Frequency Sensitivity of mobile HF vertical antennas.
 

My setup is Hustler MO-2 with center loading coils and top stinger [appropriate
for each band].

For the sake of this discussion I'll refer to my 60 meter antenna. The antenna
is tuned to resonance at 5.3715 MHz using an MFJ-259B. Adding approximately 4
inches [small spring for coil strain release] immediately below the coil and at
the top of the MO-2 has negligible effect on resonance [a few KHz]. Now, varying
the stinger above the loading coil by 4 inches causes very large changes in
resonant frequency of over 300 KHz.

What's happening in the world of Antenna Physics to account for the change in
sensitivity as a function of position above or below the loading coil?

Having asked the question, I'll slide back into the internet ether, wait for
sunspots to improve, and read the opinions and comments that follow.

Frequency Sensitivity of mobile HF vertical antennas.
 

"Dave" wrote in message
. ..
My setup is Hustler MO-2 with center loading coils and top stinger
[appropriate for each band].

For the sake of this discussion I'll refer to my 60 meter antenna. The
antenna is tuned to resonance at 5.3715 MHz using an MFJ-259B. Adding
approximately 4 inches [small spring for coil strain release] immediately
below the coil and at the top of the MO-2 has negligible effect on
resonance [a few KHz]. Now, varying the stinger above the loading coil by
4 inches causes very large changes in resonant frequency of over 300 KHz.

What's happening in the world of Antenna Physics to account for the change
in sensitivity as a function of position above or below the loading coil?

Having asked the question, I'll slide back into the internet ether, wait
for sunspots to improve, and read the opinions and comments that follow.


I don't know the "Q" of your particular antenna, but changing the whip
length will definitely have an effect! The voltage portion of the antenna
(bottom) won't make much difference, but the "current" (amperes of RF
current) will make a difference because we "tune" to resonance to match
*frequency*, and the coil and whip are what control that. Maybe I don't
explain it very well, but, I am sure others will elaborate! :)

73

Jerry

Frequency Sensitivity of mobile HF vertical antennas.
 

ORIGINAL MESSAGE:

On Fri, 11 Aug 2006 20:05:58 -0400, Dave wrote:


What's happening in the world of Antenna Physics to account for the change in
sensitivity as a function of position above or below the loading coil?

------------ REPLY SEPARATOR ------------

That behavior is perfectly normal. Length above the coil has far more
effect than length below the coil. Mobile operators have observed that
effect forever. Perhaps someone else can explain the physics involved?

Bill, W6WRT

Frequency Sensitivity of mobile HF vertical antennas.
 

Bill Turner wrote:

Dave wrote:
What's happening in the world of Antenna Physics to account for the change in
sensitivity as a function of position above or below the loading coil?

That behavior is perfectly normal. Length above the coil has far more
effect than length below the coil. Mobile operators have observed that
effect forever. Perhaps someone else can explain the physics involved?

A loading coil on an electrical 1/4WL mobile antenna can be
conceptually understood as an electrical 1/4WL stub. Consider
the following two electrical 1/4WL stubs.

Source-----Z01------+-------Z02----------open (example 1)

Source----Z02----+----Z01----+----Z02----open (example 2)

Assume Z01 = 4000 ohms, VF1 = 0.02; Z02 = 600 ohms, VF2 = 1.0

Although somewhat counterintuitive, the length of the piece
of Z01 line in the second example needs to be longer than
the piece of Z01 line in the first example to achieve an
electrical 1/4WL stub. Laying it out on a Smith Chart will
uncover the reasons.

In fact, if we create an electrical 1/4WL stub like this:

Source-----Z02-----+-----Z01-----open (example 3)

the sum of the electrical lengths of the two sections will be
electrically *longer than 1/4WL*. This is equivalent to putting
the loading coil at the very top of a mobile antenna with no
stinger.

Degrees of antenna are gained at a Z01--Z02 discontinuity,
i.e. at the loading coil to stinger transition point in
example 1 above.

Degrees of antenna are lost at a Z02--Z01 discontinuity,
i.e. at the base element to loading coil transition point
in example 2 above.
--
73, Cecil http://www.qsl.net/w5dxp

Frequency Sensitivity of mobile HF vertical antennas.
 

ORIGINAL MESSAGE:


That behavior is perfectly normal. Length above the coil has far more
effect than length below the coil. Mobile operators have observed that
effect forever. Perhaps someone else can explain the physics involved?

A loading coil on an electrical 1/4WL mobile antenna can be
conceptually understood as an electrical 1/4WL stub. Consider
the following two electrical 1/4WL stubs.


Source-----Z01------+-------Z02----------open (example 1)

Source----Z02----+----Z01----+----Z02----open (example 2)

Assume Z01 = 4000 ohms, VF1 = 0.02; Z02 = 600 ohms, VF2 = 1.0

Although somewhat counterintuitive, the length of the piece
of Z01 line in the second example needs to be longer than
the piece of Z01 line in the first example to achieve an
electrical 1/4WL stub. Laying it out on a Smith Chart will
uncover the reasons.

In fact, if we create an electrical 1/4WL stub like this:

Source-----Z02-----+-----Z01-----open (example 3)

the sum of the electrical lengths of the two sections will be
electrically *longer than 1/4WL*. This is equivalent to putting
the loading coil at the very top of a mobile antenna with no
stinger.

Degrees of antenna are gained at a Z01--Z02 discontinuity,
i.e. at the loading coil to stinger transition point in
example 1 above.

Degrees of antenna are lost at a Z02--Z01 discontinuity,
i.e. at the base element to loading coil transition point
in example 2 above.

------------ REPLY SEPARATOR ------------

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".

Bill, W6WRT

Frequency Sensitivity of mobile HF vertical antennas.
 

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

Frequency Sensitivity of mobile HF vertical antennas.
 

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?

Frequency Sensitivity of mobile HF vertical antennas.
 

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

Frequency Sensitivity of mobile HF vertical antennas.
 

Dave wrote in
:

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

Frequency Sensitivity of mobile HF vertical antennas.
 

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