Jerry Martes wrote:
I have a severly limited capacity for understanding alot of theoritical
presentations. I did read some of Cebik's information, and I still wonder
if the velocity of propagation of the "twin lead" used for the folded dipole
can be ignored. I wonder if the VP of the twin lead is an important
consideration when designing a folded dipole.

The VF of twin-lead used for a folded dipole is approximately the same
as the VF of insulated wire used for a dipole. I'm just not sure how
much "end effect" actually exists in a folded dipole since there is,
technically, no end.

If the resonant frequency of a folded dipole is identified as that
frequency where the input (feed point) impedance is R+/-j0, it seems that
the 1/4 wave stubs that shunt the feed point might strongly effect the input
impedance.

There seems to be some confusion about exactly how the feedline connects
to the folded dipole. Here is the correct way:

+------------------------------------------------------------------+
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+-------------------------------+ +-------------------------------+
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Here is the incorrect way:

|
|
+-------------------------------+--------------------------------+
| |
+-------------------------------+--------------------------------+
|
|

For the incorrect way, the feedline is indeed seeing two shorted 1/4WL
stubs in parallel. The currents for the incorrect way would be 180 deg
out of phase and defeat the purpose of the antenna.

However, for the correct way, the currents in the adjacent wires are
in phase and there is a current phase reversal (current minimum point) at
each end of the antenna. After all, a folded dipole is just a one-wavelength
loop with the conductors brought close together. When Mr. Moore invented
the Quad beam, he envisioned a folded dipole with its conductors being
separated incrementally by a distance until it came out to be a square.

The key to understanding the folded dipole is that there is an electrical
phase reversal at the same point as a physical 180 degree reversal.
180 + 180 = 360 degrees, i.e. in phase.

I recognize that there is a good chance that my reasoning is wrong. It
may be that the "stubs" that I consider to be shunting the fed point, are
not acting the same as a 'non disipative' stub. But, this is where my mind
could benefit from having some "lab data" which is what I refer to as "real
life" data.

A classic stub is a current-balanced device with the currents 180 degrees out
of phase. That is not true for a folded dipole antenna. Therefore, a folded
dipole antenna is not composed of true stubs. Semantics strikes again. A series
"stub" is different enough from a parallel "stub" that we probably should not
use the same word for the two of them.
--
73, Cecil http://www.qsl.net/w5dxp



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The currents in the wires of a folded dipole or monopole are neither in
phase nor 180 degrees out of phase, as you can easily see from EZNEC.
Because they obey superposition, you can, for convenience, consider them
as two separate sets of currents, common mode (or "antenna" current) and
one differential mode (or "transmission line" current). Neither one is
zero. The phase of the antenna current is a function of the velocity
factor of the two wires excited in parallel. For TV twinlead, for
example, this would be something like 3 percent slower than for bare
wire -- about the same as any other typical insulated wire. In contrast,
the phase of the transmission line current is dictated by the velocity
factor of the two wires excited out of phase as a transmission line. In
this mode, there's an intense field between the conductors, so the
dielectric between the conductors has much more impact. The velocity
factor for this mode is more like 0.8, so the transmission line will be
electrically considerably longer than a quarter wavelength.

The TV twinlead "antenna" will be resonant, then, at a length about 3%
shorter than if it were made from two parallel bare wires -- not
because the currents in the two wires are in phase, but because the
common mode part of the currents are in phase -- by definition, in fact.
But the effect of the transmission line stub also affects the feedpoint
impedance, and its velocity factor has to be accounted for in the
calculation of its contribution. I've seen a recommendation that the
conductors of a twinlead folded dipole be shorted about 80% of the way
from the center. What this does is to make the transmission line nearly
a quarter wavelength long, so its contribution to the feedpoint
impedance is negligible. Then you don't need to make any adjustment of
the length to compensate for the transmission line. Alternatively, you
can short circuit the wires at the ends in the normal fashion, and
slightly adjust the length to compensate for the impedance change caused
by the transmission line.

Roy Lewallen, W7EL

Cecil Moore wrote:
Richard Harrison wrote:

My ARRL Antenna Book (19th edition, page 8) says:

"Since the antenna section (of 300-ohm twin-lead) does not operate as a
transmission line, but simply as two wires in parallel, the velocity
factor of twin-lead can be ignored in computing the antenna length."

I wish the author had said:

"---the transmission line velocity factor of twin-lead can be
ignored---."


The phase of the currents in the adjacent sections of twinlead is what
is important. If the phase of the adjacent currents is 180 degrees, the
twinlead is acting like a transmission line and T-line VF must be taken
into account. If the phase of the adjacent currents is zero degrees, the
twinlead is acting like an antenna and the VF is considerably higher,
essentially equal to insulated wire. If the phase of the adjacent currents
is zero degrees, all the current is "common-mode current", something not
desirable for transmission lines but something most desirable for antennas
since common-mode currents do not inhibit radiation.

Bottom line: The currents flowing in a folded dipole antenna are common-
mode currents which radiate, not transmission line currents which do not
radiate (much), and that's a very good thing for an antenna.
--
73, Cecil http://www.qsl.net/w5dxp

Jerry Martes wrote:
I had thought of a folded dipole as an antenna with a pair of 1/4 wave
shorted stubs across its feed point.

"Stub" has more than one meaning and therefore, more than one response.

Your definition of "stub" seems to assume the currents are 180 degrees
out of phase. Therefore, you should not use the word "stub" on a folded
dipole antenna where the currents are in phase.

It's a semantics problem. If you revise your definition of "stub" to
include stubs with in phase currents, you must give up on your present
definition of stubs with only out of phase currents.

Most of us have a feel for the difference between a parallel stub
fed from a line with balanced currents and a series stub where the
currents can have any phase relationship. "Series stubs" is a very
confusing topic and could support a technical article of some kind.
I don't remember it ever being explained in detail before, at least
in the amateur literature.
--
73, Cecil http://www.qsl.net/w5dxp



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Roy

Thanks for the clear and meaningfull response to my post. I'm not
qualified to enter this discussion. I'm in the learning mode. I did wonder
if the dielectricly loaded "shorted stub" transmission line (1/2 the folded
dipole) wuld have a shortning capability for determining antenna length. As
I read it, the VP of the twin lead does effect the folded dipole's length to
aceive resonance. And, the amount of "shortning effect" is somewhere
between 20 percent and 3 percent in your example of twin lead with VP=0.8
..Please correct me if I'm wrong, but, I'd expect the "shortning effect" to
be much closer to the 3 percent end of the scale for reasons that would be
too confusing for me to try to explain. ( besides, I'm probably wrong in my
thinking)

Jerry




"Roy Lewallen" wrote in message
...
The currents in the wires of a folded dipole or monopole are neither in
phase nor 180 degrees out of phase, as you can easily see from EZNEC.
Because they obey superposition, you can, for convenience, consider them
as two separate sets of currents, common mode (or "antenna" current) and
one differential mode (or "transmission line" current). Neither one is
zero. The phase of the antenna current is a function of the velocity
factor of the two wires excited in parallel. For TV twinlead, for
example, this would be something like 3 percent slower than for bare
wire -- about the same as any other typical insulated wire. In contrast,
the phase of the transmission line current is dictated by the velocity
factor of the two wires excited out of phase as a transmission line. In
this mode, there's an intense field between the conductors, so the
dielectric between the conductors has much more impact. The velocity
factor for this mode is more like 0.8, so the transmission line will be
electrically considerably longer than a quarter wavelength.

The TV twinlead "antenna" will be resonant, then, at a length about 3%
shorter than if it were made from two parallel bare wires -- not
because the currents in the two wires are in phase, but because the
common mode part of the currents are in phase -- by definition, in fact.
But the effect of the transmission line stub also affects the feedpoint
impedance, and its velocity factor has to be accounted for in the
calculation of its contribution. I've seen a recommendation that the
conductors of a twinlead folded dipole be shorted about 80% of the way
from the center. What this does is to make the transmission line nearly
a quarter wavelength long, so its contribution to the feedpoint
impedance is negligible. Then you don't need to make any adjustment of
the length to compensate for the transmission line. Alternatively, you
can short circuit the wires at the ends in the normal fashion, and
slightly adjust the length to compensate for the impedance change caused
by the transmission line.

Roy Lewallen, W7EL

Cecil Moore wrote:
Richard Harrison wrote:

My ARRL Antenna Book (19th edition, page 8) says:

"Since the antenna section (of 300-ohm twin-lead) does not operate as a
transmission line, but simply as two wires in parallel, the velocity
factor of twin-lead can be ignored in computing the antenna length."

I wish the author had said:

"---the transmission line velocity factor of twin-lead can be
ignored---."


The phase of the currents in the adjacent sections of twinlead is what
is important. If the phase of the adjacent currents is 180 degrees, the
twinlead is acting like a transmission line and T-line VF must be taken
into account. If the phase of the adjacent currents is zero degrees, the
twinlead is acting like an antenna and the VF is considerably higher,
essentially equal to insulated wire. If the phase of the adjacent
currents
is zero degrees, all the current is "common-mode current", something not
desirable for transmission lines but something most desirable for
antennas
since common-mode currents do not inhibit radiation.

Bottom line: The currents flowing in a folded dipole antenna are common-
mode currents which radiate, not transmission line currents which do not
radiate (much), and that's a very good thing for an antenna.
--
73, Cecil http://www.qsl.net/w5dxp

When the transmission line portion is nearly a quarter wavelength long,
it has very little effect on the feedpoint impedance, so the resonant
length will be about the same as it would be if the transmission line
didn't exist. In the case of TV twinlead, though, if shorted at the
ends, the transmission line can make a noticeable impact on the
feedpoint impedance. It would take only a few minutes with EZNEC, using
the method I described earlier of modeling a folded dipole or monopole
as an unfolded dipole or monopole with a transmission line stub in
parallel with the source, to see how great the impact is. With this
model, you can give the transmission line whatever velocity factor or
length you choose, independently of the "antenna" portion of the model.
If you use the same twinlead to make antennas for different bands, the
impact of the transmission line will be different on each band because
the transmission line isn't being scaled with frequency. Therefore, any
conclusion you reach about the impact of the transmission line on the
antenna length will be quantitatively correct only at one frequency
(and, of course, only the assumed type of twinlead).

I don't have the time right now to do the modeling, but if you're truly
interested, you won't mind taking the time to do it yourself.

Roy Lewallen, W7EL

Jerry Martes wrote:

Roy

Thanks for the clear and meaningfull response to my post. I'm not
qualified to enter this discussion. I'm in the learning mode. I did wonder
if the dielectricly loaded "shorted stub" transmission line (1/2 the folded
dipole) wuld have a shortning capability for determining antenna length. As
I read it, the VP of the twin lead does effect the folded dipole's length to
aceive resonance. And, the amount of "shortning effect" is somewhere
between 20 percent and 3 percent in your example of twin lead with VP=0.8
.Please correct me if I'm wrong, but, I'd expect the "shortning effect" to
be much closer to the 3 percent end of the scale for reasons that would be
too confusing for me to try to explain. ( besides, I'm probably wrong in my
thinking)

Jerry

Roy

I'm honored that you take time to post this information for my education.
It is almost an unbeleivable coincidence that _you_ mention EZNEC. I had
that program recommended to me by a friend who told me about it this past
week end. I want to try to understand (see) what kind of pattern I'm
getting from an antenna I made for Weather Sattelite (137 MHz) reception.
I have downloaded the EZNEC and only my other projects have kept me from
starting to learn how to use it.

Thanks
Jerry


"Roy Lewallen" wrote in message
...
When the transmission line portion is nearly a quarter wavelength long,
it has very little effect on the feedpoint impedance, so the resonant
length will be about the same as it would be if the transmission line
didn't exist. In the case of TV twinlead, though, if shorted at the
ends, the transmission line can make a noticeable impact on the
feedpoint impedance. It would take only a few minutes with EZNEC, using
the method I described earlier of modeling a folded dipole or monopole
as an unfolded dipole or monopole with a transmission line stub in
parallel with the source, to see how great the impact is. With this
model, you can give the transmission line whatever velocity factor or
length you choose, independently of the "antenna" portion of the model.
If you use the same twinlead to make antennas for different bands, the
impact of the transmission line will be different on each band because
the transmission line isn't being scaled with frequency. Therefore, any
conclusion you reach about the impact of the transmission line on the
antenna length will be quantitatively correct only at one frequency
(and, of course, only the assumed type of twinlead).

I don't have the time right now to do the modeling, but if you're truly
interested, you won't mind taking the time to do it yourself.

Roy Lewallen, W7EL

Jerry Martes wrote:

Roy

Thanks for the clear and meaningfull response to my post. I'm not
qualified to enter this discussion. I'm in the learning mode. I did
wonder
if the dielectricly loaded "shorted stub" transmission line (1/2 the
folded
dipole) wuld have a shortning capability for determining antenna length.
As
I read it, the VP of the twin lead does effect the folded dipole's
length to
aceive resonance. And, the amount of "shortning effect" is somewhere
between 20 percent and 3 percent in your example of twin lead with
VP=0.8
.Please correct me if I'm wrong, but, I'd expect the "shortning effect"
to
be much closer to the 3 percent end of the scale for reasons that would
be
too confusing for me to try to explain. ( besides, I'm probably wrong
in my
thinking)

Jerry

No, it's no coincidence that I mention EZNEC. It's my ticket out of the
cube farm, and my means of escape from the Dilbert cartoon.

Roy Lewallen, W7EL

Jerry Martes wrote:

Roy

I'm honored that you take time to post this information for my education.
It is almost an unbeleivable coincidence that _you_ mention EZNEC. I had
that program recommended to me by a friend who told me about it this past
week end. I want to try to understand (see) what kind of pattern I'm
getting from an antenna I made for Weather Sattelite (137 MHz) reception.
I have downloaded the EZNEC and only my other projects have kept me from
starting to learn how to use it.

Thanks
Jerry

Jerry Martes wrote:
"I`d like to get some "real life" data."

End effect on dipoles and monopoles has little do with comparitive
performance. See Cebik. It`s more associated with where the antenna is
with respect to ground, and how fat or slim the conductors are.

Jerry also wrote:
"I want to understand (see) what kind of pattern I`m getting from an
antenna I made for Weather Satellite (137 MHz) reception."

That might be for the NOAA polar orbiters. On page 19-8 of the 19th
edition of the ARRL "Antenna Book" is a section on Antennas for
Satellite Work. Circular polarization is considered ideal. A couple of
NOAA polar orbiting satellites are active and others may be functioning
as backups. Active NOAA orbiters may fly high over the horizon a couple
of times a day, so you need a program to tell you where, when, and how
high above the horizon the flyover will occur. Some are available at no
cost, I hear. 15 minutes of visibility, 7 minutes coming and 7 minutes
of going may be available on a pass.

On page 19-10 of the "Antenna Book" are shown the patterns of individual
dipoles and as used together as an omnidirectional turnstile.

The quadrifilar helix is also considered good as a weather satellite
receiving antenna. This antenna is shown on page 292 of J.D. Kraus`
third edition of "Antennas for All Applications" in Figure 8-67.

No need for concern about the pattern obtained from folded dipoles if
you are OK with the pattern of an ordinary open-circuit dipole. They are
exactly the same. The difference is only in the inherent impedance
transformation of the folded antenna, dipole or monopole. That`s it.
Antenna catalogs list gain as 0 dBd for open-circuit dipole, folded
dipole, ground plane, or folded monopole. Patterns shown are the same
for open-circuit and folded dipoles. Patterns shown for a folded
monopole are identical with those shown for an open-circuit monpole
(ground plane).

One catalog lists a thin-wire common dipole as having 34% bandwidth and
a 60-ohm feed resistance. The folded dipole has a 45% bandwidth and a
300-ohm feedpoint because it is fatter and folded. The pattern plots are
identical.

Enjoy your EZNEC. The resuts it should prduce on ordinary and folded
dipoles are already published in many places. Makes it easy to see if
you did the EZNEC right.

Best regards, Richard Harrison, KB5WZI