"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.]

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.

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.

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.

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.

AM Radiator Efficiencies, 1kW input
(for equal distances)

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.

RF

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