Saturday, November 26, 2016

Three Eight Wave Ground Plane Antenna for 10 Metres

Over the years, I have collected many old magazine articles and it seems to be a waste to have them buried away in a folder. This one is for a 3/8 wave ground plane antenna for 28 MHz and hopefully it might encourage someone to experiment and build one.

The principle is pretty easy. A normal dipole in free space normally has a 72 Ohm feed point impedance. A quarter wave ground plane is half this...about 36 Ohms.

By extending the antenna from a quarter wave to a three-eight wave, the maximum current point moves from the base to further up the radiating element. This has the effect of lowering the angle of radiation which is a plus.

The longer three-eight wave antenna now has a reactive feed point (inductive) as it's too long. This is tuned out with a series capacitor and it should be able to get close to 50 Ohms.

This antenna would suit someone who can only put up a quarter wave ground plane for 10 metres and wants better performance.

The article from the March 1983 edition of Short Wave Magazine is shown below...

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