"Richard Fry" wrote in message ...
"Bob Haberkost" wrote these clips:

The larger fields generated by the longer radiators makes
for more power transferred (which also explains why a taller
radiator has a higher intrinsic impedance,

Have to disagree with that. The reason that a 180 degree MW vertical
generates a stronger ground wave than a 90 degree vertical (other conditions
equal) is due SOLELY to the shape of their respective elevation patterns.
Their radiation efficiency or "power transferred" has nothing to do with
their base impedances.

And yet it does. How, if I may ask, do you think that the radiation pattern of a 180
degree vertical element is lower than a 90 degree radiator? As you mention, it's
current distribution, but it's not as simple as you've characterised. The field is
generated by the summation of the currents over the length of that antenna that
combine to provide the "pull-down" effect you mention, and in the process, since the
infinitesimal slices of the radiator, each contributing its own part to the overall
field, also interact with each other in much the same way as separate elements in a
directional array interact, the phasing and amplitude over the length of the radiator
serve to enhance the direction towards the horizon and reduce radiation towards the
sky. Now, the reason why the base impedances are different for these two examples is
the same as why the effective impedance for one element in a directional array
changes when a second element is introduced into the nearspace around that first
element, because the interactions between the infinitesimal slices serve to increase
the "coupling" of the radiator to space. It's all calculus, with a heaping serving
of trigonometry thrown in for good measure.

If properly matched to their transmission lines, both of them radiate the
same total power. But the elevation pattern of the 180 degree radiator has
more intrinsic gain in the horizontal plane -- which produces the stronger
ground wave of the two.

I long ago recognised that, in the physical world, you don't get something for
nothing (a concept which, it's pretty clear, the current administration in Washington
doesn't get...or maybe they do?). Nothing in what I've discussed is ignorant of
this, although admittedly it's not explicitly stated. We broadcast engineers tend to
look at radiation patterns as they relate to the potential audience, knowing that the
areas we've pulled power from won't miss it, and then pat ourselves on the backs for
having designed an antenna system with "gain."

so 1kW into a 90 degree stick will be about half as effective
as a 180 degree stick.)

Not following that conclusion. Using the FCC's numbers, a 180 degree MW
radiator with 1 kW input produces a groundwave field of 237 mV/m at one
mile, while a 90 degree radiator produces 190 mV/m. So for same input power
and other conditions, the 90 degree radiator produces 80% of the field
strength of the 180 degree radiator.

Put another way, the input power to the 90 degree radiator would have to be
increased about 1.56X in order to produce the same ground wave field at one
mile as the 180 degree radiator.

Well, there you have it. 1.56 times, while not exactly 2, is closer to 2 than it is
to one. Consider that, since radiated field is over an area for our purposes, a
radiator half as effective as the reference would have 70.7% as much field, or the
reciprocal of the square root of two. It was a gross approximation, Richard. From
what I've seen of broadcast engineers, many have only a practical knowledge of the
underlying theoretical concepts...whether it's the understanding of modulation theory
(how many people do you know who think that amplitude modulation actually manipulates
the amplitude of the carrier? Or that FM actually changes the centre frequency?) or
antenna design, or solid state theory...never mind quantum theory. I don't believe
that getting down to this level would serve any practical purpose in this newsgroup,
however, especially since I'm not prepared to start introducing mathematical
equations into a text-based format.
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