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Endfire Loop Array Patterns on Medium Wave

loop endfire array patterns

My small changes to phasing creates remarkably different endfire loop array patterns. I can demonstrate these with empirical results and analytical modeling.

Earlier this month, I spent the evening listening to regional AM broadcast DX on the medium waves. I found myself on 730 kHz listening to CKDM in Dauphin, Manitoba, nearly 1,000 kilometers to the east. Pretty good signal for a low power 5 kW night pattern. Then, as I adjusted phasing, the Dauphin station disappeared and was replaced by CHMJ in Vancouver, about 700 kilometers west from over the Rocky Mountains.

This experience provided a perfect demonstration of my endfire loop array patterns with a diversity receiver. My endfire configuration consists of two, 1 meter diameter active loops spaced 30 meters apart, and oriented E-W. (Well, actually my baseline is more like 290º – 110° but close enough.) Dauphin has a bearing of 83° and Vancouver is 256° from my location.

By advancing the phase δ on the reference loop by +25°, I create a pattern to the east. Similarly, by retarding the phase δ by -25º, I get a pattern to the west. My endfire loop array patterns are shown above. Pretty neat, eh?

Keep in mind that we have not made any physical changes to the antenna. The loops and feedlines remain constant. I simply advanced or retarded the phase on one of the channels in the receiver by a few degrees. Mathematics took over to force creative and destructive sine wave additions resulting in patterns.

Now, obviously even a small beverage antenna would produce similar patterns and stronger signals, but would require much more real estate. I find it amazing how much you can accomplish with a few small loops and a diversity receiver like the Afedri AFE822x or SDRplay RSPduo.

Endfire Loop Array Patterns Created in Software

I created the graphics shown above using MMANA-GAL antenna modeling software. It’s free in the basic version and easy to use. I just drew two 1 meter diameter loops spaced 30 meters apart along the Y axis. You can see these as small blue squares in the picture. I fed each loop with a current source capable of simulating different phases.

Having built the basic models (one for endfire and another for broadside) I can estimate the endfire loop array patterns at any medium or short wave frequency.

Other tests on stations in Seattle (KIRO 710 at 241°), Kamloops (CHNL 610 at 268°), Spokane (KQNT 590 at 213°) and Saskatoon (CJWW 600 72°) provided similar results. My experiment showed each of these stations disappearing off the back of my beam.

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