Using commercially-available equipment is cheating anyway. What does it
take to modify PCS cellular phone firmware for digital communication on
902? Is it just a matter of firmware or isn't the RF section able to tune
that far out of band?

At the time, cell phone equipment was not readily available.

To oversimplify a bit: Low frequencies (like AM broadcast) pass through
the body without being absorbed. Microwave frequencies bounce off the body
without being absorbed.

ROTFFL!!!

Why not PROVE your ridiculous theory by putting your head into a microwave oven!

........yeah. I didn't think so.

"Bob Haberkost" wrote in message
...

"David Eduardo" wrote in message
...

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.

This concludes your antenna theory class for the day. ;-)

Very interesting discussion. Thanks.

How about introducing the masses to franklin antennas, such as the system
still in use at KFBK in Sacramento. I believe KDKA once had one, too. I saw
one in South America that had no real ground system, just the two elements
of the Frnklin and a ground mat at the base.

"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!-

Mister Fact wrote:

Certainly my reply was not related to any physical
harm which might result from radio waves themselves-
but since the message board has to do with radio
broadcasting in general- I thought it would be a good
opportunity to inject ANOTHER TYPE OF MEDICAL HARM
which I see inherent in AM broadcasting today.

Isn't that what Rush Limbaugh was doing? Injecting another
type of harmful medicine?
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Within these hallowed halls, Truth of added the
following to the collective conscience:
To oversimplify a bit: Low frequencies (like AM broadcast) pass
through the body without being absorbed. Microwave frequencies
bounce off the body without being absorbed.

ROTFFL!!!

Why not PROVE your ridiculous theory by putting your head into a
microwave oven!

.......yeah. I didn't think so.

I was actually wondering what this post of yours had to do with
broadcasting. Even if
it was off topic but informative or entertaining, it would have been better
than just
being a post about being rude to someone.

"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.

"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

Truth wrote:
Harris wrote:
To oversimplify a bit: Low frequencies (like AM broadcast) pass through
the body without being absorbed. Microwave frequencies bounce off the body
without being absorbed.

ROTFFL!!!

Why not PROVE your ridiculous theory by putting your head into a microwave oven!

I said this was a simplification. The point is that maximum absorption
occurs in the 30 to 300 MHz range. Microwave frequencies are used for
cooking because they are more practical to produce, not because they
are more effective at heating.

See the ANSI (American National Standards Institute) exposure limits
curve below:

http://www-training.llnl.gov/wbt/hc/.../slide34lg.gif

Greatest rf absorption (minimum allowable exposure) is in the 30 to
300 MHz range.

Art H.

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.