RadioBanter - Is AM Radio Harmful?

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"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.
--
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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
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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

Visit http://rfry.org for FM broadcast RF system papers.

"Bob Haberkost" wrote (clip):
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.

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

The above calls for a bit of discussion, IMO

The base current of a MW vertical radiator depends on the resistive term of
the base impedance of that radiator, according to the equation I =
sqrt(P/R), where P is the applied power and R is the base resistance.

However current is not uniform over the height of the radiator. It must
satisfy the physical reality that a current node (minimum) always must exist
at the top of the tower. Other current nodes occur at 1/2 -wave intervals
below the top, if the tower is tall enough.

Between the nodes, current rises to a loop, or maximum, at intervals of 1/4
wave. The maximum current present at the loop(s) is a function of the
amount of power applied to the base of the radiator -- not to the base
impedance of the radiator.

So for a given input power, the same absolute value of current will be
present starting 1/4 wave below the tower top, and repeating every 1/2 wave
below that -- regardless of the base impedance of the radiator.

Far-field radiated EM waves from MW vertical radiators of any height are
identical in that they all have equal electric and magnetic vectors at right
angles to each other. A ferrite receiving antenna performs well on MW
frequencies for reasons unrelated to the ratio of the E & H fields in which
it is immersed.

RF

Visit http://rfry.org for FM broadcast RF system papers.

900 MHz are NOT microwaves

Interesting. Especially since the older microwave ovens operated on 800 and
900 Mhz.

Why are people who are uneducated in certain subjects always the first to jump
in on a conversation and tell everyone else how they think things really are?

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 once worked with a guy who claimed he could hear microwaves. He said
that he could tell when the radar system was in operation and when it
wasn't by the sound. We did a simple blind test up on the roof, and it
became pretty clear that he could tell.

Turns out that what he was hearing was conducted noise from his skull
expanding due to heating effects. Admittedly this was with well over a
megawatt ERP. But it was definitely being absorbed.

What a very bright individual. It frightens me that someone like that was allowed
to get close to that equipment in the first place. They put all the rest of us at
risk.

SOME microwave frequencies get absorbed very well by water, some do not.
The body being mostly water,

Especially the EYES in our heads, so close to the cell phone antennas.

Truth wrote:

900 MHz are NOT microwaves

Interesting. Especially since the older microwave ovens operated on 800 and
900 Mhz.

Why are people who are uneducated in certain subjects always the first to jump
in on a conversation and tell everyone else how they think things really are?

Because they want to appear educated.

"Truth" wrote in message ...
900 MHz are NOT microwaves

Interesting. Especially since the older microwave ovens operated on 800 and
900 Mhz.

Why are people who are uneducated in certain subjects always the first to jump
in on a conversation and tell everyone else how they think things really are?

Yeah, why is that?

http://en.wikipedia.org/wiki/Microwave

(For the non-curious, the paragraph of interest says:

Microwaves, also known as Super High Frequency (SHF) signals, have wavelengths
approximately in the range of 30 cm (1 GHz) to 1 mm (300 GHz).

)
--
-----------------------------------------------------------------------------
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!-

Is AM Radio Harmful? Only if you listen to it.

Microwaves, also known as Super High Frequency (SHF) signals, have wavelengths
approximately in the range of 30 cm (1 GHz) to 1 mm (300 GHz).

This is a pathetic attempt to avoid the real issue and turn the argument another way
to avoid the statement they were unable to dispute.

Saying microwaves magically start at 1000 Mhz, and saying 999 Mhz is not, and 800
Mhz is not, is bull**** talk.

Regardless of where you want to call it, the FACT remains that Microwave Ovens were
manufactured that cooked your food with frequencies in the 800 Mhz region.

So, since we can cook food and heat at 800 Mhz, and since cell phones are using the
same frequency range, we can make a definite connection here.

Any attempt to shift the discussion to one about where we now want to classify the
word "microwaves" to be appropriate is just childish and ignorant, and nothing more
than diversion from the point about cell phones being dangerous.

LEGAL exposure to certain RF in Russia can be different than the LEGAL limits in the
US. As if RF follows any of these laws, or as if a cell phone is not going to harm
you just because you change the definition of what can be legally called microwaves.

What was considered low blood pressure a decade ago, is now being called high blood
pressure in an attempt to sell more medication and make more money. Just the AMA
changing the imaginary boundary line did not make us all suddenly have high blood
pressure, just as your changing what is now considered microwaves has no effect on
this issue either.

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If there's nothing that offends you in your community, then you know you're not
living in a free society.

If there is nothing that offends you in your community, then you ALSO know you're
not living in a dictatorship either.

This is a stupid play on words that is meaningless!

You can NEVER have any society in which nothing offends anyone! Not under ANY
government.

Idiots.