[TowerTalk] Change in Frequency with Height above Ground

[Bob K6UJ]

That's interesting Jim.  I remember looking at a graph of the impedance 
change of a
dipole at various heights.  It doesn't like being very close to the 
ground, hihi.

Don,
FWIW,
With my 2 el 40 meter beam I found that if I tuned it for resonance 
suspended at 15 feet above the
ground then it shifted (went up) about 20khz when raised to 90 feet.

Bob
K6UJ




On 5/26/16 8:37 PM, Jim Brown wrote:
> On Thu,5/26/2016 8:24 PM, Don W7WLL wrote:
>> Lowering the height of an antenna above ground, such as a beam, will 
>> cause the resonant freq to decrease, increase?
>
> Decrease. But not always -- I have a 10M beam at about 15 ft that goes 
> up! The variable here is mutual coupling between elements and between 
> elements and ground.
>
> Another error in using SWR as the indicator of resonance is that 
> feedpoint Z changes with height, and in a very complicated way (and 
> because of mutual coupling to ground). In general, low dipoles (as a 
> fraction of a wavelength) are lower Z, high dipoles are higher. AND -- 
> feedpoint Z is also affected by the quality of the ground.
>
> SO -- we may THINK the resonance is affected by the height as 
> indicated by the low value for SWR, but it's actually the feedpoint Z 
> changing!
>
> 73, Jim K9YC
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