> Thanks for the clarification. I forgot that when the
energy becomes a
> detached EM wave, that that magnetic and electric fields
become "mutually
> causal". So given that, then, is a few more feet of
effective height on 40m
> as important as increasing radiation efficiency? In other
words, if one
> could make a heavily top-loaded antenna at the expense of
some height, would
> it be worth the loss of a few feet?
Dudley,
I'm not sure what all that means, but radiation is a product
of current over linear distance. In other words ampere-feet.
This is because radiation comes from charge acceleration. It
has nothing to do with the electric field except as that
field relates to current flow. Of course opposing radiation
fields from other conductors can cancel radiation.
http://www.w8ji.com/radiation_resistance.htm
This is why simple basic antennas have radiation resistance
proportional to the physical area.
With a very thin short straight conductor, radiation
resistance is 1/4 that of a conductor the same length with
uniform current. Voltage is no part of it.
If I double the length of a short conductor radiation
resistance also quadruples.
This means going from no top loading to uniform current is
effectively like doubling antenna height.
The antenna can go from nearly triangular current to nearly
uniform current WITHOUT moving the loading coil if the
antenna has a capacitance hat installed when that hat
dominates the distributed capacitance of the system. The
larger the hat, the less critical the loading coil location
becomes. This is because the capacitance of the hat sets the
current taper and current distribution, not the coil
position. The inductor primarily cancels reactance, bringing
voltage and current into phase. If the distributed
capacitance of the loading coil is small compared to the
distributed capacitance above the coil , the current through
the loading coil become essentially uniform.
Voltage has nothing to do with radiation, except as it
causes current to flow over the conductor doing the
radiating.
73 Tom
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