Going Magnetic Loop

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N8UH:
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?

WB2EMS:
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.  :o

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.  ;D  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.

W3GMS:
Quote from: flintstone mop 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


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

flintstone mop:
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

ke7trp:
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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