Build a Motor Driven Magnetic Loop Antenna 1.5 Meters Square for 3.5-10MHZ

By Neil Lowson, GM4XRF

Parts List:

• 4 1.5 mtrs 22mm copper pipe
• 4 22mm copper 90 degree bends
• 1 500pf 6kv capacitor
• 1 12 volt 4 RPM geared motor
• 2 flexible couplings
• 1 insulated if possible
• 1 6-1 or 10-1 reduction gear
• 2 small micro switches
• 1 5 pin SIN SKT@plug
• 1 400mm 4”plastic pipe
• 1 4”plug insert for bottom of pipe they have a rubber seal on (B)
• 1 4”seal for top this a screwed on cover for access to the capacitor (A)
• 1 60mm *350mm *4mm paxolin for motor/capacitor
• 1 75*75 4mm paxolin plate for fixing mast to loop at bottom with saddles
• 2 12volt leds 1green 1 red
• 1 25mm PVC coupling
• 1 25mm broom handle

Getting Started: 
Assembly first starts with the capacitor and motor, this is the important part of the magnetic loop. Get the strip of paxolin and mount the capacitor at one end (top). Fit the insulated coupling to the capacitor shaft and then fit the reduction gear. The brackets will need to be made from 4mm aluminium. Then make up a bracket for the 2 micro switches.

 

They have to be set as to use the flexible coupling with a long 4mm screw in one of the holes for switching at min/max capacitance, see photo and wiring diagram. Next the 100mm /300mm plastic drain pipe (length to suit cap/motor) fits the bottom section of top plug with pvc. Glue and drill 2* 25 mm + holes 30mm down from top of pipe with the holes opposite one another for the 25mm copper pipe to be inserted in about 25mm. Then bolt cap/motor assembly into pipe. Connect capacitor to copper pipe with coax braiding and 6nmm brass bolts. Get the bottom plug fit the 25 mm coupling and the din socket connect to motor and fit with self-tapping screws and sealant on main pipe. Assembly of capacitor is now complete. Fit broom handle to coupling, I use this as the mast, also to attach the coupling loop with cable ties.

Assembly of the Loop:
4*1.5 mtrs 22mm copper pipe. Take 1 length cut 30mm of the length the pipet in half — this is the top portion of  the loop. Clean the pipe and bends with wire wool and add flux. Then fit bends to pipe. Do this on flat surface and fix bends with self-tapping screws till you solder. You now have a square loop. Paint if required.

Coupling the Loop:
Coupling loop should be about a 1/5th size of main loop made from RG58U COAX but I always make it bigger. Then trim it by 15mm at each end till you get a 1-1 match (see photo). Then solder And seal with rubber tape.

Power supply:
I use small power supply unit you can buy for calculators/portable radios 3/4.5/6/9/12/volt at 1 amp — they are ideal for the small 12V motor. I built mine into a small box with the switches and the LEDs work well. There are more sophisticated control circuits that can be used but this was the easiest.

 

This article was available at http://www.standpipe.com/w2bri/article2.htm