Bill Turner wrote:
You restated what I said in much more detail, but what you have done
is expounded on the "what". I still don't know the "how" or the "why".

The Smith Chart yields the "how". Please take the "why"
up with The Creator. :-)
--
73, Cecil http://www.qsl.net/w5dxp

Cecil Moore wrote:
Bill Turner wrote:

You restated what I said in much more detail, but what you have done
is expounded on the "what". I still don't know the "how" or the "why".


The Smith Chart yields the "how". Please take the "why"
up with The Creator. :-)

Cecil --- you are begging the question!!

I reported that the addition of 3 or 4 inches above the coil produces a much
larger shift in frequency than adding the exact same length below the coil [by
almost a factor of 100:1]. That is empirical data. It is real and measurable.

My question is one of Physics. I think everyone who reads this list knows it
happens. The real question is: Why?? Why the difference in antenna resonant
frequency?

Dave wrote:
I reported that the addition of 3 or 4 inches above the coil produces a much
larger shift in frequency than adding the exact same length below the coil [by
almost a factor of 100:1]. That is empirical data. It is real and measurable.

My question is one of Physics. I think everyone who reads this list knows it
happens. The real question is: Why?? Why the difference in antenna resonant
frequency?

Dave,

Any antenna is a large network of distributed impedances. You have
series inductance in every conductor and capacitance to the outside
world. There is also some series resistance and even shunting losses in
dielectrics, but looking at the reactances is good enough to answer
your question.

Look at what we have in the short mobile antenna. Below the loading
coil we have a conductor that carries an almost uniform current,
voltage and current are nearly in phase throughout the entire length.
We have the loading coil that adds a high value of series reactance all
in one spot. Above the loading coil we have an area with very high
power factor. The current and voltage are nearly 90 degrees out of
phase and voltage is very high.

When we perturb the system below the coil, it isn't terribly sensitive
to changes in capacitance because voltage is so low. The top of the
mast below the coil is terminated in a fairly low impedance so it isn't
sensitive to shunt capacitance and the additional series inductance
added by the small additional length is very small compared to the
system's overall reactance and electrical length at that point.

Above the coil it is another story.

Now we have very high voltage (it can still have almost the same
current above as below the coil if the antenna is a good design). The
coil is terminated in a very high value of reactance that is mostly
comprised of the distributed reactance of the small whip, and that is
mostly capacitive reactance of a very high impedance.

Altering the whip length or diameter above the coil greatly changes the
system resonance because the small value (very high reactance) of the
whip is what actually resonates or terminates the coil.

The loading coil and the capacitance above the coil form a series
resonant circuit. The area above the coil has very high impedance, very
high power factor, and as such is very sensitive to any additional
shunting impedance caused by additional length or diameter.

The area below the loading coil has almost unity power factor, has low
impedance, and has much lower levels of electric field so it isn't
nearly as sensitve to shunt impedance (distributed capacitance)
changes.

73 Tom

Dave wrote in
news
Cecil Moore wrote:
Bill Turner wrote:

You restated what I said in much more detail, but what you have done
is expounded on the "what". I still don't know the "how" or the
"why".


The Smith Chart yields the "how". Please take the "why"
up with The Creator. :-)

Cecil --- you are begging the question!!

I reported that the addition of 3 or 4 inches above the coil produces
a much larger shift in frequency than adding the exact same length
below the coil [by almost a factor of 100:1]. That is empirical data.
It is real and measurable.

My question is one of Physics. I think everyone who reads this list
knows it happens. The real question is: Why?? Why the difference in
antenna resonant frequency?

Let me try. If you look at the antenna as a set of lumped circuit bits,
the lower mast portion is mostly just a wire with radiation losses. The
coil and stinger form a series resonant circuit back to ground, thus a
small change in either will result in a different resonant frequency.
The antenna only presents a low, resistive load, at resonance, so
changing the stinger length (thus varying the capacitance to ground)
changes the resonance rapidly. Changing the length of the high-current
lossy feeder pipe below the coil does little, though it will increase
efficiency if you make it as tall as possible.

--
Dave Oldridge+
ICQ 1800667

Dave Oldridge wrote:
My question is one of Physics.

Lest anyone think I have invented something new, I'm sure
Walter Maxwell was doing these sorts of Smith Chart
calculations while I was in high school.

Given the following 1/4WL resonant stub with Z01 = 600
ohms and Z02 = 4000 ohms:

Source----Z01A----x----Z02----y----Z01B----open

Boundary conditions a The feedlines are lossless. The
source sees zero ohms. The Z01A length is 45 degrees. The
Z02 length is 45 degrees. How long is the Z01B length?

This is a problem easily solved by using the outside circles
of two Smith Charts, one normalized for 600 ohms and the
other normalized for 4000 ohms. The procedure is:

Assuming the source is seeing zero ohms, calculate the
impedance at 'x'. Plot that impedance on a Z02 Smith Chart.
Calculate the impedance at point 'y' of the Z02 length. Plot
that impedance on the first Smith Chart and see how long
Z01B needs to be to reach to infinity.
--
73, Cecil http://www.qsl.net/w5dxp

Dave wrote:
Cecil --- you are begging the question!!

Of course I am, nobody knows *why* things are the way they
are. There is no 'why' built into quantum mechanics. There
is only probability. Why does one photon wind up in an
inner interference ring and one wind up in an outer
interference ring? Nobody knows.

I reported that the addition of 3 or 4 inches above the coil produces a
much larger shift in frequency than adding the exact same length below
the coil [by almost a factor of 100:1]. That is empirical data. It is
real and measurable.

My question is one of Physics. I think everyone who reads this list
knows it happens. The real question is: Why?? Why the difference in
antenna resonant frequency?

I thought my stub examples would answer that question. Anything
done below the coil affects the number of degrees subtracted
from the antenna by the bottom element to coil interface. Anything
done to the stinger affects the number of degrees added to the
antenna by the coil to stinger interface. For a given element delta
length, the number-of-degrees-added effect is greater than the number-
of-degrees-subtracted effect.

Let's take a lossless resonant electrical 1/4WL stub where Z01 = 600
ohms and Z02 = 4000 ohms.

Source-----Z01A-----+-----Z02-----+-----Z01B-----open

Let's assume that Z01A is 45 degrees and Z02 is 45 degrees.
How many degrees does Z01B have to be to make the stub an
electrical 90 degrees long? Hint: 36.5 degrees is lost at
the Z01A to Z02 junction.
--
73, Cecil http://www.qsl.net/w5dxp

Cecil Moore wrote:
Dave wrote:

Cecil --- you are begging the question!!


Of course I am, nobody knows *why* things are the way they
are. There is no 'why' built into quantum mechanics. There
is only probability. Why does one photon wind up in an
inner interference ring and one wind up in an outer
interference ring? Nobody knows.


Methinks you have left the world of deterministic Physics and moved into Quantum
Physics. Such comments do not contribute to the answer. You are not using
Quantum mechanics to try to explain to me what is happening. You are using
transmission line models. Stick to the transmission line model.

I thought antenna EM Physics was deterministic.

- - - -

your model.

You stated:

"Anything done below the coil affects the number of degrees subtracted
from the antenna by the bottom element to coil interface. Anything
done to the stinger affects the number of degrees added to the
antenna by the coil to stinger interface. For a given element delta
length, the number-of-degrees-added effect is greater than the number-
of-degrees-subtracted effect."

The 4 inch spring is approximately 0.646 degrees long at 5.3 MHz.

Original: Source-----Z01A-----+-----Z02-----+-----Z01B-----open

Case #1: Source-----Z01A-----+-----Z02-----+-0.646-+-----Z01B-----open

Case #2: Source-----Z01A-----+-0.646-+-----Z02-----+-----Z01B-----open

Z01A has not changed. Z02 has not changed. Z01B has not changed. [The L/D ratios
for these components have remained the same and the Zo of each element is
therefore the same.] The physical characteristics of 0.646 have not changed. The
Capacitance to the coil is the same in either case. Only the location has changed.

Do you claim the complex impedance of 0.646[case 1] is different from the
complex impedance of 0.646[case 2]. Why?

Or, do you claim the net Z01B + 0.646[Case 1} is now electrically longer, in
degrees, than Z01A + 0.646[case 2]. Why?

Dave wrote:
Or, do you claim the net Z01B + 0.646[Case 1} is now electrically
longer, in degrees, than Z01A + 0.646[case 2]. Why?

I already explained how to prove to yourself why those
two cases are different. Please perform the Smith Chart
gymnastics and then report your findings.
--
73, Cecil http://www.qsl.net/w5dxp

Cecil Moore wrote:
Dave wrote:
Or, do you claim the net Z01B + 0.646[Case 1} is now electrically
longer, in degrees, than Z01A + 0.646[case 2]. Why?

I already explained how to prove to yourself why those
two cases are different. Please perform the Smith Chart
gymnastics and then report your findings.

If one takes one foot of stinger and transfers it to the
base element section, the antenna length remains the same
and the resonant frequency increases. I assume you already
knew that. So of course, 0.646 is electrically longer when
it is on top of the coil than when it is on the bottom of
the coil. The Smith Chart exercise will show why that is
true.
--
73, Cecil http://www.qsl.net/w5dxp