On Mon, 24 Dec 2007 11:42:03 -0600, AD5VJ Bob wrote:
>Will an 1/8 wave vertical work for top band or do you have to start
>with 1/4 wave? Perhaps top loading it would work??
The taller the antenna, the better it will work (up to at least 1/4
wave). Top loading can make a short antenna more closely approach
the performance of a quarter wave.
Think of it this way, Bob. It's a series circuit that includes the
Radiation Resistance (Rr) and R loss, where R loss = Rg + Rw Rg =
ground system Rw is the wire, including skin effect. Rr represents
the power that is radiated, and becomes larger as the antenna gets
longer. Rr is on the order of 30 ohms for a quarter wave. There are
graphs of Rr vs antenna height in the ARRL Antenna Book (and
probably in the Handbook).
A loading coil adds loss resistance, while top loading adds very
little. Both make the antenna "look" longer to the transmitter (that
is, closer to 30 ohms). R loss adds to that, so an antenna with a
lousy ground system will be closer to 50 ohms, and thus have less
VSWR, but the ground system will burn more of the transmit power.
My 160 vertical is currently 70 ft tall with enough top loading to
make it look like a quarter wave to the transmitter. I'm working on
making it taller, which will require less top loading. Obviously,
you can make the height less important by reducing R loss - use
bigger wire, parallel conductors, and a beefier radial system.
Hope this helps.
73,
Jim K9YC
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