Ham radio operator
ON4AKH
OS4S
Copyright 2015 on4akh.be. All Rights Reserved.
Since 2003 I have been experimenting with magnetic loops. My first loops where made
out of soft copper tubing about 12mm in diameter.
Soon I learned that the bigger the diameter of the tubing, the higher the efficiency of the loop.
In 2007 Bart ON2BA got in contact with a metal shop. The shop was able to bend aluminum
tubes with diameter of 50mm. We had a couple of 6 meter tubes bent into circels.
Unfortunantely, due to other occupations, the loops have been in my garden for about 4 years
without touching them. Now I finally found the time to finish the first loop.
The loop with the loop to mast plate already attached
As a capacity to tune the loop, I used a vacuum variable capacitor made in Russia.
It has a maximum capacity of 500pF and can handle 10K Volt.
This is the beginning of the the construction to hold the capacitor. Two bracelets to
connect water pipe of 60mm to the wall on a plexiglass plate. To minimize galvanic
resistance between the capacitor and the loop, I made from copper sheet plate
loops to go around the vacuum capacitor and the magnetic loop.
The capacitor and motor in place. The motor I picked up from a Hamfest.
It has a reduction, so it rotates with a speed of about 10 rpm. There is a teflon piece
to isolate the motor from the capacitor. There is a spring type connection to connect
the motor to the capacitor. This to not put strain on the capacitor when the motor and
shaft of the capacitor are not 100% aligned. A multi turn potmeter shows the position of the motor.

The finished enclousure for the capacitor. This will be painted black.
Notice the pieces of copper sheet that will go around the aluminum tube of the loop

Detail of the mast to loop plate
Detail of the mast to loop plate
Detail of the gamma match.
This is a design of Jean-Pierre ON6YC.
The measures are for this loop are:

Length : loop length / 8 = 64,7 cm

Tube diameter : Loop tube
diameter /3 = 16 mm

Distance from the loop :
Circumference of the loop/70 = 8 cm

Detail of the gamma match connection to the PL259. A piece of copper rod
soldered to the connector and fixed with hot glue. The rod is threaded so a nut
can be used to fix it to the gamma match tube
The loop with the capacitor encloser mounted on top.
Detail of the copper sheet wrapped around the magnetic loop tube.
Enforced to stay in place with a stainless steel water hose clamp
A hole on the bottom of the loops has two purposes. Drain water out of the loop.
It also is a way out for the cable that controls the motor and reads the multi
potmeter coming from the capacitor enclousure
With the capacitor of 500 pF the loop can be tuned from 40 meter till 80 meter band and beyond. However to obtain a perfect SWR, the stub on the gamma match needs to be moved. If you put the stub inthe middle, SWR < 1.5 on both bands.
I hope this page brings you some ideas on how to build a magnetic loop. Understandable you won't be able to find the same motor or even vacuum capacitor. However for a motor you can use a stepper motor. With the correct software you can know the position of such a motor so there is no need for a potmeter to read the position of the motor.

My next step is to create some software and interface the motor and potmeter with my computer. This will allow me to change the position of the vacuum capacitor based on thefrequency of my radio.
But that will be described in another project page when finished.