Thursday, February 17, 2011

The bi-square array for 18 MHz



The diamond-shaped bi-square beam is much larger than the delta loop, but provides about 3-dB gain. This is a great antenna to try if you have the space. It's shown in fig. The loop is a half wavelength on a side and open at the top. The feedpoint impedance at the bottom of the loop is about 2900 ohms; I use a twowire 600-ohm quarter-wave stub to provide a more reasonable impedance value of about 122 ohms. Match it to a 50-ohm coax line by adding a quarter-wave transformer made of 75- ohm coax. Wind the 75-ohm line into a coil about 6 inches in diameter to reduce RF currents flowing on the out-side of the coax. Resonate the loop and stub to 18.1 MHz with a dip meter. Temporarily close the stub at the bottom using a movable short with a I-turn loop in the middle.

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