> Just remember, if you run radials at a 45 degree
> angle from the base of the vertical, it will effectively
> shorten the electrical length of the vertical by the same
> amount of elevation. That is, if the radials are 20' above
> the feedpoint, the vertical will need to be 20' taller
> to be resonant at the design frequency.
I'm not sure exactly what feed system is being described, but with
the original system he was talking about and inverted groundplane.
Say we build a groundplane antenna, either inverted or not, and have
four radials at right angles to the vertical section. We measure the
resonant frequency as 1.9MHz.
Now we slope those radials in the direction of the radiator, so they
form a 45-degree angle without changing any lengths.
The total resonant frequency change is only about 3% or so. The
radiation resistance is reduced substantially, however.
The primary change is in radiation resistance rather than resonant
frequency.
I'd be cautious inverting the antenna, because a very strong electric
field will appear near the earth. That would still require a ground
screen or small radial system of sorts to prevent the strong electric
field and resulting current flow from creating loss.
Once again we should remember, the last thing we ever want to do is
concentrate a strong magnetic, electric, or radiation field in a
small cross-section of a lossy media (like earth with no screen or
radials). Depending on soil conditions and capacitance between the
radiator and earth, there could be substantial loss. Especially since
the voltage would be very high at the open antenna end when loop
radiation resistance is low at the current maxima.
73, Tom W8JI
W8JI@contesting.com
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