My first antenna was a magnetic loop, which was placed on top of the dormer-window which is about 10m above ground.
I constructed my first magnetic loop as a octogonal made of 12mm copper tube, 8 x 500 mm tubing. The feeding loop is made of ordinary installation wire, running along the straight top-section of the octagon.
The capacitor was placed inside a PVC tube at the bottom of the octagon.
The PVC tube is made of three parts: the central part with rubber sleeves
which
take up the two other parts: tubes of different lengths with sealed end-caps.
During assembly it is necessary to put some grease on the rubber
sleeves otherwise it will be very
difficult
to deassemble it for maintenance!The tube assembly is connected to the octagon and to the main pole by tie-wraps. The center part has a side-duct glued onto it. Inhere small holes are drilled to let the wires pass for connecting the capacitor to the loop and for the motor.
The central pole was glued into a 100 mm tube. This whole assembly is put into a 120mm tube that is fixed in a bucket of stone and concrete as a solid base. Up to now it withstood all the wind and storms.
Inside the tubeassembly I put the 2 x 500pF capacitor with motordrive.
The motor has a gearbox of 1:3000.
This is necessary to tune the loop adequately to the correct freqency.
Tuning is very precise and makes the difference between a QSO yes or no.
This picture shows the capacitor with motor drive before the wiring was done and before any weatherproof mounting took placeThe loop I described above served for over a year on my roof. I took it down after more than a year and the capacitor was as brandnew, no moist had come in despite of rain, snow, frost, wind etc. etc.
Tuning is done from the shack with two pushbuttons. First find maximum noise-level, then apply a low power signal and tune for minimal SWR.
Tuning is a critical process as you see from the SWR graph that I recorded when tuned to 3784 kc.:
