Experimenting with magnetic loops

[mw0uzo]

I made a test magnetic loop for 10m today, about 1.5m in diameter. Seems to work well, the loop is just made from twisted up solid copper strand, not very thick diameter on a plastic pipe frame on a stand made form a thrown away cupboard door. I don't have a suitable capacitor yet, but will hopefully get one soon.

Anyone else here made magnetic loops? I want to build one that will TX on 80m as well as get rid of all the nasty mean noise I suffer from. I understand that the size of the loop is proportional to the bandwidth, and the 1/resistance of the loop proportional to how far down it will tune with the capacitor and still be efficient. Could be wrong here, please correct me. Edit: I have a few conflicting sources of information - size affects the efficiency too. Larger diameter of pipe is good.

So to get those resistances down - large diameter pipe. Copper pipe readily available. Is thin, large diameter and lightweight pipe easily available?

[mw0uzo]

Hmmm, I reread that middle paragraph and it sounds like gibberish. Brain really not in gear tonight.

[ab1ga]

First, some things you should know:

1. Small loops have a very low radiation resistance, so any ohmic resistance in the wire is a significant source of loss. That's the reason for the large diameter tubing, although lots of folks have made such loops using coaxial cable.

2. Small loops are high Q systems, which means they have a narrow bandwidth, and require tuning as you move about in the band. This may or not be a problem.

3. Because of their high Q, a small loop antenna has high peak currents and voltages. This poses a challenge when selecting the tuning capacitor. Even at modest power (100W), you have to be careful to avoid arcing across the capacitor plates, or hot spots due to high currents through ohmic losses.

"Small" means on the order of a tenth of a wavelength or so in circumference or less.

For a good overview, perhaps more relevant to your environment, try to beg, borrow, or steal the following book:

Backyard Antennas
by Peter Dodd, G3LDO
RSGB Publications

Chapter 5 has lots of good stuff on loop antennas, and there's even a plan for an 80m loop (by PA2JBC), including a homebrew capacitor!

73 and have fun,

[W5GA]

And copper pipe is readily available, but not cheap.

[kd8dey]

Never did a small loop, But I did do a full wave loop for 6 meters using a home brew 4:1 coax balun.
used a pop can for the coil form then used electricians tape to hold the windings together.

1.5:1 match across the entire band. Didn't really get a chance to use it before those bad storms a couple years ago ripped the house service feed I had one of the support ropes tied to off the side of the house and took it with it.

The only antenna I have now is laying across the roof with busted chimney straps so I'm off the air.

[NQ6U]

And copper pipe is readily available, but not cheap.

Type M copper is cheaper than aluminum, usually, and easier to work with (you can solder it) but it is a lot heavier.

[W5GA]

Type M copper is cheaper than aluminum, usually, and easier to work with (you can solder it) but it is a lot heavier.
What is type M copper pipe?

[kd8dey]

What is type M copper pipe?

Copper comes in three grades, M for thin wall pipe used mainly inside homes, L for thicker wall pipe, used mainly outside for water services and K, the thickest, used mainly between water mains and the water meter.

[W5GA]

Copper comes in three grades, M for thin wall pipe used mainly inside homes, L for thicker wall pipe, used mainly outside for water services and K, the thickest, used mainly between water mains and the water meter.
OK, thanks! The last time I looked at the price of copper pipe at either Lowes or Home Despot I about had a heart attack. No idea what grade it was though, just the stuff in the rack in the plumbing section.

[W3WN]

OK, thanks! The last time I looked at the price of copper pipe at either Lowes or Home Despot I about had a heart attack. No idea what grade it was though, just the stuff in the rack in the plumbing section.Try and deal with a wholesaler. Depot & Lowes are not exactly the cheapest, although they have high quality.

[mw0uzo]

Thanks for your replies, I'll find some suitable pipe and post back with progress/pics/results. :)
It was pointed out to me that my test loop is actually a full wave loop on 10m, so its not in magnetic loop territory yet. There's a capacitor on its way so hopefully will be able to get it into magnetic loop mode and see how well the noise is suppressed.

ab1ga .. thanks for the pointers in the right direction :-D

[HUGH]

I've started using self-adhesive copper tape on fibreglass rods to keep the weight and cost down. Some of the rods were salvaged from a burnt-down frame tent that someone abandoned. 8mm and 10mm rods or tubes are useful because you can still use soldered plumbing fittings to join them.

The tape was widely sold for slug deterrent but it's cheaper to buy from the internet.

[mw0uzo]

Yeah I've seen the copper tape trick, did you post a thread about it a while ago?
I nabbed some manky but thick coax the other day for making a better loop. I'm going to make it smaller this time for portable use, with a faraday loop.
For the high voltage capacitor for the larger version, I've got some glass and copped clad board that I'm going to turn into a variable capacitor. Two pieces of copper clad at 20cmx20cm will be around 1000pF, with one plate mounted on a screw thread for adjustment.

[n4aud]

I built a mag loop using copper tubing, but for receiving only. I used a capacitor from an old Hammarlund s-38 and a small stepper motor to adjust it remotely. Worked OK, never got around to build one to xmit.

[KG4CGC]

Is there a page that has a magnetic loop calculator?

[ab1ga]

Is there a page that has a magnetic loop calculator?

Here's one, a collection of generally good stuff, but some of the codes were written by someone who is now an SK, so probably not kept current with current versions of OS/Browser, etc:

http://www.smeter.net/antennas/single-turn-loop.php

[KG4CGC]

Here's one, a collection of generally good stuff, but some of the codes were written by someone who is now an SK, so probably not kept current with current versions of OS/Browser, etc:

http://www.smeter.net/antennas/single-turn-loop.php
Thanks. I used to check out that site a lot.

[mw0uzo]

Fed up trying to find decent capacitors, I dismantled my manual ATU. I changed the loop material to a 4.7m circumference of RG-213 coax and got rid of the gamma match and used an unshielded Faraday loop. Two of the variable capacitors in parallel got it resonant on 80m. I can now hear stations clearly when before they were completely swamped in noise and inaudible. There is still quite a lot of noise however, approx S7.

Further improvement expected to improve RX S/N further is use of a shielded Faraday loop. Not tested on TX yet - a further version made from large diameter copper tube is likely necessary as well as a custom made butterfly capacitor and motor drive for convenient use and 100w of handling power.

It really is quite surprising, the S/N improvement over a wire dipole. Its not the holy grail, but it is significantly improved. (YAY!)

[KG4CGC]

Fed up trying to find decent capacitors, I dismantled my manual ATU. I changed the loop material to a 4.7m circumference of RG-213 coax and got rid of the gamma match and used an unshielded Faraday loop. Two of the variable capacitors in parallel got it resonant on 80m. I can now hear stations clearly when before they were completely swamped in noise and inaudible. There is still quite a lot of noise however, approx S7.

Further improvement expected to improve RX S/N further is use of a shielded Faraday loop. Not tested on TX yet - a further version made from large diameter copper tube is likely necessary as well as a custom made butterfly capacitor and motor drive for convenient use and 100w of handling power.

It really is quite surprising, the S/N improvement over a wire dipole. Its not the holy grail, but it is significantly improved. (YAY!)
Excellent! Could you possibly include some pictures? Visuals help. :)

[mw0uzo]

Excellent! Could you possibly include some pictures? Visuals help. :)

Yeah - I'll take some photos today.

Noise is so strong here that the small unshielded faraday loop of 96cm circ. removed from the main loop picks up s2 to s3 of noise! :o

[mw0uzo]

Loop on top of my garage
http://dl.dropbox.com/u/5626002/NetPics/SV202381.JPG

Faraday loop feed point
http://dl.dropbox.com/u/5626002/NetPics/SV202372.JPG

Testing lash up with caps removed from MFJunk atu
http://dl.dropbox.com/u/5626002/NetPics/SV202374.JPG

Caps inside box and connections soldered as much as my iron could give...
http://dl.dropbox.com/u/5626002/NetPics/SV202384.JPG

Specs: Main loop ~1.5m diameter, 470cm circumference. Faraday loop 94cm circumference. Power handling is more than expected ~75W (hot dry weather!). SSB was a lot more sparky when testing than CW, I found only very occasional flashover with 100W of CW on 80m.

Had some 20dB over 9 reports on 40m this evening, which is bloody great :)

The loop works on 80m, 40m and 30m.

[mw0uzo]

Very pleased with the performance of the loop, even though according to a calculator it should be only 2% efficient on 80m and 19% on 40m. Adding some radials and raising the height improved TX performance considerably. TX performance is better than my wire antenna.

Made a small 10m loop yesterday for testing. It looks like some sort of bell-end on my roof! Hopefully I'll get a few contacts today and see if its any good.

Need to made a remote controlled high voltage cap. Probably going to go for servo control with a coarse and fine butterfly capacitor on the same axis with servo control from either side. Problem is getting high capacitance for 80m and minimising capacitance when capacitor and its assembly is fully open so the loop has good range.