[TowerTalk] Change in Frequency with Height above Ground

[Steve Hunt]

If you derive the expression for SWR in terms of antenna R and X you can 
differentiate with respect to frequency and determine the conditions for 
minimum SWR analytically.

You'll find that when dX/df >> dR/df - the case for most HF wire 
half-wave dipoles at any height above any type of ground - minimum SWR 
occurs very close to X=0 (resonance)

It becomes less true when the wire diameter is an appreciable fraction 
of a wavelength, and even less true for something more complex such as a 
beam. In the case of my hexbeam, maximum gain, maximum F/B and minimum 
SWR all occur at different frequencies than one another, and other than 
resonance. Shooting for resonance at a particular frequency would be the 
least useful tuning goal.

Steve G3TXQ