[TowerTalk] Will Resonance Go Up or Down?

[Jim Lux]

At 02:56 PM 3/13/2006, doc wrote:
>Is it fair to generalize that proximate objects
>will almost always detune an antenna to a higher
>frequency?

No...

imagine putting a giant sheet of metal near a dipole.  It will tend to 
reduce the resonant frequency because it increases the apparent capacitance.

Or, for the example originally given (a dipole close to the ground), the 
ground lowers the resonant frequency. So, when you raise it, the resonant 
frequency goes up.

>Or are there standard rules of thumb which help
>to ID when that is expected or not?

Things that are close to a half wavelength, or multiple thereof, are more 
likely to cause problems.

Things that are similarly aligned (as in parallel wires) are more likely to 
cause problems.

Things that are either conductors or have high dielectric constant 
(permittivity) are more likely to cause problems. (Water is a good example 
of the latter, notoriously, water manifested as the sap in trees)

However hanging a horizontal antenna from a tree, while bad for the latter, 
is good from the former (the tree is at right angles to the antenna)


Assessing this sort of thing is where computer modeling is wonderful.  You 
don't care about the exact results, but you just want to know if something 
has an effect.  You put a rough model of your antenna in (who cares if it 
resonates at exactly the right frequency) and put your test article in the 
model as some wires, and see if there's any current in the wires.  If 
there's current, there's a problem.