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Magnetic Loop Feed Point – Many Choices

magnetic loop feed point

A magnetic loop feed point is the method you use to apply radio power to the antenna.

There are several methods for connecting a magnetic loop to your radio. All involve a coaxial cable, followed by some sort of arrangement to transfer radio signals to and from the main loop. Frank N4SPP has a great web site which describes the inductive, capacitive and auto-transformer methods.

To start out, my loop will use inductive coupling. This is done by connecting the coax to a second loop (or inductance) and placing it in the plane of the main loop. This second loop is generally made to be 1/5 the diameter of the main loop, and placed at the side opposite the capacitor.

In the photo above, you can see my magnetic loop feed point as the small loop at the bottom. It is resting on a piece of Styrofoam for now. This is to keep the small loop flat and in the same plane as the main loop. It is made out of a short length of RG-58 coax, which is in turn connected to the antennas coaxial feed.

Hams generally use a measurement called SWR (Standing Wave Ratio) to determine how well a feed is matched with the antenna. The lower the SWR (ideally 1:1) the better the match. The higher the SWR (say 5:1 or 10:1) the poorer the match.

With this early arrangement, I am able to get an SWR of less than 2:1 at resonance, which is good. However, I have noticed that the placement of the smaller feed loop is critical. A few inches either way and the match becomes poor. This tells me that I have to design some way of keeping the feed loop in proper shape and make some adjustments to its placement.

Magnetic Loop Feed Point with a Faraday Loop

The inductive feed system I have chosen is called a Faraday loop. It is a shielded rather than bare wire loop. The literature says that a shielded loop should reduce local electrostatic noise.

An inductive magnetic loop feed point is basically a transformer with two windings. The smaller winding is the feed loop, and the larger winding is the main loop. In any event, this approach is a simple starting point.

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