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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