"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.
--
-----------------------------------------------------------------------------
If there's nothing that offends you in your community, then you know you're not
living in a free society.
Kim Campbell - ex-Prime Minister of Canada - 2004
-----------------------------------------------------------------------------
For direct replies, take out the contents between the hyphens. -Really!-

"Bob Haberkost" wrote in message
...

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!

At medium wave (AM Broadcast) many Class A stations (formerly "clear
channel") use antennas of about 190 or 200 degrees tall. The FCC requires a
minimum antenna effectiveness for that class which is higher than for the
other classes of stations. The base impedance of these sticks near a
half-wave tall is going to be pretty high - and all but one US Class A
station run 50 kW. One of the factors in deciding AM tower height is to
place the first null in the vertical pattern such that the nighttime skywave
interferes as little as possible with the ground wave toward the edges of
the groundwave coverage.

Of course Class A AM stations are a Big Deal and generally have very good
ground systems.

At medium wave (AM Broadcast) many Class A stations (formerly "clear channel")
use antennas of about 190 or 200 degrees tall.


A Clear Channel is a Clear Channel is a Clear Channel. Any of 540, 640, 650,
660, etcetera.

The average height over all ND-U Class As is 195 degrees.

The real goal here is to get 400 mVm/kW at 1 km, or better, without also having
high-angle radiation which could cancel the groundwave in the fringe area ...
that area where the primary service area ends and the secondary service area
begins.

Taller than about 200 degrees requires sectionalization to do this.

225 degrees is a real killer for a Class A, but is perfectly fine for a Class B
or C, which doesn't have a large primary service area, anyway.

The best performing Standard Broadcast radiator is 360 degrees tall, and
consists of a 180 degree bottom section, and a 180 degree top section.



The FCC requires a minimum antenna effectiveness for that class which is higher
than for the other classes of stations.


362.10 mV/m/kW at 1 km for Class A.

281.63 mV/m/kW at 1 km for Class B and D.

241.40 mV/m/kW at 1 km for Class C.

Of all lower 48 Class As, two don't have conforming radiators, and both of
these are in San Francisco.

Of all Alaska Class As, only one has a conforming radiator.



The base impedance of these sticks near a half-wave tall is going to be pretty
high - and all but one US Class A
station run 50 kW.


The only such Class A in the lower 48 is 1560 in Bakersfield, CA.

There are numerous such Class As in Alaska.

"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.

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.

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.

RF

"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.
--
-----------------------------------------------------------------------------
If there's nothing that offends you in your community, then you know you're not
living in a free society.
Kim Campbell - ex-Prime Minister of Canada - 2004
-----------------------------------------------------------------------------
For direct replies, take out the contents between the hyphens. -Really!-

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 mi² and 2,550 mi² 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

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