Tuesday, February 15, 2011

What is a magnetic loop antenna?

Child and Radio 1925              Franz Fiedler (1885-1956)    

I will try to explain in few words how the small magnetic loop antenna works.

1) The magnetic loop antenna is an RF transformer. The primary is the small inner loop that is 1/5 of the size of the main loop. The secondary is the main loop, and is the actual antenna. This outer conductor is closed by a variable tuning capacitor. The two loops are isolated, with no electrical contact between the inner loop and the outer loop.

2) The magnetic loop antenna is a high “Q” device, so the band width is very narrow. Even minor frequency changes will require the user to retune the antenna, but in exchange the antenna provides superior rejection of unwanted signal reception, the best receiving gain and the lowest SWR in transmit.

3) The magnetic loop antenna can be used for either vertical or horizontal polarization, and the small size makes it easy to alternate between the two.

4) In most radio activities we use the loop in the vertical position -- providing HORIZONTAL polarization -- supported by a small pole. The antenna can be turned freely in both directions to minimize or even completely eliminate man made noise coming from other directions. This position provides a "figure 8" radiation pattern similar to larger dipole antennas.

For condo windows, hotel balconies and other space-limited locations, we use the loop in the horizontal position, which (of course) gives vertical polarization and an omnidirectional radiating pattern.

5) The small magnetic loop works perfectly at low heights. A one meter diameter loop works perfectly one meter above the soil.

6) The small magnetic loop doesn’t need counterpoises, making it ideal for temporary operating locations, or for situations in which there is no room available for counterpoise wires.

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