That magnetic loop on other bands

The magnetic loop
antenna

As you may remember, I have been experimenting with my tiny magnetic loop antenna on the shack windowsill. It is at most about 50cm diameter and the wire is not very thick. To my amazement, I am getting spots with about 10W on 20m FT8. It seems to match on most bands above 20m. If stuck for a "proper" antenna, I can always try this!

20m FT8 TX spots in 30
minutes on the tiny loop

Magnetic loops

Although narrowband, these antennas can be remarkably effective. They are ideal for modes that do not require rapid retuning such as WSPR, PSK31 or FT8. It is some time since I tried one.

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

Magnetic Loops

A few years ago I built a magnetic loop TX antenna. With QRP powers, the very high voltages that exist if the loop is efficient are less problematic. The key to efficiency is low loss. For example, the loop should be made of thick copper pipe or coax, if possible. The capacitors used to resonate the loop should also be low loss. Having said this, I have had good results with normal air spaced variable capacitors and trimmed coax as capacitors. Of course, magnetic loops tend to be very sharply tuned, but for things like WSPR, JT65 and PSK31 where frequency changes are small they are fine. In fact they work remarkably well even indoors. They can be small too as the photo shows. This loop was tuned to 10m WSPR but the same sized loop was used on 20m where I was spotted in Japan - from the back of a chair in the bedroom!

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

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 .