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.

"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

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

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

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

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?