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Author Topic: Going Magnetic Loop  (Read 40284 times)
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flintstone mop
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« on: October 18, 2011, 08:27:50 PM »

I have been reading a lot about magical antennas, like magnetic loops and find them quite fascinating.
I just installed a magnetic loop 3 foot diameter antenna for listening. Got it on a TV rotator and it is a pleasure to pick something out of the grass...the noise, local noise, ma nature. I can see the noise floor go up over 10dB by switching between the conventional dipole or vertical to the loop.

I'm looking at some designs for a transmitting loop.........12 foot square, using aluminum pipe....rotating might become a problem. BTW this is probably the one antenna that becomes directional for transmit. A dipole only 40 feet off the ground on 75 or 160 is not directional. But a magnetic loop antenna is. It may not be beamed like a Yagi and have gain, but it does exhibit direction.
They can take legal limit, but the vacuum variable needs to be 30kv. A lot of hype about a magnetic loop antenna being just as effective as a dipole 1/2 wave up in the air and more efficient.
QST has an article about a Ham who built a 2 loop antenna.
Fred
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Fred KC4MOP
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« Reply #1 on: October 18, 2011, 08:38:27 PM »

Fred,

If you come up with what looks like a workable design for 160, I'd sure love to know about it. I live on a leetle, tiny city lot with not even enough room for a decent antenna garden, much less a farm. I'm using a coil-loaded dipole on 160 right now, but I need to improve the 160 antenna, so all ideas are welcome.

ldb
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Ed/KB1HYS
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« Reply #2 on: October 18, 2011, 10:11:42 PM »

How big is your lot? Any trees or just the house/roof?  I have the same problem with our 100x100ft lot, But I have some very tall trees (65 ft or so white pine and a few oak) that make great ant supports.  One day I will mess around with actually using the Tree itself as a vertical ant on 160 meters.  It's been done and is supposed to work, tested by the Army. 
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73 de Ed/KB1HYS
Happiness is Hot Tubes, Cold 807's, and warm room filling AM Sound.
 "I've spent three quarters of my life trying to figure out how to do a $50 job for $.50, the rest I spent trying to come up with the $0.50" - D. Gingery
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« Reply #3 on: October 18, 2011, 10:28:33 PM »

If you are very careful to minimize I2R losses, an STL (Small Transmitting Loop) can actually outperform a dipole mounted higher than the loop. Using large diameter copper pipe and a special capacitor is a must.

There's some good links and references on STLs at the bottom of http://www.w0btu.com/magnetic_loops.html.

I just received my vacuum variable (eBay, Russian military surplus) last week for my own STL project.

As for the QST article, there's a typo: it says the loop can approach the performance of a vertical, but "dipole" (more gain) should have been there instead of "vertical". Or, they could have said exceeds the performance of a vertical.
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73 Mike 
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« Reply #4 on: October 18, 2011, 10:37:23 PM »

If you come up with what looks like a workable design for 160, I'd sure love to know about it.

Here's one guy who did that: http://www.bulldogtrust.com/index12.htm

And I've seen others, I think they're in the links on my STL page I mentioned.
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73 Mike 
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Steve - K4HX
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« Reply #5 on: October 18, 2011, 10:54:42 PM »

This guy has already done it.

http://amfone.net/Amforum/index.php?topic=12451.msg91347#msg91347
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ke7trp
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« Reply #6 on: October 19, 2011, 12:04:54 AM »

This guy gets on 75 with us.

http://www.qrz.com/db/K8NDS

His Signal was right there with the rest of the Dipoles on the band. 


C
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« Reply #7 on: October 19, 2011, 12:21:28 AM »

This guy gets on 75 with us. http://www.qrz.com/db/K8NDS

I've worked Rich on 40 SSB, and he's loud. But if he runs the legal limit on AM with that loop, that's impressive. If there were a lot of losses, it would smoke.

Keep in mind that loop is only five feet off the ground! :-)
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73 Mike 
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« Reply #8 on: October 19, 2011, 01:17:57 AM »

How are magnetic loops at rejecting line noise?
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Radio Candelstein - Flagship Station of the NRK Radio Network.
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« Reply #9 on: October 19, 2011, 03:42:04 AM »

How are magnetic loops at rejecting line noise?

They were almost meant for that purpose. The rejection of a single, local RFI source at very low angles is excellent. You would probably want to use a rotor.

The null is much less pronounced at higher angles, but can be >50 dB at 1 or 2 degrees elevation. You can see that in the EZNEC 3-D model below. Notice the deep null along the X axis at ~5 degrees which decreases rapidly as you go up. (This model is a 6' diameter loop 5' high on 40 meters.)

I will probably not rotate mine (at least initially) since I have Beverages and very little line noise. But the null is quite sharp, and if you need to null line noise, etc. then a rotor is almost a necessity.


* EZNEC 6ft loop 5ft high 7MHz.gif (20.01 KB, 546x422 - viewed 622 times.)
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73 Mike 
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vincent
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« Reply #10 on: October 19, 2011, 05:06:54 AM »

I posted this link because maybe (if interested) part of the argument, I do not know the performance of these antennas and I don't know the manufacturer/seller.

http://www.ciromazzoni.com/English/Loop%20Antenna.htm
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flintstone mop
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« Reply #11 on: October 19, 2011, 06:48:53 AM »

How are magnetic loops at rejecting line noise?
That's the beauty of magnetic loops.....The line noise is electrical which is part of our Electromagnetic radio signal. The loop is only listening for the magnetic part. I'm not into the good tech talk using $.25 cent words. But when I switch to my magnetic loop on receive the noise floor goes down over 10dB and those little signals lost in the grass are now readable.

Thanks for the reminder link Steve.........3 inch pipe starts to be real nice. What I picked out of the QST article was losses. You need to make extremely good mechanical/electrical connections and you have to deal with distributed capacitance. That is why the Ham in QST made a 2 turn loop for his project.
And legal limit is possible. You need a 30kv vacuum variable......the only expensive part.......I'm looking at these other links......
EDIT....looking at Steve's link for one of our guys and his loop. The 3 inch aluminum pipe and big vac variables were a lot more than I thought for cost. And there were some complications to get it on the air.........
I have a nice big lot, but nothing high to hang a decent antenna for 160 or 75M.
Fred
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Fred KC4MOP
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« Reply #12 on: October 19, 2011, 10:45:37 AM »

Thinking whilst driving a school bus....
WHAT IF:
instead of using expensive metal for the loop.......make the loop FORM using 3 inch dia. Fiberglass or even thick cardboard tubes epoxied together keeping the 3 inch dia. in mind
WRAP the entire circular or octagon or rectangular Fiberglass form with overlapping 2 inch wide copper strap??
Overlapping will allow one to SOLDER the overlaps to make it like one continuous piece of copper. It would be lighter than using copper pipe or even aluminum pipe. The goal seems to be big diameter pipe and big size of the actual loop.

Please read warnings about RF exposure. These magnetic loops at legal limit can be lethal. Something about running a vacuum cleaner 200 feet from the loop antenna from the radiated RF
here's the link again: http://www.bulldogtrust.com/index12.htm
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Fred KC4MOP
Steve - K4HX
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« Reply #13 on: October 19, 2011, 10:52:39 AM »

"I have a nice big lot, but nothing high to hang a decent antenna for 160 or 75M."


What do you have up for 160 now?

A mag loop is not going to give you any better signal than a dipole. An inverted-L with a good ground system may out perform the loop.
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« Reply #14 on: October 19, 2011, 11:53:56 AM »

WHAT IF: instead of using expensive metal for the loop.......make the loop FORM using 3 inch dia. Fiberglass or ... WRAP the entire circular or octagon or rectangular Fiberglass form with overlapping 2 inch wide copper strap?? ...

That's pretty much what K8NDS did, and I was thinking of doing the same. But when I priced all that, it cost (and likely weighed) more than 1" (actual OD is 1.25") type M copper water pipe. Two 10' sections plus the elbows is <$100 at Lowe's or Home Depot.

Having said that, perhaps your idea would work great for a ~3" OD loop.

Other ideas (which I haven't pursued) are paralleling two loops made from copper pipe, or using large Heliax.

Keep in mind that the bigger the loop diameter, the smaller pipe you can use for the same efficiency. You can see that by downloading AA5TB's loop calc spreadsheet and playing with the numbers.
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73 Mike 
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« Reply #15 on: October 19, 2011, 03:36:56 PM »

I'm watching this thread with great interest.

The limitations of my current location prevent me from putting up a traditional antenna for the low bands. And I have 15KV and 20KV Jennings caps that are waiting for a good project...

My question is about doing multi-turn loops. I read the QST article this month about mag loops, and the author made it sound as if doing a multi-turn loop gives you roughly the same efficiency as a single turn, larger loop using the same amount of material. Is there any way to calculate this?

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-Tim
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« Reply #16 on: October 19, 2011, 04:32:33 PM »

I'm curious about the multi turn loops also, and how does the feed change when the loop has more than one turn. If you're using a 1/5 size coupling loop, is that 1/5 size based on the diameter of the multiple turns, or on the effective diameter if the total circumference was converted to one turn? (my experiments suggest it's the former)

I think wrapping a flat piece of copper around a form and soldering the edges is likely to add resistive loss that you may not want, since the resistive losses play a very large role in the efficiencies of these loops. You may be ahead just using the flat material on edge as the loop material - some of the MFJ loops did that, using a band of steel for the element. I keep looking at 4-6 inch wide roof flashing as something to possibly make a low band loop with suspended inside of a small wooden cabin (or maybe wrapped around the outside). The aluminum is fairly cheap, the copper, not so much so.  Shocked

I've done some of my experiments using old hard line, both aluminum and some copper jacketed, as the element. Cheaper than buying and soldering up copper pipe.  Grin  Depending on the dimensions, you can use the capacitance between the inner conductor and outer shield  (about 27 pf per foot) as part of the tuned circuit. I have a 30 meter loop built that way, just needs a small trim cap to adjust the tuning across the band. That may work up to around 100 watts, for QRO you'll need the vacuum variable for sure.

I would still like to get my hand on one of those QMAC mobile loops that tune down to 3 Mhz and are supposed to be 10 db more effective than a loaded whip in that frequency range.

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73 de Kevin, WB2EMS
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« Reply #17 on: October 19, 2011, 04:33:39 PM »

I have been reading a lot about magical antennas, like magnetic loops and find them quite fascinating.
I just installed a magnetic loop 3 foot diameter antenna for listening. Got it on a TV rotator and it is a pleasure to pick something out of the grass...the noise, local noise, ma nature. I can see the noise floor go up over 10dB by switching between the conventional dipole or vertical to the loop.

I'm looking at some designs for a transmitting loop.........12 foot square, using aluminum pipe....rotating might become a problem. BTW this is probably the one antenna that becomes directional for transmit. A dipole only 40 feet off the ground on 75 or 160 is not directional. But a magnetic loop antenna is. It may not be beamed like a Yagi and have gain, but it does exhibit direction.
They can take legal limit, but the vacuum variable needs to be 30kv. A lot of hype about a magnetic loop antenna being just as effective as a dipole 1/2 wave up in the air and more efficient.
QST has an article about a Ham who built a 2 loop antenna.
Fred

Fred,

Which loop did you install?  I have seen several commercial ones for sale.  Years ago, I built one designed by Doug Demaw W1FB (sk).  It was for 160 M and was 5 ft per side.  The loop was parallel resonated with a mica trimmer cap.  It was just used on receive.

Joe, W3GMS
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Simplicity is the Elegance of Design---W3GMS
flintstone mop
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« Reply #18 on: October 19, 2011, 07:20:50 PM »

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/

Mike W0BTU, the helical winding has a purpose. It is not to reduce the weight. Please read on below.
If you QRZed K8NDS you will see links to his website and the youtube vids.
K8NDS, Rich, is using copper flashing 3 inch wide and helically winding as opposed to one long piece of pipe. This winding creates inductance and you need less C for tuning. It also lowers the HV. A 15KV vac variable and you're there legal limit.
The multi turn loop in QST was to try to distribute the capacitance. Rich thought that there were short comings to that design. He was leaving on a business trip and will return NOV 1st.
Here is his website for the helically wound loop and some YouTube videos of his antennas and actual on air QSO's.
http://www.hlmagneticloopantennas.com/

Fred
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Fred KC4MOP
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« Reply #19 on: October 19, 2011, 08:04:10 PM »

My idea was this:

Get a Childs hoola hoop at Walmart. 
Cut a 3 inch section out of the hoola hoop on one side. 
Buy some Tinned copper braid.
Slip braid over and around hoola hoop pull the braid tight. (Like chinese hand cuffs)

Mount to board or plexi.  Feed with Coax and use a simple Gamma with a 30Kv air cap.

You can get the braid at home depot or at Georgea copper:

http://www.gacopper.com/Braid.html

Cheap TV rotator, I have a homeade 15 ft tower I can Drag around the yard for experiments like this.  If only it was not 100 outside still.
 

C
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K5WLF
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« Reply #20 on: October 19, 2011, 11:01:41 PM »

How big is your lot? Any trees or just the house/roof?  I have the same problem with our 100x100ft lot, But I have some very tall trees (65 ft or so white pine and a few oak) that make great ant supports.  One day I will mess around with actually using the Tree itself as a vertical ant on 160 meters.  It's been done and is supposed to work, tested by the Army. 

Ed,

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
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« Reply #21 on: October 19, 2011, 11:02:27 PM »

... My question is about doing multi-turn loops. I read the QST article this month about mag loops, and the author made it sound as if doing a multi-turn loop gives you roughly the same efficiency as a single turn, larger loop using the same amount of material. ...

I got kind of excited about that, too, until I saw that the two-turn loop in the 11/11 QST actually had significantly more loss than the single-turn one. But maybe I misread that.

Multi-turn loops with conductors too close together have the "proximity effect" issue. The conductor in the front may have a certain current value at a given point on that conductor; but the turn behind it next to that point may have a current that is quite different. This causes increased current in a given area. You can spread the turns out to reduce this effect, but the bottom line is that losses can be much higher than in single-turn loops.
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73 Mike 
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« Reply #22 on: October 19, 2011, 11:17:50 PM »

... Mike W0BTU, the helical winding has a purpose. It is not to reduce the weight. ...

I realize that, Fred. I've studied K8NDS's material about his loops extensively, and have his info printed and bound in a 3-ring binder along with a lot of other mag loop info. I know what he's claiming. I was just saying that I added up the cost of the copper and the pipe, and for a reasonable efficiency, Type M copper water pipe was not only cheaper, it was easier to build.

There's a lot of controversy about Rich's design. He wrote an article about his helical loop on eHam, if you're interested. The comments to his article there points out that his helical design may have additional losses, but I think that's a little overblown.
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73 Mike 
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« Reply #23 on: October 19, 2011, 11:33:37 PM »

... Get a Childs hoola hoop at Walmart.  ... Buy some Tinned copper braid.
Slip braid over and around hoola hoop pull the braid tight. ...

That would be fine for a receiving loop, but it would have way too much resistance for a transmitting loop. Small loops like this have been made from coax like RG-8, and they get extremely hot at 1500 watts. Almost all the power is lost in heating the copper braid. (Depending on the loop diameter, the loop current can exceed 100 amps.)

The reason coax braid is not a good choice for an STL is that the radiation resistance (Rrad) of an STL like this is in the milliohms, and the resistance of the braid can exceed Rrad of the loop itself.

That's why we have to use what might seem to be a ridiculously large conductor to have a loop with any kind of reasonable efficiency.

Even people who have made small receiving loops from, say, #16 copper wire find out that the signal level is waaaay down from one made from a much larger conductor. The Rrad is so small that the losses in the wire are enormous.

There was a good discussion on this at http://forums.qrz.com/showthread.php?311422-HF-magnetic-Loop-antenna-questions.
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73 Mike 
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« Reply #24 on: October 19, 2011, 11:52:25 PM »

Interesting.  I can see why Rich used that thick copper strap!

C
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