10 Sep 2015

Magnetic Loops on HF TX

It is some while since I used a magnetic loop antenna on HF TX, but these antennas work well, if you can accept their very narrow bandwidth. In fact the sharper the tuning, the more efficient it is likely to be. I used a small magnetic loop made with small-bore central heating copper pipe and was amazed how well it worked with WSPR spots from Japan on 20m with the loop in the shack, which was an upstairs bedroom at the time. Ideal applications are where frequent retuning is not needed such as PSK31, JT65 or WSPR. On 10m they can be very efficient. Even on 20m I was very impressed. Bandwidths are usually measured in kilohertz. Losses have to be kept low if they are to be efficient. Even thin wire will work but the antenna will not be as efficient. As a minimum coax cable or copper pipe should be used for the loop. At QRP powers ordinary air-spaced capacitors work fine to bring the loop to resonance. At 100W these may well arc over.

See https://sites.google.com/site/g3xbmqrp3/antennas/magloop .

4 comments:

PE4BAS, Bas said...

Hello Roger, I'm impressed by the results as well. See:

http://pe4bas.blogspot.nl/2015/09/experimental-wspr-30m-setup.html

73, Bas

Roger G3XBM said...

Thanks Bas.Yes indeed these antennas really do work.

John said...

Hello Roger. I have had good results with a loft mounted loop (MFJ 1786) but have found it to be far more directional than is generally recognised. To get the very best out of a Mag Loop I think one should find some means to rotate it. One of my plans over next few months is to relocate the loop into the garden (use good quality co-ax) and install it on a rotator. As I only operate QRP I think this will be worthwhile.
Regards.
John - GW3OIN.

Roger G3XBM said...

Yes John, they are classic loops so will have best results in the plane of the loop and be much less good at right angles to the loop. A rotator is not a bad idea.