"Richard Fry" wrote in message ...

"Bob Haberkost" wrote

Huh? AM stations essentially always have vertical radiators,
especially in Europe where there are so many high powered
stations. In general, AMs don't work very well otherwise.

H-Pol radiators have little to no ground wave.

H-pol would not be used on VHF and above (FM/TV broadcast etc) if that was
true. A linear, horizontal dipole antenna at MW or any other band generates
its maximum field strength at all angles perpendicular to its longitudinal
centerline -- which includes all angles from below the antenna out to the
radio horizon; i.e., a "ground" wave. [Free-space radiation with respect to
the dipole itself is the same whether its axis is horizontal or vertical.]

H-Pol is used on VHF, such as TV and FM, not because there's no ground wave (which
there still isn't) but because, in historical times, the antennas used to receive TV
and FM were H-Pol (most still are, if you look around). However, vhf broadcasters
(you know?) have been allowed to used V-Pol (to the limits of the H-Pol
authorisation) or elliptical or C-Pol as well since the early 70s, due to the number
of portable receivers coming into use at the time whose antennas are, frequently,
vertically-oriented. And while all dipole radiators have the characteristic
radiation pattern you describe, this isn't a "ground" wave since there's no bias for
radiation along the horizontal plane when the radiator is oriented horizontally -
it's only when this radiator is vertical that the omnidirectional radiation
perpendicular to the centreline is a "ground" wave, as significantly less power goes
skyward, in conformance with your description. Further, since medium wave radiation
has a significantly larger wavelength when compared to the size of the earth, the
diffractive effects make for over-the-horizon transmission, further enhancing the
phenomena called "ground wave propagation".

The reason h-pol is not used for MW is because path losses are much higher
for h-pol than v-pol in that part of the radio spectrum.

And, as noted above, because for the same amount of coverage, more power would be
necessary, since well over half of the radiated power goes uselessly skyward.

This is why a vertical radiator is sometimes called a "ground plane"
antenna, snip for those installations on the ground, this counterpoise
is usually buried.


The radial ground system used with MW broadcast antennas reduces antenna
system losses (I^2R), and keeps maximum radiation directed more toward the
the horizontal plane, rather than at some elevation angle above the
horizontal. The FCC defines the minimum efficiency of radiators licensed
for MW broadcast in terms of producing a field strength of so many mV/m at 1
km from the antenna, per kW of antenna input power. These efficiencies
cannot be met without using a good ground system.

Right....but how is this information inconsistent with my description, which is to
say that a vertical radiator needs a ground plane? You also fail to note that the
rules specify different minimum efficiencies for differring antenna lengths.

Those familiar with 11-meter Citizens Band know this antenna
in its 27MHz form, snip the reason why this particular configuration has
these radials at a 45-degree angle from the horizontal is because a ground
plane antenna has an intrinsic impedance of about 30 ohms....the farther
towards being vertical, the more it's like a dipole, with a dipole's
characteristic 72 ohm impedance. Thus, at 45 degrees or so, the
ground planes typically used for C-Band are about 50 ohms without
the need for a matching network.)


Possibly more important is the point that drooping the radials also tends to
lower the angle of maximum radiation, which can improve field strength for
receiving antenna sites at/near ground level.

Perhaps. But isn't it interesting that the angle selected is the same angle as what
produces a 50-ohm impedance? If the effect were more pronounced at a different
angle, one would think that that angle would be preferred, and then using a matching
network, bring it back to 50 ohms. Of course, there would be some loss in that
network, which might overwhelm the additional advantage gained by dragging down the
lobe.

The nice thing about the low radiating impedance of a vertical radiator is
that the high base current necessary for a given power means that the
magnetic vector is bigger than the electrostatic vector, and since
ferrite loops used in most AM radios respond to the magnetic
vector, the "connection" is more intimate.

?? The table below shows the efficiencies for MW vertical radiators with a
good ground system. The self-impedance of a 90 degree vertical is about 50
ohms, and for a 180 degree vertical it is over 100 ohms. So for the same
input power, base current is lower in a 180 degree radiator than in a 90
degree radiator. Yet the efficiency of the 180 degree radiator is higher --
the opposite of the above quote statement.

The ground wave field strength of a MW vertical radiator per kilowatt of
input power is related only to the current distribution in the radiator, not
its base impedance. Whatever the base impedance is, it can be matched to 50
ohm line at the tower base, using the right network. But the network doesn't
affect the relative field radiation pattern of that radiator.

But....I've seen (and fortunately NOT had to deal with) antenna systems with very
high base impedances (one, if my memory serves me correctly, was 800 ohms! Not much
current, but do the math...any appreciable power, like 3 or 4 kW, and there's a real
danger of getting tangled in with some pretty high voltages). While it's not a
scientific survey, I can tell you that those systems, watt-for-watt, perform worse
than lower impedance systems, and that's not even counting the difficulties in having
1kV base voltages!

And it's more than just current distribution that affects efficiencies. It's the
integral of the loop currents, which is why your chart shows better efficiencies for
those taller radiators. 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, as you have above, so 1kW into a 90 degree stick will be about
half as effective as a 180 degree stick (actually, shy of twice, due to the
I-squared-R losses you mention..

AM Radiator Efficiencies, 1kW input

Twr Hgt, Deg Effic
70 182mV/m
90 190
100 195
180 237
190 246
225 274

Note here that "efficiency" is the FCC definition for MW broadcast.
Efficiency falls for short radiators because the ohmic loss even in the best
ground system becomes a bigger percentage of the resistive term of the
radiators base impedance.

I appreciate the effort and time you've made trying to teach me something about
antenna theory, but be assured that there's not much more that need to know, and I
sincerely doubt that going into much more detail than this is warranted for this
particular thread.
--
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