ORIGINAL MESSAGE:
On Thu, 18 Jan 2007 21:24:19 -0800, "Dave Tipton" <dave@lodave.org>
wrote:
>(468/Original Frequency) - (468/New Frequency) = Length to shorten in Feet
------------ REPLY FOLLOWS ------------
While the above method will probably work well enough, there is a
slightly more accurate method, since it does not depend on the
accuracy of the 468 factor to begin with. The 468 factor can be
affected by the closeness to earth and nearby objects and/or antennas.
The following method takes those things into account automatically:
Divede the actual resonant freq by the desired resonant freq to get a
factor, typically between .98 and 1.02 or so. Multiply the original
length by that factor to get the new length. Add or subtract wire as
needed, more wire if the desired freq is lower, less wire if the
desired freq is higher.
For example, if the actual resonant freq is 7150 and the desired freq
is 7010, the factor is 7150/7010=1.02. If the original length was 66
feet, the desired length is 66*1.02=67.32 feet.
This method works best for non-inductively loaded antennas. If there
is a loading coil present, you will probably have to cut and try since
the effect of the loading coil magnifies the effect of changing the
overall length. Or you could use an antenna modeling program such as
EZNEC. EZNEC is much more time consuming than cut and try, but it
gives you a better feel for what is happening and that can be a
valuable thing too.
Have fun!
Bill, W6WRT
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