John wrote:
. . .
I thought it was supposed to be backwards from the usual unfolded monopole
such that it would go up in resistance and become inductive.?.

Why would it do that?

Roy Lewallen, W7EL

"Roy Lewallen" wrote in message
...
John wrote:
. . .
I thought it was supposed to be backwards from the usual unfolded
monopole
such that it would go up in resistance and become inductive.?.

Why would it do that?

Roy Lewallen, W7EL


Well, you said earlier that the folded monopole could be modeled as an
unfolded monopole with a shorted transmission line in parallel. I thought I
understood. When I modeled the unfolded monopole, I saw it do as usual when
the element was varied in length. But when I included the shorted section of
transmission line and varied it directly with the element, I thought I saw
the terminal reactance go inductive as the length was decreased below
1/4-wave resonance and I thought the terminal resistance went up. So, I was
expecting the same from EZNEC by modeling the folded version.

I guess I'm really lost here.

John

I'll once again separate the "antenna" from the "transmission line" to
make it easier to see what's happening.

If you're dealing with an air-dielectric folded dipole, the transmission
line stub is nearly a quarter wavelength long. So at resonance, its
impedance is high and it doesn't have much effect on the feedpoint
impedance. As you lower the frequency or shorten the antenna, the
resistance of the antenna (as opposed to the transmission line) drops
fairly slowly, and the reactance becomes negative relatively quickly.
This is in parallel with the transmission line, whose reactance becomes
more positive as the line gets electrically shorter. If you look at the
net result of this parallel combination, you get a feedpoint impedance
that has a rising resistance as frequency drops or the antenna shortens,
and a reactance that gets more negative.

At some frequency below resonance, the increasing positive reactance of
the transmission line equals the negative reactance of the antenna,
creating a parallel resonant (sometimes called anti-resonant) circuit.
Just before this happens, the resistance skyrockets and the feedpoint
reactance heads positive. Exactly at parallel resonance, the reactance
is zero (by definition of resonance) and the resistance is very high.
And just below that frequency, the reactance heads rapidly to a high
positive value, then begins decreasing as the frequency drops below
that. The frequency or length where you hit anti-resonance depends on
the impedance of the transmission line. I fished up a model of a 17.56
foot high folded monopole with #12 conductors spaced 6 inches apart
which I had lying around. It's resonant at about 13.25 MHz., where its
feedpoint impedance is 143 ohms. It hits anti-resonance at about 8.5
MHz, where its feedpoint resistance is about 15k ohms. Below that, the
feedpoint reactance is positive, and decreases as the frequency is lowered.

If you want to model a folded monopole as a separate unfolded monopole
and transmission line (which is a way to model one made from twinlead,
since you can separately adjust the transmission line length to account
for the reduced velocity factor of the transmission line mode), here's
what you have to do.

First, make the unfolded monopole from two wires, connected in parallel
at the bottom and top, or from a single wire of equivalent diameter.
Next, choose the impedance of the transmission line to be 1/4 the
impedance of the actual line. You have to use a transmission line model
for this, not a transmission line made from wires. Make sure it's in
parallel, not series, with the source at the base of the monopole. In
EZNEC, a transmission line is connected in parallel with a source if
they're on the same segment. Finally, multiply the reported feedpoint
impedance by four to find the Z of the actual folded monopole.

Roy Lewallen, W7EL

John wrote:
"Roy Lewallen" wrote in message
...

John wrote:

. . .
I thought it was supposed to be backwards from the usual unfolded

monopole

such that it would go up in resistance and become inductive.?.

Why would it do that?

Roy Lewallen, W7EL



Well, you said earlier that the folded monopole could be modeled as an
unfolded monopole with a shorted transmission line in parallel. I thought I
understood. When I modeled the unfolded monopole, I saw it do as usual when
the element was varied in length. But when I included the shorted section of
transmission line and varied it directly with the element, I thought I saw
the terminal reactance go inductive as the length was decreased below
1/4-wave resonance and I thought the terminal resistance went up. So, I was
expecting the same from EZNEC by modeling the folded version.

I guess I'm really lost here.

John

"Roy Lewallen" wrote in message
...
I'll once again separate the "antenna" from the "transmission line" to
make it easier to see what's happening.

If you're dealing with an air-dielectric folded dipole, the transmission
line stub is nearly a quarter wavelength long. So at resonance, its
impedance is high and it doesn't have much effect on the feedpoint
impedance. As you lower the frequency or shorten the antenna, the
resistance of the antenna (as opposed to the transmission line) drops
fairly slowly, and the reactance becomes negative relatively quickly.
This is in parallel with the transmission line, whose reactance becomes
more positive as the line gets electrically shorter. If you look at the
net result of this parallel combination, you get a feedpoint impedance
that has a rising resistance as frequency drops or the antenna shortens,
and a reactance that gets more negative.

At some frequency below resonance, the increasing positive reactance of
the transmission line equals the negative reactance of the antenna,
creating a parallel resonant (sometimes called anti-resonant) circuit.
Just before this happens, the resistance skyrockets and the feedpoint
reactance heads positive. Exactly at parallel resonance, the reactance
is zero (by definition of resonance) and the resistance is very high.
And just below that frequency, the reactance heads rapidly to a high
positive value, then begins decreasing as the frequency drops below
that. The frequency or length where you hit anti-resonance depends on
the impedance of the transmission line. I fished up a model of a 17.56
foot high folded monopole with #12 conductors spaced 6 inches apart
which I had lying around. It's resonant at about 13.25 MHz., where its
feedpoint impedance is 143 ohms. It hits anti-resonance at about 8.5
MHz, where its feedpoint resistance is about 15k ohms. Below that, the
feedpoint reactance is positive, and decreases as the frequency is
lowered.

If you want to model a folded monopole as a separate unfolded monopole
and transmission line (which is a way to model one made from twinlead,
since you can separately adjust the transmission line length to account
for the reduced velocity factor of the transmission line mode), here's
what you have to do.

First, make the unfolded monopole from two wires, connected in parallel
at the bottom and top, or from a single wire of equivalent diameter.
Next, choose the impedance of the transmission line to be 1/4 the
impedance of the actual line. You have to use a transmission line model
for this, not a transmission line made from wires. Make sure it's in
parallel, not series, with the source at the base of the monopole. In
EZNEC, a transmission line is connected in parallel with a source if
they're on the same segment. Finally, multiply the reported feedpoint
impedance by four to find the Z of the actual folded monopole.

Roy Lewallen, W7EL



I can see I did some things improperly. I'll go back and try again. Thanks a
lot for explaining.

John

John wrote:
"I`ll go back and try again."

John has the best help there is in Roy Lewallen, the creator of EZNEC.
The idea of breaking the behavior of a folded dipole or unipole into its
differential (transmission line)-mode and common (antenna)-mode
behaviors goes back according to Paul H. Lee in "The Amateur Radio
Vertical Antenna Handbook" to W.V. Roberts, "Input Impedance of a Folded
Dipole", RCA Review, Vol.8, No.2, June 1947, p. 289.

Around the 1/4-wave length, the folded monopole`s resistance is steadily
rising with frequency. High radiation resistance as compared with loss
is good. This happens with the open-circuit 1/4-wave vertical too.

Around the 1/4-wave length, the folded monopole undergoes an abrupt
change from inductive reactance when it is too short for resonance to
capacitive reactance when it is too long for resonance. The open-circuit
whip undergoes a similar change but it has a capacitive reactance when
it is too short for resonance and an inductive reactance when it is too
long for resonance..

One contributor to this folded monopole thread said he found a coil
shunted across the feedpoint of an Andrew Corporation folded monopole.
On page 26-12 of my 19th edition of the "ARRL Antenna Book" is described
a matching technique using such a coil. It`s called the "helical
hairpin" (with tongue in cheek). This method seems convenient, in
conjunction with length adjustment of the folded monopole, to get a 50 +
j0 impedance at the specified operating frequency. I am not privy to
Andrew`s actual practice as we just placed the orders and the antennas
worked as advertised.

Figure 17 on page 6-9 of my 19th edition of the "ARRL Antenna Book" is
very similar in appearance to the Andrew Corporation folded monopole.
There is a lot of good information in the Antenna Book on folded
antennas, and more.

Best regards, Richard Harrison, KB5WZI

"Richard Harrison" wrote in message
...
John wrote:
"I`ll go back and try again."

John has the best help there is in Roy Lewallen, the creator of EZNEC.


I agree wholeheartedly.


The idea of breaking the behavior of a folded dipole or unipole into its
differential (transmission line)-mode and common (antenna)-mode
behaviors goes back according to Paul H. Lee in "The Amateur Radio
Vertical Antenna Handbook" to W.V. Roberts, "Input Impedance of a Folded
Dipole", RCA Review, Vol.8, No.2, June 1947, p. 289.

Around the 1/4-wave length, the folded monopole`s resistance is steadily
rising with frequency. High radiation resistance as compared with loss
is good. This happens with the open-circuit 1/4-wave vertical too.


This is what I'm trying to see using EZNEC. I agree with the resistance
trend, but I keep seeing capacitive reactance below 1/4-wave resonance and
inductive reactance above 1/4-wave resonance.


Around the 1/4-wave length, the folded monopole undergoes an abrupt
change from inductive reactance when it is too short for resonance to
capacitive reactance when it is too long for resonance. The open-circuit
whip undergoes a similar change but it has a capacitive reactance when
it is too short for resonance and an inductive reactance when it is too
long for resonance..


I see no difference in the trends.


One contributor to this folded monopole thread said he found a coil
shunted across the feedpoint of an Andrew Corporation folded monopole.
On page 26-12 of my 19th edition of the "ARRL Antenna Book" is described
a matching technique using such a coil. It`s called the "helical
hairpin" (with tongue in cheek). This method seems convenient, in
conjunction with length adjustment of the folded monopole, to get a 50 +
j0 impedance at the specified operating frequency. I am not privy to
Andrew`s actual practice as we just placed the orders and the antennas
worked as advertised.

Figure 17 on page 6-9 of my 19th edition of the "ARRL Antenna Book" is
very similar in appearance to the Andrew Corporation folded monopole.
There is a lot of good information in the Antenna Book on folded
antennas, and more.

Best regards, Richard Harrison, KB5WZI


My copy of the book is the 18th edition.

John

"John" wrote in message
...

"Richard Harrison" wrote in message
...
John wrote:
"I`ll go back and try again."

John has the best help there is in Roy Lewallen, the creator of EZNEC.


I agree wholeheartedly.


The idea of breaking the behavior of a folded dipole or unipole into its
differential (transmission line)-mode and common (antenna)-mode
behaviors goes back according to Paul H. Lee in "The Amateur Radio
Vertical Antenna Handbook" to W.V. Roberts, "Input Impedance of a Folded
Dipole", RCA Review, Vol.8, No.2, June 1947, p. 289.

Around the 1/4-wave length, the folded monopole`s resistance is steadily
rising with frequency. High radiation resistance as compared with loss
is good. This happens with the open-circuit 1/4-wave vertical too.


This is what I'm trying to see using EZNEC. I agree with the resistance
trend, but I keep seeing capacitive reactance below 1/4-wave resonance and
inductive reactance above 1/4-wave resonance.

John,
For a 1/4 wave folded monopole working above a ground plane, you have to go
below the frequency where the monopole is 1/8 wavelength before it goes
inductive. For a folded DIPOLE it is 1/4 wavelength. You are already doing
EZNEC, spend another 3 minutes with it.

Tam/WB2TT

Around the 1/4-wave length, the folded monopole undergoes an abrupt
change from inductive reactance when it is too short for resonance to
capacitive reactance when it is too long for resonance. The open-circuit
whip undergoes a similar change but it has a capacitive reactance when
it is too short for resonance and an inductive reactance when it is too
long for resonance..


I see no difference in the trends.


One contributor to this folded monopole thread said he found a coil
shunted across the feedpoint of an Andrew Corporation folded monopole.
On page 26-12 of my 19th edition of the "ARRL Antenna Book" is described
a matching technique using such a coil. It`s called the "helical
hairpin" (with tongue in cheek). This method seems convenient, in
conjunction with length adjustment of the folded monopole, to get a 50 +
j0 impedance at the specified operating frequency. I am not privy to
Andrew`s actual practice as we just placed the orders and the antennas
worked as advertised.

Figure 17 on page 6-9 of my 19th edition of the "ARRL Antenna Book" is
very similar in appearance to the Andrew Corporation folded monopole.
There is a lot of good information in the Antenna Book on folded
antennas, and more.

Best regards, Richard Harrison, KB5WZI


My copy of the book is the 18th edition.

John

"Richard Harrison" wrote in message
...


Around the 1/4-wave length, the folded monopole`s resistance is steadily
rising with frequency. High radiation resistance as compared with loss
is good. This happens with the open-circuit 1/4-wave vertical too.

Around the 1/4-wave length, the folded monopole undergoes an abrupt
change from inductive reactance when it is too short for resonance to
capacitive reactance when it is too long for resonance. The open-circuit
whip undergoes a similar change but it has a capacitive reactance when
it is too short for resonance and an inductive reactance when it is too
long for resonance..


Hey, Richard -

Take a look at Roy's second paragraph:

"If you're dealing with an air-dielectric folded dipole, the transmission
line stub is nearly a quarter wavelength long. So at resonance, its
impedance is high and it doesn't have much effect on the feedpoint
impedance. As you lower the frequency or shorten the antenna, the
resistance of the antenna (as opposed to the transmission line) drops
fairly slowly, and the reactance becomes negative relatively quickly.
This is in parallel with the transmission line, whose reactance becomes
more positive as the line gets electrically shorter. If you look at the
net result of this parallel combination, you get a feedpoint impedance
that has a rising resistance as frequency drops or the antenna shortens,
and a reactance that gets more negative."

What Roy is saying is also what I'm seeing with EZNEC. You are saying the
opposite reactance occurs with a folded monopole.

John

John wrote:
"What Roy is saying is also what I`m seeing with EZNEC. You are saying
the opposite reactance occurs with a folded monopole."

On Fri. Apr. 23. 2004, 4:19 pm (CDT-2) Roy Lewallen wrote:
"This can be resonated as Richard Harrison recently pointed out, with a
series capacitor."

Why? look above in Roy`s posting:
"---EZNEC shows a feedpoint impedance of 46.1 + j1893 ohms."

The + j1893 is inductive, not capacitive. It`s the reactance shown by a
too short (less than 1/4-wave) folded monopole or short-circuit stub.

I believe I am on the same page with Roy.

Best regards, Richard Harrison, KB5WZI

(Richard Harrison) wrote ...
Around the 1/4-wave length, the folded monopole`s resistance is steadily
rising with frequency. High radiation resistance as compared with loss
is good. This happens with the open-circuit 1/4-wave vertical too.

Around the 1/4-wave length, the folded monopole undergoes an abrupt
change from inductive reactance when it is too short for resonance to
capacitive reactance when it is too long for resonance. The open-circuit
whip undergoes a similar change but it has a capacitive reactance when
it is too short for resonance and an inductive reactance when it is too
long for resonance..


Not to be picky and unncessarily perpetuate this discussion, but it's
already been correctly stated in this thread that a folded monopole or
dipole exhibits the same impedance characteristics around resonance as
a conventional antenna. That is, when it's too short for resonance,
reactance is capacitive, and is inductive if too long. And resistance
is 4 times the resistance of a conventional antenna, and actually
*increases* on either side of resonance, according to models.

The above statements are only true in the region of operation around
1/4 wavelength (folded monopole). As frequency or length is decreased
more significantly, past the anti-resonant point (something that
doesn't happen with conventional 1/4-wavelength monopoles), its
characteristics take on a completely different twist, where reactance
suddenly becomes (and stays) inductive and decreasing, and resistance
decreases rapidly.

A folded monopole (or folded dipole) is, in some respects, like two
different antennas, with two different sets of characteristics,
depending on whether you are operating above or below anti-resonance.
One of the problems in discussing folded monopoles/dipoles is because
of just this reason -- you simply can't make general statements about
how it works unless you also provide some of the parametric
assumptions.

Al WA4GKQ