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Maximundo

Antenna length formula

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If I’m not mistaken RF radiation travels at the speed of light 300,000 kilometers per sec divided by 2400,000,000 (frequency used) = 0.000125 x 1000 = 0.125 meters x 100 = 12.5 centimeter (full wave) divided by 4 = 3.125 cm x 10 = 31.25mm for a ¼ wave antenna.

Please correct me if my formula is incorrect.

RE to:

http://www.yb2normal.com/antenna3.html

“The ground legs are cut to exactly 3cm in length, measured from the bend. The signal wire is also cut to 3cm”

Has anyone tried trimming this ground plane antenna and ended up with a decline in efficiency? I’m not trying to nit-pick but with antennas this short, it could make a difference when trying to get the lowest SWR reading.

How about doubling the length to a ½ wave antenna?

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(1) RF does not travel at the speed of light in a metallic medium. It is faster, so antenna element lengths are made shorter to allow for proper resonance. For a half-wave groundplane antenna, Yb2normal's recommended 30mm is a fine length to start with.

The EXACT length for best operation may require some minor tweaking, but without a Network Analyzer it is tough to eek out the last drop of gain. And in the end, the effort would probably not be noticed.

(2) Doubling the length will not improve the traditional groundplane antenna. Because it is designed for use above physical (earthly) ground, the proper impedance and good resonance is obtained with the 1/4 wave lengths (hence, 1/2 wave operation). I believe that doubling the size for the proposed full wave groundplane operation would substantially increase feedpoint impedance (which is not a good thing). It's hard to tell, but my antenna book's impedance plot examples show that it would be close to 8K ohms!

If you want to use the 1/2 wave length elements then a traditional monopole design can be used. However, you will earn a little more gain if you build the Groundplane design as-is.

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I seem to remember reading someware that if you increase the size to 3/4wave the impedence returns to 50R .

Terry

UK

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I know in the 10-meter band, commonly available ground plane antennas were ¼ wave and 5/8 wave. So maybe specific fractions of the wave can in fact have considerable influence on impedance. From what I gather RF is not fully understood, a bit of a mystery.

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