Antennas

My main antenna is a horizontally mounted triangular loop at about 9 to 10m high. It’s fed by an automatic ATU against a comprehensive ground system and is resonant in our 80m band.

I modelled it using EZNEC, and results broadly bear out these predictions.

I found that leaving the end of the wire opposite the feed point unconnected gave the best results for my site, so the antenna could be regarded as a rather bent quarter wave end fed. It seems to behave like a loop however.

Predicted high angle radiation on 10m was disaapointing and also found in practice on air. Adding a co-axial cable trap about 5/8 wavelength from the feed point has dramatically improved DX performance on this band, with apparently little or no effects on other bands.  The trap effectively isolates the rest of the loop so the antenna behaves as a vertical on this band.

I chose 5/8 deliberately as this length theoretically gives some slight gain on 10m, but more importantly gives a medium impedance for the ATU to match. This reduces loss within the ATU, improving overall efficiency.

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