Sunday, February 13, 2011

Magnetic Loop Antenna


One solution for an indoor shortwave antenna is a magnetic loop antenna. I started this project because it is not very difficult to build this antenna type. Like most antennas it is cheap. The most critical point is to build - or find - a useful loop of copper tube.
I was lucky and found a 18 mm copper tube bound to a ring in a property market. It was circa 200 cm long and formed more or less a circle. I cutted the tube into 2 parts, polished and clear coated the surface. From plywood I made a stand. Two ends of the tubes were mounted with screws on a piece of wood, the both other ends hold with a piece of plastic tube together. I used a Hammarlund variable capacitor of 15 to 325 pF to close the circle at one side. For precise tuning I use an 6:1 slow motion drive, which allows better fine tuning into the targeted frequency range.
To close the other side of the copper ring and to couple out of the antenna ring I took a ferrite toroid T 80-2 and formed 2 windings of 0.5 mm copper wire. One winding with 20 turns is closing the circle of the copper tubes, the other winding of 5 turns connects to a TNC-plug. So we have a Balun with a 4:1 ratio.
This loop antenna works down to circa 6.000 kc and up to 23.000 kc.

Schematic of the loop antenna:

The balun at the top of the antenna:

Hammarlund variable air-cap with slow-motion drive

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