and runs our vacuum cleaner at a speed proportional to the carrier power. You don't want to even think about what it would do to a pacemaker.

  Ok, Good discussion. The strong H field is something to respect, and I can see some transformer action going on between the loop and other metal things that could masquerade as a transformer secondary. Still, I am having trouble with that vacuum cleaner comment. These use motors that are universal like an electric drill where the brush wiring is in series with a field coil. To complete the circuit wouldn't the vacuum power switch need to be ON, and the power cord terminals shorted out? Then to make DC we need rectification somewhere in the loop. Unmodulated RF at 160M or 80M shouldn't do anything to a motor like that.

   I am sure I must be missing something. Help me out guys. How does the RF field couple power into that vacuum cleaner motor? Maybe some of the new vacuum cleaners use an electronic controller where rectification from RF is possible? This is like Tesla lighting incandescent lamps from far away.

Jim
WD5JKO