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SSB Antenna theory

G

Gary Schafer

Jan 1, 1970
0
This tries to explain how short SSB antennas operate and why.
The discussion is concerning antennas that are shorter than a quarter
wavelength.

TUNING TO A QUARTER WAVE

I looked through several older handbooks and antenna handbooks and
found most of them professing what Larry is saying about "tuning an
antenna to an electrical quarter wave."

No wonder so many people have it wrong! The ARRL has been preaching
this stuff for years. But in the same paragraphs they will speak to
the "electrical length being very close to the physical length". Can’t
have it both ways! Even the 2000 ARRL handbook has it wrong.

They finally got it right in their antenna handbook. Not once did I
see reference to "tuning an antenna to an electrical quarter wave
length.

It may seem like semantics but there are a lot of people that get
confused and think that when making the system resonant with a
shorter antenna that the antenna is really the same as a quarter wave
length antenna when there is a loading coil. It is far from that. Its
radiation resistance and its feed point resistance are both much
lower.

An antennas electrical length is what it is by itself. Adding a coil
to it to make it resonant will not change that.

Also a lot of people think that the antenna has to be resonant in
order to radiate well. That is also far from true. Using a coil to
cancel out the reactance of the antenna forms a resonant circuit with
the antenna which must be done in order to get power to the antenna.
This is a different thing than the antenna being resonant itself. But
the antenna itself will perform the same whether it is resonant or
not. The problem is getting power to the antenna as you will see
below.


CURRENT DISTRIBUTION

With a full quarter wave length antenna the current distribution on
the antenna is more like a sine wave curve. Larger at the bottom and
tapering slowly as you go up the antenna.

With an antenna shorter than a quarter wave length the current is
still maximum at the bottom feed point and smallest at the top.
However the shape of the distribution is different.

With a short base loaded antenna the current distribution is about
linear. In other words it drops in direct proportion to the length of
the antenna.

With center loading or top hat loading the current distribution is
"pushed up" the antenna. It has a higher almost constant current at
the lower part of the antenna and it drops off very fast at the top.

RADIATION RESISTANCE

Current distribution on the antenna determines radiation resistance.
Radiation resistance determines efficiency of the antenna system. The
higher the radiation resistance the higher the efficiency of the
system.

With a more constant current distribution that center loading gives
over base loading, the radiation resistance is greater on the antenna.
This allows more power to be put into the antenna to radiate.

Radiation resistance is not to be confused with feed point resistance.
"Radiation resistance is defined as the resistance that would
dissipate the same amount of power that is radiated by the antenna."

As an antenna is made shorter (less than a quarter wave length) the
radiation resistance drops. As radiation resistance drops you must
increase the current to maintain the same amount of power to radiate.
(OHMS LAW)

REACTANCE

Note that the larger a capacitor is the less reactance it has.
The larger an inductor (coil) is the more reactance it has.

A short antenna looks like a capacitor and like any capacitor it has
capacitive reactance. That reactance is AC resistance. In order to get
power into that antenna you must have an equal amount of inductive
reactance in the circuit to cancel out the capacitive reactance. When
the two are equal the circuit is said to be resonant and purely
resistive.

Note that when adding the inductance it changes nothing about the
antenna itself. Only the reactance / impedance seen at the feed point
which is the transmitter end of the coil.

The shorter the antenna (less capacitance presented) the higher the
capacitive reactance and thus the larger the coil required to cancel
it. This means more wire in the coil. The more wire in the coil the
more resistance the coil will have. (not to be confused with
reactance) The more resistance it has the more loss it will have.

It gets worse, because as the antenna gets shorter its radiation
resistance gets smaller as the coil resistance is getting larger. The
coil resistance can be 10 times or higher in resistance than the
radiation resistance of the antenna. Because they are in series the
same current that flows in the antenna also flows in the coil. The
coil will therefore absorb most of the power. (ohms law again)

By center loading the antenna rather than base loading it the current
distribution is shifted in the antenna and that increases the
radiation resistance of the antenna.
However it is not a free lunch. The higher up you raise the coil on
the antenna the more coil is required. This increases coil loss again.
But the radiation resistance of the antenna goes up faster in
proportion to the coil resistance so you end up with less system loss.

CURRENT IN THE ANTENNA, VOLTAGE ON THE COIL

When a short antenna is used some think that the current requirement
is less rather than more for the antenna. This is related to the fact
that the voltage at the coil-antenna junction (output terminal on your
tuner) is much higher with a short antenna. Therefore the thought is
"if the voltage is higher the current must be lower".

Well it isn’t! The reason the voltage is so high is because of the
high inductive reactance of the coil in the tuner. Because the
inductive reactance is high (lots of coil) the voltage goes high at
that point.

Here are some numbers to illustrate what happens when a coil is used
with a short antenna:
With a 10.5 foot antenna at 3.5 mhz the capacitance of the antenna is
around 30 pf.
The radiation resistance is about .55 ohms
This takes a 62.5 microhenry coil to equal the capactive reactance.
With a Q of around 200 the coil will have a resistance of about 6.88
ohms.
The coil and antenna radiation resistance will provide a load of 7.43
ohms at the feed point. (6.88 + .55 = 7.43)

Additional matching will be required to get it to 50 ohms. But if you
apply 100 watts to the 7.43 ohms you will have a coil / antenna
current of 3.67 amps. (I squared R = 100W)

Now that reactance of the coil will be 1375 ohms. So 1375 times 3.67
amps = 5046 volts rms or 7137 volts peak across the coil!! (V= IR)
Who says you can’t get zapped from 100 watts!

This is where your high voltage comes from with a low impedance
antenna.

HIGH VOLTAGE NOT IN PHASE

Note that there is a phase shift across the coil so the current
through the coil and the voltage across it are not in phase. That is
what allows the voltage to rise so high. You can’t use ohms law here
to calculate power without allowing for the phase difference.
Otherwise it would look like 6900 watts was being delivered to the
antenna.

As you increase the length of the antenna the capacitance it
represents becomes higher thus making its capacitive reactance lower.
That also makes the need for inductive reactance lower and reduces the
coil size and inductive reactance. By reducing inductive reactance you
also reduce the voltage seen across the coil.

Also increasing the length of the antenna increases its radiation
resistance which requires less current through it for the same amount
of power. With less current through the antenna you will have less
current through the coil. So with less coil impedance and less current
through it, the voltage developed across it will also be less for the
same amount of power applied to the circuit.

CAPACITY HATS

Using a capacity hat on a short antenna increases the amount of
capacitance that the antenna represents in the circuit. That decreases
the capacitive reactance which increases its radiation resistance.
Increasing its radiation resistance as above increases the efficiency
of the system. Also less inductive reactance is needed and the
associated benefits are also realized.


COAX AS A FEED LINE

Some have advocated using coax between the tuner and whip antenna or
long wire antenna as a feeder rather than an open piece of wire.
That would be ok if the antenna were not short for the frequency being
used. Here is why it doesn’t work with a short antenna like a whip or
long wire that is short for the frequency.

With the same antenna in the example above if we used just 1.5 feet of
RG 58 coax which has 21 pf /ft capacitance would give us about 30 pf
capacitance. The same amount as out whip. Putting that in parallel
with the antenna would drop the radiation resistance in half. This
would cut the efficiency of the antenna in half!

Examples above are from the 2000 ARRL handbook.


After we worry about all the losses above there are the ground losses
that are also in series with the antenna feed point. Those losses can
be several times greater than the antenna losses. You can begin to see
that a short antenna can be very inefficient.

After reading this if still interested, reread my other earlier post
about short SSB antennas and it may make more sense.

Regards
Gary
 
G

Gary Schafer

Jan 1, 1970
0
I tried to give an abbreviated synopsis of the subject and dispel some
myths. I did not cover all the details.

You need look no further than the ARRL itself for references. If you
read the whole post you would see that near the bottom I credited the
ARRL 2000 edition of their handbook, antenna section. The definition
of "radiation resistance" is from there and the calculations of the
example antenna with impedance's and voltages developed are directly
from there.

If you also look in the 18th edition 1997? of the ARRL antenna
handbook chapter 16 "mobile and marine antennas", and probably later
versions, you will see where they properly discuss loaded short
antennas and what the coil does.

Also if you look at the earlier post titled "Notes on short SSB
antennas" you will find a reference to W8JI's web site where he
discusses these very items in detail. He and many other engineers will
tell you the same thing.

I don't mean to discredit the ARRL but their statements in regard to
tuning an antenna to a quarter wave in their older publications are
misleading as evidenced by all the misconceptions that fly around.

While that is a simplified explanation of what happens with the
antenna matching, I suppose it was easier to propagate that (no pun
intended) term for simplicity. But if you really want to understand
what is going on it will get you into trouble in understanding as it
is a conflict with what really happens.

Regards
Gary
 
G

Gary Schafer

Jan 1, 1970
0
I take it that you are trying to learn and not just be argumentative.

If you read what I wrote, I said that in the antenna handbook that
they finally have it right.

Read the first three paragraphs under "TUNING TO A QUARTER WAVE"
below.

Regards
Gary
 
G

Gary Schafer

Jan 1, 1970
0
I am not sure just what you are missing here. Or maybe I am not
understanding your question.

Again I am posting the reference pages below in the antenna handbook.
Not once could I find in there that they stated that a loading coil on
an antenna made it into a quarter wave antenna as did earlier versions
of the antenna handbook and the regular handbook. That is why I say
they finally got it right. Maybe you are questioning which one is
right.

In the earlier handbooks the subject was more or less glossed over
with poor explanation of what happens in the antenna matching. The
newer antenna handbook goes into more detail.

I even tell you the pages!

Also if you look at my earlier post "Notes on short SSB antennas"
there is a link to W8JI's web site where he discusses the same stuff
that I have. He tells you why a loaded antenna is still the same
length electrically as an unloaded antenna. In that post there is a
copy of part of his article that deals with this topic as I credited
him with.
For more details look at his web site.

REFERENCE 1
If you also look in the 18th edition 1997? of the ARRL antenna

REFERENCE 2
 
R

-rick-

Jan 1, 1970
0
"Doug Dotson" wrote ...
I guess I'll pick up the latest Antenna Handbook and start reading.

Doug, k3qt
s/v Callista

The fundamental work on "small" antennas was done by a guy named Wheeler. After
digging in the filing cabinet I found his paper from the proceedings of the
I.R.E. (institute of radio engineers?) that preceded the IEEE.

"Fundamental Limitations of Small Antennas" by Harold A. Wheeler fellow, I.R.E.
December 1947

One insight is that a small antenna can theoretically be nearly as efficient as
a 1/4 wave element but it is difficult to match to the small radiation
resistance. (actually you match to the sum of the radiation and loss
resistance). The efficiency is simply the ratio of radiation resistance to the
sum of radiation plus loss resistance. A small loop antenna which looks
inductive makes the job easier as you can build a low resistance loop and use
high Q capacitors for tuning/matching.

regards,
-rick-
 
G

Gary Schafer

Jan 1, 1970
0
The square of the turns ratio is 9:1 so the antenna's impedance is
somewhere around 6 ohms or so at the feedpoint. The 650 W amp melted the
solder joints on the core using #12 wire for the turns, so I went to #10
If one were to forego the old untuned wire/tuner marine antenna
configuration and go with a real tuned vertical, this toroid
autotransformer will very efficiently match the very low base impedance to
the 50 ohm transceiver across the 2-30 Mhz HF band.
Larry W4CSC

Some day I might try this antenna using the handrails of the boat as ground
plane. It's gotta work better than the stupid untuned backstay and
inefficient antenna tuner. It certainly results in much better signal
reports.


Your feed point resistance may be 6 ohms but about 5.8 to 5.9 ohms of
that are coil resistance. The radiation resistance of the 15 foot whip
on 3.5 mhz is in the order of .1 ohm.

So about 97% of your power is going up in heat in the coils. Only a
couple percent of the power is making it to the antenna to be
radiated.
Of 650 watts only around 20 watts makes it to the antenna.

A full quarter wave length vertical has a radiation and feed point
resistance of around 36 ohms. Much easier to get power into than a .1
ohm 15 foot antenna.

Oh, don't forget to add in all the ground loss resistance too. Less
power to the antenna yet.

If you can get your feed point resistance down to around 1 ohm then
you will get about 10% of your power into the 15 foot antenna!

Regards
Gary
 
G

Gary Schafer

Jan 1, 1970
0
I've never met anyone so full of pure bullshit in my entire life as you,
Gary. It's simply incredible.

One hopes noone in their right mind will hire you as an engineer and suffer
the consequences.

I doubt 20 watts would make a signal 800 miles away at 20 over S9 in any
conditions, but we're, I'm sure, gonna hear more bullshit from you about it
in the near future.

Larry W4CSC

What class licenses and degrees do you hold, anyways? I've been a 1st
phone licensee since the 1960's, an avid ham operator since 1957 when I was
10 and graduated with honors from many military electronics schools run by
the US Navy because Vietnam's draft kinda got in the way of college in
1964.

Stop by some time and I'll let 20 watts burn your ass for you....(c;
I've never seen 20 watts produce a corona in air over 8" long....
How many kilovolts is that in air at sea level?


Heh heh, still haven't figured it out Larry?

Regards
Gary
 
G

Gary Schafer

Jan 1, 1970
0
I've never met anyone so full of pure bullshit in my entire life as you,
Gary. It's simply incredible.

One hopes noone in their right mind will hire you as an engineer and suffer
the consequences.

I doubt 20 watts would make a signal 800 miles away at 20 over S9 in any
conditions, but we're, I'm sure, gonna hear more bullshit from you about it
in the near future.

Larry W4CSC

What class licenses and degrees do you hold, anyways? I've been a 1st
phone licensee since the 1960's, an avid ham operator since 1957 when I was
10 and graduated with honors from many military electronics schools run by
the US Navy because Vietnam's draft kinda got in the way of college in
1964.

Stop by some time and I'll let 20 watts burn your ass for you....(c;
I've never seen 20 watts produce a corona in air over 8" long....
How many kilovolts is that in air at sea level?


What part do you deem to be "bullshit" Larry? The parts you don't
understand?

I know that you like sensationalism in big arcs and bragging rights of
running high power. That's fine if that's your thing but you let it
cloud reality. Being able to pull a big arc from an antenna tells you
nothing about its efficiency or how well it will radiate. It does make
for good show though. A tesla coil will produce some pretty high
voltage and corona too.

By the way, time in grade doesn't count either. Being a corporal for
20 years doesn't automatically make one an expert at anything. Not
that you can't be an expert, time in grade just doesn't contribute.

Note that I haven't said that you are full of bullshit. I am giving
you the benefit of the doubt.

If you went to tech school as you claim, you have forgotten some of
the basics or you were asleep through many parts.

I would have thought that basic AC circuit theory would have been part
of your education. Maybe not.


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.

Here are a few questions:

1. Do you understand that when you have a capacitor and a coil in
series in an AC circuit that the voltage across either can be much
greater than the applied voltage to the circuit? (basic AC theory)

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

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

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

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.

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?

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

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

If you do not understand any of the above questions please let us
know. That may be the reason that you are not understanding what goes
on in your antenna system.

What I previously wrote pertaining to the voltage developed across the
coil and how low the radiation resistance can be on a short antenna
was quoted directly from the 2000 ARRL handbook. If you have it look
at HF mobile antennas. Page 20.46, 3rd column on the page. It explains
why the voltage is 5000 volts rms across the coil with just 100 watts
applied to an antenna with a radiation resistance of less than an ohm.
Isn't that amazing! Must be black magic huh Larry? Or just maybe it
has something to do with the above questions.

Regards
Gary
 
J

John F. Hughes

Jan 1, 1970
0
I'm gonna jump in here -- not as an expert on antennas, but
merely as someone who understands maxwell's equations
and some mathematics.

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.


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.

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.

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.
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).
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.
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...
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."
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.

----------------------------------------------------------

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...
<statement about antennas paraphrasing ARRL ahndbook deleted>
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.

--John Hughes
 
G

Gary Schafer

Jan 1, 1970
0
-------------------------------------------------------------------------
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.
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.
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.
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.
"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.
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.
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.
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. :>)
----------------------------------------------------------

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.

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

Vito

Jan 1, 1970
0
Larry W4CSC said:
I've never seen 20 watts produce a corona in air over 8" long....

Awww .... my buddy's 4-watt CB does it all the time. Of course he had it
peaked up at the truck stop but ... (c;

73, K3DWW
 
V

Vito

Jan 1, 1970
0
Gary Schafer said:
On Tue, 04 May 2004 15:08:08 -0000, Larry W4CSC



So about 97% of your power is going up in heat in the coils.

If Gary's right then 97% of 650 Watts otta get the coil so hot it melts
solder and ... OY!

That sounds dangerous to me, Larry. I think you otta send the whole rig to
me so I can check it out. With a DD degree I may be the best qualified (c:
 
J

John F. Hughes

Jan 1, 1970
0
Well I don't profess to be any kind of expert either.

You claimed to be expert enough to tell that Larry doesn't
know what he's talking about, and to be able to distinguish
the "right" material from the "wrong" as published by ARRL.
That *sounds* like a claim...
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.

Bzzt. The voltage applied is irrelevant only for linear components.
But if you want to assume that antennas (and/or coils) are linear
over the range of frequencies and voltages you're considering, I'm
willing to go with that.

Now I'll rephrase the last half of what you wrote:

Every component has an impedance that may be frequency-dependent.
We'll be working at a single frequency, f. The real part
of the impedance of a coil at frequency f will be called
the "series resistance."


Series resistance = Real ( Impedance(f))

There's an equation. It DEFINES the term on the left in terms
of two things on the right -- the "real part" function, which
was known to Cauchy for instance, and the Impedance, which you
can find in Horowitz and Hill, for instance. Write some
more things like that, and I'll be running right along with you.
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.

I don't think I was making any assumptions. The only term I
found DEFINED was "feedpoint impedance"; I would HOPE that
the definition of feedpoint resistance would be "real part of
feedpoint impedance," but not all engineering is consistent
in its terminology.
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.

Uh...I read the originals. No equations, no definitions, and
the websites pointed to had the same sort of blather.
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.

Uh...I'm going to sound stupid here, but how do you measure "power
dissipated?" And does it include, for instance, the heat generated
by the wiring, etc.? Certainly that's power dissipated, but
somehow it doesn't seem to capture the sense of the thing you
describe above. Perhaps you could give me a definition of THAT
term as well.
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.

Hunh? All I know about is impedance, I'm afraid, when talking
about AC signals. Can you express this thing in terms of
impedance? All I want is a simple equation...
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.

Ah...so now we have a circuit. It looks something like this:


AC+ -----###---%%%%------%%%%%%----AC-

where the first set of "%%%" signs represent a component
with the (presumed) linear behavior of the atmosphere
and the second resistor (it was all I could draw) represents
the (presumed) linear behaviour of the earth. And the ###
is some coil at the bottom of the antenna perhaps.

Maybe I've got this circuit wrong -- please correct me
if this isn't the model you're using. But if it IS the model
you're using, then each of the three components above has
a reactance at frequency f, and you can start writing out
the equations. [I should say "One could start writing out
the equations"; I'm getting the sense that you cannot.]
Me either that's why I ask the question. But first you must understand
what radiation resistance is. See above.

Now that I "understand" that radiation resistance is a resistance
that could be substituted for some part of the circuit and would
dissipate the same power as the replaced part did, BUT is
not actually a resistance of any part of the circuit, I cannot
see how any current flows in it.
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.

No. A diode doesn't have resistance per se. It's true that the
voltage/current curve is (probably) differentiable at most points,
so one could speak of a "local resistance," but that doesn't
mean that you can say anything relevant with Ohm's law, except
if you're talking about very very tiny changes in voltage and
the corresponding tiny changes in current.

I *don't* believe we're talking about two linear resistors. I know
I must be stupid, but if antennas were just pairs of resistors,
no one could make a living designing them. I *do* suspect we're
talking about some sort of collection of impedances, but
I've lost any hope that you know anything about them.
Also true. But it also has a real phase shift.

Uh...are you telling me that there are two different ways to
express a complex number, one of them in terms of the real
and imaginary parts that sum up to give the number, and the
other in terms of a magnitude and an argument? If so, deMoivre
beat you to it by a few years.
Rr = 395 x (h/lambda) squared

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

OK. It's an equation. THIS I can work with. Presumably since you
said so above, this is an equation for a vertical whip antenna.
And it's certainly true that in this equation, the term Rr increases
with the square of the height. I'm going to guess that one
of two things is true:

(a) This is your a DEFINITION of the symbol Rr, in which case
your conclusion in statement 8 is true, but not interesting, or

(b) You got this equation from somewhere where it is either
(i) empirically observed for a wide range of values of h and lambda,
and where Rr is actually defined so that it can be measured, or
(ii) proved, based on some assumptions about the circuit in question
(does it include a loading coil, for instance???) and a clear
definition of Rr,

In case b, I'd love to see the data and/or proof, but even more,
I'd love to see the definition of the thing being measured (or
appearing in the proof, as the case may be).
(referenced from the ARRL antenna handbook) like it or not. :>)

Ah. Excellent. Forgive me for not having it with me; I'm in
France. But if you'd type in, verbatim, their definition (NOT
description) of radiation resistance, that would be great...
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.

OK. I read 'em. The "explanations" are blather, and so I
guess I'm not gonna take your word for it.

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.

I've read the original post. I think I grasped every germ
of truth in it.

And I've looked at W8JI's web page. The discussion in his
radiation_and_fields.htm page is particularly entertaining.
It's true that it ignores just a few things (like, say, Maxwell's
equations, and the relativistic relationship between the electric
and magnetic fields, and a few other things you can read about
in, say, Purcell's lovely book on Electricty and Magnetism,
semester 2 of the Berkeley Physics series; but what the hell
does Purcell know? After all, he's only got a Nobel prize in
physics, not a radio license...) but the gist is not uniformly
awful. The part where he says that there are electric fields,
magnetic fields, and electromagnetic fields is a pleasure to
look at. When *I* look at Maxwell's equations, I see phi,
the electric potential, and E, the electric field, and B, the
magnetic field. (And if you feel giggly, you can add, say,
zeta, the magnetic potential, and then declare it to be zero
everywhere). No mention of a THIRD field. Live and learn,
I always say. But I don't think that I want to live and
learn from that particular source...

By the way, he mentions a formula for radiation resistance,
too (claims it's a definition, but since he's given other
definitions above, this must certainly NOT be the definition.
Or maybe he's just a crappy writer. Anyhow, his formula
(for which he provides no proof) looks like yours. But
the constant differs by a factor of five. Go figure!

--John
 
G

Gary Schafer

Jan 1, 1970
0
-----------------------------------------------------------------
You claimed to be expert enough to tell that Larry doesn't
know what he's talking about, and to be able to distinguish
the "right" material from the "wrong" as published by ARRL.
That *sounds* like a claim...
Your definition of an expert is obviously different than mine.
Bzzt. The voltage applied is irrelevant only for linear components.
But if you want to assume that antennas (and/or coils) are linear
over the range of frequencies and voltages you're considering, I'm
willing to go with that.

Old debating tactic. Tell me I'm wrong, then agree.
Now I'll rephrase the last half of what you wrote:

Every component has an impedance that may be frequency-dependent.
We'll be working at a single frequency, f. The real part
of the impedance of a coil at frequency f will be called
the "series resistance."


Series resistance = Real ( Impedance(f))

There's an equation. It DEFINES the term on the left in terms
of two things on the right -- the "real part" function, which
was known to Cauchy for instance, and the Impedance, which you
can find in Horowitz and Hill, for instance. Write some
more things like that, and I'll be running right along with you.

Thanks but I'll stick with my definition.
I don't think I was making any assumptions. The only term I
found DEFINED was "feedpoint impedance"; I would HOPE that
the definition of feedpoint resistance would be "real part of
feedpoint impedance," but not all engineering is consistent
in its terminology.


Uh...I read the originals. No equations, no definitions, and
the websites pointed to had the same sort of blather.

I suppose it seems so when you have little understanding on the
subject and have a closed mind to such. But then I see your main
interest is to try and display your debating skill.
Uh...I'm going to sound stupid here, but how do you measure "power
dissipated?" And does it include, for instance, the heat generated
by the wiring, etc.? Certainly that's power dissipated, but
somehow it doesn't seem to capture the sense of the thing you
describe above. Perhaps you could give me a definition of THAT
term as well.

Reread the above definition of radiation resistance carefully.
Hunh? All I know about is impedance, I'm afraid, when talking
about AC signals. Can you express this thing in terms of
impedance? All I want is a simple equation...

Keep reading.
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.

Ah...so now we have a circuit. It looks something like this:


AC+ -----###---%%%%------%%%%%%----AC-

where the first set of "%%%" signs represent a component
with the (presumed) linear behavior of the atmosphere
and the second resistor (it was all I could draw) represents
the (presumed) linear behaviour of the earth. And the ###
is some coil at the bottom of the antenna perhaps.

Maybe I've got this circuit wrong -- please correct me
if this isn't the model you're using. But if it IS the model
you're using, then each of the three components above has
a reactance at frequency f, and you can start writing out
the equations. [I should say "One could start writing out
the equations"; I'm getting the sense that you cannot.]

Yes by golly you have it! I think anyway, if I understand your
writing.
Yes each component has reactance as well as resistance. But when the
circuit is at resonance the reactance is tuned out. It is then a pure
resistive load containing only the resistive elements.
Resonance is when the capacitive reactance is equal to the inductive
reactance in the circuit. But then you knew that.
And it doesn't matter whether I can or can't write out any formula.
Now that I "understand" that radiation resistance is a resistance
that could be substituted for some part of the circuit and would
dissipate the same power as the replaced part did, BUT is
not actually a resistance of any part of the circuit, I cannot
see how any current flows in it.

No imagination? :>)
It is part of the circuit in the form of the antenna that radiates the
equivalent amount of power.
No. A diode doesn't have resistance per se. It's true that the
voltage/current curve is (probably) differentiable at most points,
so one could speak of a "local resistance," but that doesn't
mean that you can say anything relevant with Ohm's law, except
if you're talking about very very tiny changes in voltage and
the corresponding tiny changes in current.

I *don't* believe we're talking about two linear resistors. I know
I must be stupid, but if antennas were just pairs of resistors,
no one could make a living designing them. I *do* suspect we're
talking about some sort of collection of impedances, but
I've lost any hope that you know anything about them.

In simple terms, any time you have a voltage applied to something that
does not conduct infinite current that something is said to have
resistance. It's resistance may not be stable and may change with
amounts of applied voltage but it is still resistance and good old
ohms law still applies for each static point. But then again this is
completely irrelevant to the rest of this.
Nice diversion attempt though.

You seemed to have formed your opinion long ago of how things work.
More debate tactics.
Uh...are you telling me that there are two different ways to
express a complex number, one of them in terms of the real
and imaginary parts that sum up to give the number, and the
other in terms of a magnitude and an argument? If so, deMoivre
beat you to it by a few years.
Running low on tactics?
OK. It's an equation. THIS I can work with. Presumably since you
said so above, this is an equation for a vertical whip antenna.
And it's certainly true that in this equation, the term Rr increases
with the square of the height. I'm going to guess that one
of two things is true:

(a) This is your a DEFINITION of the symbol Rr, in which case
your conclusion in statement 8 is true, but not interesting, or

(b) You got this equation from somewhere where it is either
(i) empirically observed for a wide range of values of h and lambda,
and where Rr is actually defined so that it can be measured, or
(ii) proved, based on some assumptions about the circuit in question
(does it include a loading coil, for instance???) and a clear
definition of Rr,

In case b, I'd love to see the data and/or proof, but even more,
I'd love to see the definition of the thing being measured (or
appearing in the proof, as the case may be).


Ah. Excellent. Forgive me for not having it with me; I'm in
France. But if you'd type in, verbatim, their definition (NOT
description) of radiation resistance, that would be great...

Being in France is a lame excuse. I am sure if you wanted to you could
find a copy there but I am sure that you can also find it in many
other books that deal with antenna theory. But then that would end
the argument too easily wouldn't it.

Already done several times.
OK. I read 'em. The "explanations" are blather, and so I
guess I'm not gonna take your word for it.

Meaning you don't understand them.
I've read the original post. I think I grasped every germ
of truth in it.

Careful you don't get infected by it.
Truth can be an awful thing to deal with.
And I've looked at W8JI's web page. The discussion in his
radiation_and_fields.htm page is particularly entertaining.
It's true that it ignores just a few things (like, say, Maxwell's
equations, and the relativistic relationship between the electric
and magnetic fields, and a few other things you can read about
in, say, Purcell's lovely book on Electricty and Magnetism,
semester 2 of the Berkeley Physics series; but what the hell
does Purcell know? After all, he's only got a Nobel prize in
physics, not a radio license...) but the gist is not uniformly
awful. The part where he says that there are electric fields,
magnetic fields, and electromagnetic fields is a pleasure to
look at. When *I* look at Maxwell's equations, I see phi,
the electric potential, and E, the electric field, and B, the
magnetic field. (And if you feel giggly, you can add, say,
zeta, the magnetic potential, and then declare it to be zero
everywhere). No mention of a THIRD field. Live and learn,
I always say. But I don't think that I want to live and
learn from that particular source...
I see you are trying hard to sneak in some of your credentials without
trying to make it obvious but you have failed. Are they imaginary or
real?
By the way, he mentions a formula for radiation resistance,
too (claims it's a definition, but since he's given other
definitions above, this must certainly NOT be the definition.
Or maybe he's just a crappy writer. Anyhow, his formula
(for which he provides no proof) looks like yours. But
the constant differs by a factor of five. Go figure!

I think that he has quoted well known antenna professors in his
definition. Ones you may not be familiar with since RF and antennas
are foreign to you as you have stated.
Are you now the new expert?


You have written many words and have said little of any substance.
Only to try and twist and turn statements to make them seem
unscientific.
You tell me that I am wrong in my explanation and go on to say in
the same paragraph you agree with me. (debate 101)

Maybe you view the discussion as a contest but I don't. I am only here
to try and help others understand a little about antennas.
You seem more interested in winning a debate rather than providing any
useful information to others.

Forgive some of my seemingly snide remarks. However I think they are
fitting with some of the amateur debating tactics you have presented
which add nothing to the discussion of antennas.
There is an entertainment value though.

Regards
Gary
 
J

John F. Hughes

Jan 1, 1970
0
I suppose it seems so when you have little understanding on the
subject and have a closed mind to such.

Nah. I think that it's pretty clear that the original posts had no
equations. I happen to think that mathematics is the language of
science, and so when words get vague and ambiguous, I ask for
mathematics. That's just my idiosyncracy.
In simple terms, any time you have a voltage applied to something that
does not conduct infinite current that something is said to have
resistance.

Ah. Now I can throw away my copy of Horowitz and Hill.
[description of website vs. E&M book by Purcell]
I see you are trying hard to sneak in some of your credentials without
trying to make it obvious but you have failed. Are they imaginary or
real?

Nah. I don't have any credentials here. Just an interest in
science. *Purcell* has credentials. I'm just a guy who likes
equations. You apparently don't. That's OK. We'll go about
understanding phenomena differently. It's a big world.

I have no idea whether you're right, Larry's right, or both
are wrong about loaded antennas. But I do know that you're
the wrong person for me to try to learn from, because you
aren't willing to write definitions or equations, and those
are what I use to understand science.

Fortunately, there are other sources of information out
there...

-John
 

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