Tom,
Just a personal observation concerning short monopoles. When I model a
50 foot vertical with one 200 foot top hat wire using EZNEC 3.0 on 1830 kc,
then add two more 200 foot top hat wires, the near electric field in V/m RMS
increases, the total electric field at 1 mile increases and the feedpoint
impedance increases a little. When I conduct the same modeling exercise on
180 kc I see the same results as at 1830 kc but cannot verify it in the
field.
With no top hat wires attached a 50 foot vertical antenna obviously has
capacitice reactance and therefore inductive top loading wires are needed or
a linear load or at a last resort a lossy coil. With the 50 foot vertical
and three 200 foot top hat wires the antenna feedpoint becomes inductive and
feedpoint impedance high enough for the necessity of a parallel matching
network. When you feed a 90 degree monopole at it's ground end the feedpoint
impedance and radiation resistance are basically synonymous, lengthen the
monopole to 135 or 180 deg and of course feedpoint impedance and radiation
resistance become different but the added "electrical" length does seem to
increase radiation resistance.
Last year a friend of mine using professional broadcast measuring
equipment took field measurements and found that the near electric field in
V/m RMS increased and that the total field at 1 mile also increased with
three top loading wires versus none or one, verifying the modeling data in
my opinion.
The measurements were also made with only one 1/4 wave radial and then
sixty 1/4 wave radials and no difference in the measurements were noted.
This implied to me that the radiation resistance of the 50 foot vertical
with three 200 foot top hat wires was high. You can make your eyes heavy
grappling over the mathematical definition of radiation resistance but it's
easier just to look at radiation resistance as a measure of antenna
efficiency.
73,
Thomas Giella,
KN4LF/NNN0HUS
Plant City, FL, USA
KN4LF 160 Meter Amateur Radio Resources And More:
http://www.kn4lf.com
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