Bruce Bowman wrote:
This antenna, and I'm confident any 1/4 wave vertical, is very
sensitive to grounding. I've had to mount mine on the roof, and the
vertical doesn't like being mounted up there (13' up in our case with
the radials are layed out on our flat roof). I was warned by Fluid
Motion (the mfr) this elevation would be an issue unless I could get
it > 0.2 wave lengths off ground; that means more than 8m up to use
the 40m band. At anything over 8m elevation, sloping the radials will
improve the performance. It'a all in the report available on their
site.
As you get closer to the ground, the system behaves more like a
ground-mounted vertical, and so you need to treat the radial system as a
capacity-coupled counterpoise, as opposed to the resonant radials of a
ground plane antenna. This means that you need more radials.
Otherwise, much of the ground return current will flow in the lossy real
ground. If I were putting a BigIR or similar on a low roof, I would
start with about 16 radials.
Yes, the KAT2 matches the complex
component of impedance, but it doesn't do anything about matching the
real component of impedance... that takes a matching transformer. Simply
getting a low SWR doesn't necessarily mean you're getting power out to
the antenna.
I would put it differently. The matching is fine -- the problem is that
the RF current to the antenna divides between the radiation resistance
of the antenna and the effective ground resistance. If the radiation
resistance of the antenna is very low, then most of the power ends up
heating the ground.
--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco
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