[TowerTalk] dipole length adjustment

[Jim Lux]

At 06:15 AM 1/19/2007, Bill Turner wrote:
>ORIGINAL MESSAGE:
>
>On Thu, 18 Jan 2007 21:24:19 -0800, "Dave Tipton" <dave at 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.


One caveat, which seems obvious once you know it, is that this works 
only for changes of a few percent.  Don't go trimming your 80 meter 
dipole for 40 meters by doing this...

Jim, W6RMK