Magnetic loop antenna for PSKMail backbone frequency 10.148 Mhz

Magnetic loop antenna "without" variable capacitor.

In magnetic loop antennas, variable capacitor is needed to tune antenna to frequency currently used. Because high voltages (easily thousands of volts), suitable capacitor may be hard to find (or expensive).
If needed tuning range is little, loop itself can be used as tuning capacitor.

This antenna is made from 5 meter long piece of 50 x 5 mm aluminium. (Some 6000-series alloy, maybe 6061, not sure.)
Ends of aluminium are mounted to form a capacitor. There is M8 bolt for fine tuning.

Feeding is usual little loop inside big one, made of aluminium.
Diameter of loop is about 1.3 meter. It is chosen just to be biggest one I can put in trunk of my Land Rover. (Hopefully it fit, I have not tried.) Thats why 5 meter was chosen...very scientific...

Some estimations were made with these online calculators:
Capacitor calculator and loop antenna calculator. In first try, antenna tuned to somewhere 7 Mhz, so I shortened capacitor section to be about 70 cm, original calculations showed 1 meter with average distance between plates (ends of aluminium) something like 5 mm. i used wire diameter 3 cm in loop calculator. In other words, when used to 50 mm aluminium, result was 30% lower than expected (in frequency of "sweet spot").

That is the capacitor section of capacitorless loop and fine tuning bolt.
It is easy to make threads to aluminium: just a little oil to bolt and drill bit 1mm smaller than bolt is needed. Bolt can be turned in hole and it forms nice threads. Bolt is insulated from other side of antenna with 3 mm plexiglass which is simply taped to aluminium. Also ends of "capacitorplates" Are insulated with same plexiglass and taped to stay together. Between aluminium plates is also 3mm nylonrope, to avoid plates touching each others.
Turning 1 round of this bolt changes tuning point about 30 khz.

Eventually this antenna will turn your SWR-meter to be a rain is quite easy to tune, and tuning will last stable s long as weather stays exactly same.
I made this antenna for field playing with PSKMail. If you need stationary antenna, you should add some kind of remote tuning motor for comfortable use. I could imagine, this ring would be stable enough if placed in attic and it could be a good stealth antenna, not prone to interference caused by household machines.

And that is how loop itself is made. This must be a traditional blacksmith's tool for making loop antennas...


All The Best!
Tarmo, OH6ECF