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On Wed, 5 May 2004 09:46:52 +0000 (UTC), "John F. Hughes"
wrote:



I'm gonna jump in here -- not as an expert on antennas, but
merely as someone who understands maxwell's equations
and some mathematics.

Well I don't profess to be any kind of expert either.


On 2004-05-05, Gary Schafer wrote:

If you are interested in how this stuff really works it might help us
to understand some of the things that you have misconceptions about.

I don't think that it'll help at all to know what (if anything)
he has misconceptions about. But it *would* help to have a
mathematical model and a definition of terms.

The fact that ALL this stuff has been gone over several times and he
still makes blanket statements of how it is wrong makes one wonder
where the problem really is.



OK. Gary says that "your history doesn't matter; it doesn't matter
what tech school you went to, etc.", and I agree to some extent. Since
I didn't go to any tech school, I've got nothing to be embarassed
about. I just happen to know a bit of math and physics.

Not really what I said. I said that "time in grade doesn't matter".
Not trying to belittle anyone's education pro or con.



Here are a few questions:

2. Do you understand that a coil has series resistance as well as
reactance?

Wow. That's interesting. I'm just going to take a shot in the dark
and assume that by "series resistance" you mean if I apply
a DC voltage across the coil, and wait until the circuit reaches
a steady state, I'll notice some current flowing; the ratio
of the voltage applied to the steady-state current flow is
what I call "series resistance." [...and you're implicitly asserting
that this ratio is independent of the voltage applied, i.e., that
the steady state current is linear as a function of the applied
voltage"...] I expect that this assertion is true.

So in other words, what you're calling "series resistance" is
the real part of the (complex number) impedance, and reactance
is the imaginary part.

From what I've seen Larry write, I'll bet he understands that.

Yes, the series resistance is the "real part". However it is not just
the DC resistance of the material in the coil. It is the AC resistance
of the material known as skin effect, which will be greater than the
DC resistance at radio frequencies. The higher the frequency the
greater the skin effect. The voltage applied is irrelevant.



3. Do you understand the difference between Radiation resistance and
feed point resistance? This is an important one!

I'm just a farmer from the country, but where I come from, there's
just impedance. I don't know how you split the real part of that
complex number into two parts. Maybe where you come from, there's
"carbon resistor" resistance and "thin-film" resistance, too, but
I'm not sure how the electrons can tell the difference.


A Google search doesn't yield any real information on "feed point
resistance," so I guess that answering for myself, I can say
"sure...radiation resistance is the real part of the impedance of an
antenna; feed point resistance is an undefined term."

There *does* seem to be widespread use of the term "feedpoint
resistance," although definitions seem to be scarce as hen's
teeth. Just being the ignorant sorta guy I am, I tend to
gravitate towards the ones that define "feedpoint impedance;"
one could then say that feedpoint resistance is the real part
of that complex impedance. But that seems strikingly similar
to the definition of "radiation resistance." How very odd.

This is the very reason I pose the question! By making presumptions
you get yourself into trouble in understanding. Don't feel alone
though, because this is probably one of the most misunderstood terms
with antennas.
Again, I don't claim to be any sort of expert here. If you read the
original post in this thread it attempts to explain it with some
references too.

But in a nut shell, "radiation resistance" is an imaginary term when
dealing with antenna radiation. It is the amount of resistance that it
would take to dissipate the same amount of power that actually is
being radiated. It is pure resistance. No reactance involved.


FEED POINT RESISTANCE, on the other hand is the resistance (assuming a
vertical whip antenna here) seen at the base of the antenna , the
feed point. It includes the radiation resistance of the antenna, the
loss resistance of any coil involved and the ground resistance. They
are all in series. This is with the reactance tuned out so the feed
point is purely resistive. Feed point impedance would be the same
thing but it may have reactance. In other words not purely resistive.


4. Do you know that Radiation resistance is in series with feed point
resistance.

Ah...now you're losing me. For me, one of those terms is undefined,
so it's hard to be "in series with" the other. And if we take
"feed point" to be "feedpoint," then since the two seem to be
the same, it's hard to admit that they're in series. But I'm sure
you can clear this up for me. Can you just write down the
equations? (with all the symbols defined -- that'll make it
much clearer).

Rr + r = r feedpoint.
See above.


5. Do you know that the same amount of current that flows at the feed
point of the antenna is the same amount that flows in the radiation
resistance of the antenna? They are in series you know.

"Flows in the radiation resistance?" I don't honestly know whether
Larry knows more or less than you do, but at least I've never seen him
write something like this.

Me either that's why I ask the question. But first you must understand
what radiation resistance is. See above.


6. I assume that you know ohms law and that if the same amount of
current flows in two series resistors that the larger resistance will
dissipate more power than the lower value resistor?

Um...Ohm's law tells me, if I recall correctly, that for certain
materials, the current flowing through them varies linearly with the
applied (DC) voltage; in these cases, the ratio of the two is called
the "resistance." If you think I'm being overly pedantic here, you can
ask "what's the resistance of a diode?" The answer is, of course,
"the current through a diode does not vary linearly as a function
of the applied voltage, so it does not have a resistance."
So you have to be careful about applying Ohm's law...

Oh the diode has resistance all right but in its case you have to
define what point on the curve you are looking at. Irrelevant here
though. Here we are talking about two linear resistors. Nothing
complicated.


7. Do you understand that there is a phase shift between current and
voltage across a coil in an AC circuit.

I would say "an inductor has a complex impedance that happens not to
be a real number, but rather one that has an imaginary part as well."

Also true. But it also has a real phase shift. All part of
understanding why there is high voltage across the loading coil.


8. Do you understand that the radiation resistance gets very low in a
short antenna?

Uh...I guess I don't "understand" that. But if you'd write
out an equation or two, I might know what you meant by it.

Rr = 395 x (h/lambda) squared

Where Rr = radiation resistance
h = radiator height in meters
lambda = wavelength in meters

(referenced from the ARRL antenna handbook) like it or not. :)


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Someone else asked a very interesting question earlier:

1. You state that some editions of some ARRL publication are wrong.
2. You state that other editions are right.

You haven't told us where one finds evidence for this
wrongness/correctness. Does one of them have an error in some
equation? Can you construct a real circuit for which the
predictions of one book are wrong and the predictions of the
other are correct? Or do we just have to take your word
for it that one is right, the other wrong? If it's the
latter, then why bring the ARRL into it? Why don't we just
agree that whatever you say is right, and whatever anyone
says that appears to contradict it is wrong? It'd save a lot
of writing...

I am not going to say it again. Please READ the former posts. I have
shown the references many times and explained what the errors were.
And please, don't take my word for it if you have any doubts.


statement about antennas paraphrasing ARRL ahndbook deleted

Here I provided a reference and you don't want to consider it?
But yet you ask for references.

Isn't that amazing! Must be black magic huh Larry? Or just maybe it
has something to do with the above questions.

Maybe...but I'd find it more compelling if it had something to
do with known (by which I mean "widely accepted and tested")
physical laws like Maxwell's equations, and an analysis of
the circuits in question.

If you are really interested in learning please read the original post
in this thread and go look at the web site of W8JI that I posted
there. He explains this very subject very well in detail. He even
throws in a little math for you.

Regards
Gary


--John Hughes