Sequence #1...
B. Haberkost:
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,
R. Fry:
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.
B. Haberkost:
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? etc
Not because of any change in base impedance. The electrical height of the
tower determines BOTH the elevation pattern it produces, AND the base
impedance of that tower. Base impedance is an effect, not a cause.
If base impedance determined efficiency and "power transferred," then a 90
degree tower should have very nearly the same elevation pattern as a 245
degree tower, because the base impedance for those two heights are very
similar (90 degree is about 63+j105 ohms; 245 degree is about 64 +j50
ohms). Yet the elevation patterns for these two verticals are greatly
different. The elevation pattern of a 245 degree vertical has two distinct
major lobes; one centered on the horizontal plane, and one at about 45
degrees. The 90 degree tower produces an elevation pattern with a single
lobe centered on the horizontal plane.
These verticals can be computer-modeled to show their shapes and intrinisic
gains in dBi. I'll e-mail you a graphic I generated in NEC to compare them
for you.
Sequence #2...
R. Fry:
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.
B. Haberkost:
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.
To help you compare geographic areas covered by a 90 degree vs a 180 degree
radiator, here are the numbers using the FCC's MW coverage program. For 1kW
input power to the tower base, a 1,000 kHz carrier, and conductivity of
8mS/m, the radial distance to the 2mV/m contour is 25.6 miles from 90 degree
tower, and 28.5 miles from the 180 degree tower. The areas covered are
2,058 miČ and 2,550 miČ respectively. So the 90 degree vertical covers
about 80% of the area served by the 180 degree vertical. Not very close to
a 2:1 difference at all.
Sequence #3:
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?)
The instantaneous frequency DOES change with frequency modulation, although
the average center frequency stays close to the unmodulated value. In fact,
a very common FM exciter design uses the incoming program audio to change
the resonant frequency of the frequency-determining components of an RF
oscillator, whose resting frequency is the stations licensed carrier
frequency.
RF
Visit
http://rfry.org for FM broadcast RF system papers.