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.