John wrote:
"To achieve resonance, non-folded dipoles/monopoles must be cut slightly
less than 1/2 or 1/4 wave due to "end effect", so I`ve read. EZNEC
agrees.

Is this true for the folded dipole/monopole?"

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---."

Wave velocity on the antenna wire is very important.

End effect is sometimes defined as the capacitive effect at the ends of
a 1/2-wave antenna.

Length in free-space wavelengths is reduced in an antenna by a "fat"
radiator. The 1/2-wave resonance point (first resonance in an ordinary
open-circuit dipole), for a thin radiator, produces a drivepoint
resistance of 72 ohms. This can be cut in half by using conductors with
a very large periphery. Radiators of large periphery slow the wave
velocity along the surface. This reduces the physical length required
for electrical resonance.

Feedpoint resistance of the center-fed 1/2-wave dipole results from
far-end reflections.In the open-circuit dipole, the high impedance at
the ends of the radiators is transformed by the 1/4-wave return to the
feedpoint into a low impedance. The short-circuit at the far ends of a
1/2-wave folded dirole is reflected to the drivepoint as a high
impedance.

Constructed of the same size wire, a folded dipole has 2 wires
effectively in parallel as radiators and their size is enhanced by
spacing so that wave velocity is reduced more than in the open-circuit
dipole.

The slower the antenna wave velocity, the shorter the length to produce
resonance. I would expect more "end effect" in the usual folded dipole /
monopole.

Best regards, Richard Harrison, KB5WZI

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



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Cecil, W6DXP wrote:
"Bottom line: The currents flowing in a folded dipole 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."

Nicely said! Agreed that it is the common-mode currents which radiate,
but differential-mode currents exist too. At the tips of the folded
dipole, some current turns the corner and flows in the opposite
direction after its U-turn. This creates a situation much like the
reflection from the open circuit ends of the common dipole. At the
center of the continuous wire which ties the tips of the driven side of
the folded dipole together, the colliding currents have traveled the
same distance at the same velocity so they are still 180-degrees
out-of-phase. This amounts to a short-circuit, and in fact this amounts
to a zero voltage point which may be grounded without consequences in
most cases.

In the folded monopole (unipole), the currents into the input terminals
flow nearly as they would flow into any 1/4-wave short-circuit stub.
The difference seems to be that the grounded side of the transmission
line feeds both its side of the folded unipole and the ground plane,
creating an opportunity for imbalance and radiation. And, radiate it
does with very nearly the same characteristics as an open-circuit ground
plane antenna.

Best regards, Richard Harrison, KB5WZI

Now that it`s posted, I see I moved Cecil back to 6-land. That would be
unfortunate and I apologize. Happily, Cecil is now back in Texas where
he can comingle with the Sidewalk Cattlemen, I believe.
Cecil`s call: W5DXP

Best regards, Richard Harrison, KB5WZI

I have absolutely no experience with analyzing a folded dipole. I would
like to take advantage of this thread to ask for some information.
I had thought of a folded dipole as an antenna with a pair of 1/4 wave
shorted stubs across its feed point. I thought the antenna length for a 1/2
wave dipole would be the same if it was fed as a 72 ohm dipole or fed as a
300 ohm folded dipole. But, if there was some dielectric loading in the
twin lead it would make it necessary to account for the VP of the twin lead
used for the *folded dipole" so the impedance of the stubs would be
accounted for.

Specifically -- If a 1/2 wave folded dipole an antenna is constructed
with a twin lead with a VP of 0.9, and the total antenna length is close to
1/2 wave, the shorted stubs will impose a capacitive reactance shunting the
feed point impedance.
But, when I take my thinking to extreme configurations, my ideas seem to
fail.

I'd sure like to get some "real life" data. I'm sure some of you guys
know if the real life folded dipole gets measurably effected by VPs of 0.8
or 0.9.

Jerry



"Cecil Moore" wrote in message
...
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



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

Personal experience testimonials are often unreliable. Modeling data are
often more complete, skillfully designed, well executed, and less
colored by the operator`s opinion, by the right modeler.

One highly educated, experienced, and competent source is L.B. Cebik,
W4RNL. The plain folded dipole is just one of the many antenna types he
has modeled and enriched his web pages with. He compares it with the
common open-circuit single wire dipole in his analysis.

Just search on "folded dipole". Cebik`s web pages will appear near the
top of your list of options. Click on the most likely of your options
and you are there.

Best regards, Richard Harrison, KB5WZI

Richard

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.
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.
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.


Jerry



"Richard Harrison" wrote in message
...
Jerry Martes wrote:
"I`d sure like to get some "real life" data."

Personal experience testimonials are often unreliable. Modeling data are
often more complete, skillfully designed, well executed, and less
colored by the operator`s opinion, by the right modeler.

One highly educated, experienced, and competent source is L.B. Cebik,
W4RNL. The plain folded dipole is just one of the many antenna types he
has modeled and enriched his web pages with. He compares it with the
common open-circuit single wire dipole in his analysis.

Just search on "folded dipole". Cebik`s web pages will appear near the
top of your list of options. Click on the most likely of your options
and you are there.

Best regards, Richard Harrison, KB5WZI

Jerry Martes wrote:
"It seems that the 1/4 wave stubs that shunt the feed point might
strongly affect the input impedance."

Not at the resonant frequency.

Don`t sweat twin lead VF in an antenna as radiation is from common-mode
current.

The marvelous 19th edition of the ARRL Antenna Book has Fig 17 on page
24-14. It`s "Lumped-constant circuit equivalents of open and
short-circuited transmission lines". Note (A), the top figure, which is
a short-circuited stub of any length less than 1/4 wavelength. It is an
inductance! Also note (C). It is a short-circuited stub of exactly
1/4-wave:

Equal to a parallel resonant circuit, a "very high" impedance. So high
in fact that many in parallel would not be noticed.

Best regards, Richard Harrison, KB5WZI

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:

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

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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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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