Folded monopole dilemma
 

Good evening, Gentlemen.

A thought experiment:

Start with a regular 1/4-wave monopole ground plane. The literature says it
looks like half the value of a dipole, about 35 Ohms, when resonant. It
would be nice to have the resistance at the terminals be a bit higher, and I
very much value a grounded element anyway, so let's let it evolve into a
folded monopole. The literature says it should now have about 4 times the
terminal resistance of the original 1/4-wave we started with (about 140
Ohms). Huh. Now it's a bit high.

They tell me that shortening the antenna below resonance will lower the
resistance and introduce capacitance. But I think I have also seen in the
literature that the antenna can be viewed as a transmission line. A shorted
portion of parallel conductor transmission line (the folded monopole) less
than 1/4-wave long looks inductive. But wait! Which will win? Will the
shortness of the antenna look capacitive or will the transmission line
dominate and the antenna will look inductive?

Even better, is there some choice of the folded section wire diameters and
spacing that will give an inductance that will exactly offset the
capacitance due to shortness? So, then, is there a folded monopole of such
dimensions that the resistance is 50 Ohms (due to being shorter than 1/4
wave) with no terminal reactance (due to the inductive design of the
"transmission line" cancelled by the shortness of the antenna's
capacitance)?

Brain hurts.

John, KD5YI

Sorry!


"The other John Smith" wrote in message
nk.net...

I've recently done some NEC-2 (MultiNEC) modeling of folded dipoles
which might help answer some of your questions. Translating the
results to folded monoploes should be fairly straightforward.

The model is a half-wave folded dipole for 14.2 MHz in freespace,
resonant at 33.15 feet using #18 wire with 2 inch spacing. The
center-fed input impedance is 289 - j0.01, which is 4 times the
resonant impedance of 72 ohms for a conventional dipole. A folded
1/4-wavelength monopole would have half that impedance, or about 144
ohms.

Examining the R-X curves for this dipole shows that it has
characteristics very similar to a 3/2-wavelength dipole, operating at
its third harmonic, and on a relatively low-slope part of the curves,
indicating a low Q and good bandwidth, similar to a fat dipole.

Shortening the antenna increases capacitive reactance, as might be
expected. However, input resistance *increases* as the length
decreases, which is contrary to our experience with common
1/2-wavelength dipoles. This is because we're on the high side of
full-wave resonance, where very high resistance values exist at its
peak. As we shorten the antenna, we're climbing the full-wave
resistance curve, which peaks when the antenna length is 22 feet. If
we further shorten the antenna past full-wave resonance, we now begin
experiencing a "normal" decrease in resistance as we "slide" back down
the low side of the full-wave resistance spike. However, capacitive
reactance has now quickly changed to inductive reactance as we crossed
full-wave resonance.

If we continue to shorten the folded antenna length, we come to a
length of about 17 feet where the input impedance is 50 + j2000 ohms.
Notice that the impedance is *inductive*, not capacitive as we are
accustomed to seeing with ordinary short dipoles. The inductive 2000
ohms can be cancelled with a series capacitor (or other suitable
matching network). Q has increased (because we're on a relatively
steep part of the R-X curves) and bandwidth has narrowed considerably
from the resonance at 33.15 feet.

So, by reducing the length of the 1/2-wavelength folded dipole from
33.15 feet to 17 feet, we have a 50 ohm resistive impedance by
matching the inductive reactance with a capacitor (or split capacitor)
instead of the usual lossy, low-Q loading coils. Gain and patterns
appear to be the same as a conventional dipole.

Translated to a monopole, the length would be a little more than half
the dipole's 17 feet, to boost feed point resistance from 25 ohms to
50 ohms. My guess is (I haven't modeled it) that this antenna
functions much like a 3/8-wavelength monopole, although much shorter.
Actually building this antenna and placing it the real world will
obviously change the above values.

Unfortunately, it doesn't appear that any combination of element size
and spacing will offset the need for impedance matching with the
shortened folded dipole or monopole.

I hope this makes sense. I'm sure Roy, Cecil, Tom, and others might
have comments/corrections that will be helpful to me and others who
are relative neophytes in the wonderful world of antennas.

Al WA4GKQ

Even better, is there some choice of the folded section wire diameters and
spacing that will give an inductance that will exactly offset the
capacitance due to shortness? So, then, is there a folded monopole of such
dimensions that the resistance is 50 Ohms (due to being shorter than 1/4
wave) with no terminal reactance (due to the inductive design of the
"transmission line" cancelled by the shortness of the antenna's
capacitance)?

Modeling a 17 foot folded dipole made from copper #18 wire spaced at 2
inches at 14.2 MHz with EZNEC shows a feedpoint impedance of 46.1 +
j1893 ohms. This can be resonated, as Richard Harrison recently pointed
out, with a series capacitor. There's no free lunch, though -- at 1 kW,
the voltage across the capacitor is almost 9000 V RMS (about 12,000
volts peak), and even at 10 watts, it's almost 900 volts RMS. Besides
concerns about arcing, you'd have to make sure the insulation across the
capacitor is very good, since even a very small leakage current will
cause significant loss. And you end up with a fairly narrow-banded
antenna, with the 2:1 SWR bandwidth of about 130 kHz. The loss due to
finite wire conductivity is 1.9 dB, which might or might not be
acceptable, depending on the particular use. Increasing the wire size
will reduce the loss, but also the bandwidth -- introducing loss nearly
always improves bandwidth, so reducing it narrows the bandwidth. Without
wire loss, and assuming the resulting 29 ohm feedpoint impedance is
transformed to 50 ohms, the 2:1 SWR bandwidth becomes 80 kHz. Like a
great number of variations, this antenna would surely be useful to some
people in some situations, and might well be better than some other
alternatives. But here's an antenna rule you can take to the bank:
Small--broad band--efficient, choose any two. Any time either a modeling
program or an antenna inventor or seller tell you any different, you
should be very, very skeptical.

Roy Lewallen, W7EL

alhearn wrote:
I've recently done some NEC-2 (MultiNEC) modeling of folded dipoles
which might help answer some of your questions. Translating the
results to folded monoploes should be fairly straightforward.

The model is a half-wave folded dipole for 14.2 MHz in freespace,
resonant at 33.15 feet using #18 wire with 2 inch spacing. The
center-fed input impedance is 289 - j0.01, which is 4 times the
resonant impedance of 72 ohms for a conventional dipole. A folded
1/4-wavelength monopole would have half that impedance, or about 144
ohms.

Examining the R-X curves for this dipole shows that it has
characteristics very similar to a 3/2-wavelength dipole, operating at
its third harmonic, and on a relatively low-slope part of the curves,
indicating a low Q and good bandwidth, similar to a fat dipole.

Shortening the antenna increases capacitive reactance, as might be
expected. However, input resistance *increases* as the length
decreases, which is contrary to our experience with common
1/2-wavelength dipoles. This is because we're on the high side of
full-wave resonance, where very high resistance values exist at its
peak. As we shorten the antenna, we're climbing the full-wave
resistance curve, which peaks when the antenna length is 22 feet. If
we further shorten the antenna past full-wave resonance, we now begin
experiencing a "normal" decrease in resistance as we "slide" back down
the low side of the full-wave resistance spike. However, capacitive
reactance has now quickly changed to inductive reactance as we crossed
full-wave resonance.

If we continue to shorten the folded antenna length, we come to a
length of about 17 feet where the input impedance is 50 + j2000 ohms.
Notice that the impedance is *inductive*, not capacitive as we are
accustomed to seeing with ordinary short dipoles. The inductive 2000
ohms can be cancelled with a series capacitor (or other suitable
matching network). Q has increased (because we're on a relatively
steep part of the R-X curves) and bandwidth has narrowed considerably
from the resonance at 33.15 feet.

So, by reducing the length of the 1/2-wavelength folded dipole from
33.15 feet to 17 feet, we have a 50 ohm resistive impedance by
matching the inductive reactance with a capacitor (or split capacitor)
instead of the usual lossy, low-Q loading coils. Gain and patterns
appear to be the same as a conventional dipole.

Translated to a monopole, the length would be a little more than half
the dipole's 17 feet, to boost feed point resistance from 25 ohms to
50 ohms. My guess is (I haven't modeled it) that this antenna
functions much like a 3/8-wavelength monopole, although much shorter.
Actually building this antenna and placing it the real world will
obviously change the above values.

Unfortunately, it doesn't appear that any combination of element size
and spacing will offset the need for impedance matching with the
shortened folded dipole or monopole.

I hope this makes sense. I'm sure Roy, Cecil, Tom, and others might
have comments/corrections that will be helpful to me and others who
are relative neophytes in the wonderful world of antennas.

Al WA4GKQ

Roy Lewallen wrote:
"Modeling a 17 foot folded dipole made from copper #18 wire spaced at 2
inches at 14.2 MHz with EZNEC shows a feedpoint impedance of 46.1 +
j1893. This can be resonated as Richard Harrison recently pointed out
with a series capacitor. There`s no free lunch though---at 1kW, the
voltage across the capacitor is almost 9000 V RMS (about 12000 volts
peak) and even at 10 watts its almost 900 volts RMS.

I agree that at 1KW input to Roy`s folded dipole the power-correction
capacitor has 8466 volts across it. That`s close enough to 9 KV for me.

No single antenna fits all applications and alterations may adapt an
antenna for more than one application.

Antennas have a voltage to current ratio (Zo) which is a function of
position along along the conductor. Zo is also a function of conductor
length to diameter ratio. Fat wires have lower Zo than do thin wires.
Low Zo means low voltage (relatively).

Also spacing the folded antenna conductors farther apart lowers
impedance and Q. This helps bandwidth.

Raising the current by lowering Zo is no panacea as the volts across the
capacitor are Amps x XC.

The capacitance of 1893 ohms at 14.2 MHz is about 0.000006 pF. If the
plate size is kept significant, the spacing should be good for 12 KV
with no problem.

The Andrew Corporation folded monopoles I am familiar with were usually
working with 500-watt VHF FM transmitters in our land-mobile operations.
Bandwidth required was 2f + 2d, if I recall, and the (f) was maximum
modulation frequency, and the (d) was the peak deviation. Bandwidth was
less than 20 KHz. Half-duplex was the communications mode so we needed
the antenna only to work at one carrier frequency.

It was a cakewalk. Antennas only flashed over on lightning strikes and
the 50-ohm Heliax saw most of the lightning as a common-mode disturbance
and rejected its passage through the coax (via counter-emf from coax
distributed inductance).

The VHF Andrew folded monopole element was similar to the slide pipe on
a trombone only made of stainless steel. It had clamps to hold its
position once set. Andrew set its length for 50-ohms at our frequency, I
suppose, and adjusted the reactance for a net zero. When we set it atop
our tower we always had about 500 watts forward and nearly zero
reflected power. Some of these are surely operating well at this moment
after 50 years or more, though they`ve surely accumulated many small
pits from countless lightning strikes.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:

. . .
The capacitance of 1893 ohms at 14.2 MHz is about 0.000006 pF. If the
plate size is kept significant, the spacing should be good for 12 KV
with no problem.
. . .

By my reckoning, a capacitive reactance of 1893 ohms at 14.2 MHz is 5.9 pF.

Roy Lewallen, W7EL

"Roy Lewallen" wrote in message
...
Modeling a 17 foot folded dipole made from copper #18 wire spaced at 2
inches at 14.2 MHz with EZNEC shows a feedpoint impedance of 46.1 +
j1893 ohms.


I didn't know I could model loops in EZNEC. But now I see that it has
problems only with small loops. I guess a 1/4-wave loop is not considered
small. I'll go back and try it.

My fall-back plan is to make a simple 1/4-wave resonant folded monopole and
feed it with a 1/4-wave length of 75 Ohm coax. There will probably be some
mismatch, but I think it will be tolerable.

Thanks, guys.

John

Roy Lewallen, W7EL wrote:
"By my reckoning, a capacitive reactance of 1893 ohms at 14.2 MHz is 5.9
pF."

Dang me and my sliderule. Neither of us keeps track of decimals very
well. 6 pF can be obtained with wide spacing and high breakdown volts
with small plates. I don`t see much of a hurdle to clear because 6 pF is
a small capacitance.

As a practical matter, Andrew Corporation used another method to tune
its folded monopoles, I believe, because they had d-c continuity. They
supplied these antennas for decades to work at VHF. To move from 10m to
20m brings problems of scale, mechanical and electrical.

The original question said that the open-circuit ground plane has a
35-ohm feedpoint (at some elevation), and a folded ground plane has
about 140 ohms as a feedpoint. Neither ground plane matches the usual
coax at the antenna resonant frequency. Commercial antenna makers
advertise and deliver open-circuit and folded radiator ground plane
antennas which are nearly 50 + j0 ohms feedpoint impedance at a
specified frequency when mounted high and in the clear.

The folded radiator offers more lightning protection than the
open-circuit radiator. The folded radiator contains the ability to
step-up feedpoint impedance in cases where an open-circuit radiator
would have an inconveniently low feedpoint. Most TV yagis, for example,
use a folded dipole as the driven eleement due to the low feedpoint
impedance caused by mutual coupling with the parasitic elements.

Most energy in a lightning strike is at lower frequencies. Tune the
bands during thunderstorm season and notice where the static crashes are
worse, though much of this is due to propagation, some is due to the
shape of the transient. Where the folded antenna loop is small in terms
of wavelength, the loop is nearly a short-circuit and differential
energy is small.

I saw lightning problems solved by replacing open-circuit antennas with
folded-element antennas. As lightning is an interference problem taken
to an extreme, folded elements are also useful in solving some other
interference problems. But there are cautions. A folded dipole has a
resonance where it is only 1/4-wave from tip to tip. Its circumference
is 1/2-wave and resonates. This gives a folded dipole twice as many
resonances as an open-circuit dipole.

I make arithmetic mistakes more frequently when I don`t know for sure
that the number I calculate is reasonable or not. I do know that 20-kV
to 40-kV sparkplug voltage does not ordinarily leap many feet through
the air. I also have a formula for capacitance:

CpF = 0.225 K A / S

CpF = capacitance in pF

K = dielectric constant

A = area of one of the 2-plate capacitor plates
(sq. in.)

S = spacing between the plates in inches

For air, K = 1.0006

For a vacuum, K = 1

6 pF is not much so it should be easy to create.

Best regards, Richard Harrison, KB5WZI

"Roy Lewallen" wrote in message
...
Modeling a 17 foot folded dipole made from copper #18 wire spaced at 2
inches at 14.2 MHz with EZNEC shows a feedpoint impedance of 46.1 +
j1893 ohms.


Now I'm confused again.

I modeled a monopole at 434 MHz. Varying the vertical element showed the
terminal impedance to get lower in resistance and become increasingly
capacitive as the vertical element is shortened.

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


John

John wrote:

I didn't know I could model loops in EZNEC. But now I see that it has
problems only with small loops. I guess a 1/4-wave loop is not considered
small. I'll go back and try it.
. . .

Because EZNEC uses NEC-2 for calculations, it has the same problems with
small loops that NEC-2 does. It's able to model any kind of antenna that
NEC-2 can, within its segment limitation.

Roy Lewallen, W7EL

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

I didn't know I could model loops in EZNEC. But now I see that it has
problems only with small loops. I guess a 1/4-wave loop is not
considered
small. I'll go back and try it.
. . .

Because EZNEC uses NEC-2 for calculations, it has the same problems with
small loops that NEC-2 does. It's able to model any kind of antenna that
NEC-2 can, within its segment limitation.

Roy Lewallen, W7EL


I don't know what NEC-2 is able to do. Does this mean I can model folded
monopoles?

John

John wrote:

I don't know what NEC-2 is able to do. Does this mean I can model folded
monopoles?

John

Sure. But you can't accurately model ones made with twinlead or window
line, since NEC-2 or EZNEC can't account for the effect of the dielectric.

Roy Lewallen, W7EL

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

I don't know what NEC-2 is able to do. Does this mean I can model folded
monopoles?

John

Sure. But you can't accurately model ones made with twinlead or window
line, since NEC-2 or EZNEC can't account for the effect of the dielectric.

Roy Lewallen, W7EL


Okay, great!

I modeled a folded monopole at 434 MHz. Varying the length down from
resonance, the element showed the terminal impedance getting lower in
resistance and become increasingly capacitive just like the unfolded
monopole.

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

John

"John" wrote in message
...

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

I don't know what NEC-2 is able to do. Does this mean I can model
folded
monopoles?

John

Sure. But you can't accurately model ones made with twinlead or window
line, since NEC-2 or EZNEC can't account for the effect of the
dielectric.

Roy Lewallen, W7EL


Okay, great!

I modeled a folded monopole at 434 MHz. Varying the length down from
resonance, the element showed the terminal impedance getting lower in
resistance and become increasingly capacitive just like the unfolded
monopole.

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

John


Did you go down to 217 MHz and below? If not, check it out. Should hit
another resonance at something like 50,000 +j0, and stay inductive below
that.

Tam/WB2TT

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

"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

"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.


Okay! I did it!

I didn't separate out the transmission line from the antenna. Instead, I
just modeled the folded monopole. I plotted on a Smith chart the resultant
terminal impedance as the vertical element varied from .23 wavelengths to
..245 wavelengths. It went from (73.31 - J 40.81) through (85.71 + J 0.7908)
to (93.59 + J 22.56). Easy!

I found that a vertical element of .2325 wavelengths gave me 76.05 - J 30.27
ohms. That's a sweet spot for this particular antenna in that feeding it
with a .143 wavelength piece of 75 Ohm coax will match to a 50 Ohm source.

To further my education, I also checked the anti-resonance point you
mentioned.

Thanks!

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

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

snip

Is it possible for someone to post the "file"/spreadsheet for this? As
someone who uses EZNEC less than some other programs, I'm not sure how
to set up the feed and the antenna in parallel.

Also, we had a nice presentation on the use of EZNEC at the 2004 Aurora
yesterday by W0ZQ. Aurora is the yearly conference of the Northern
Lights Radio Society. Other speakers included VE4MA, noted cutting edge
moonbouncer - first on 24Ghz, covering broadband over powerline.

tom
K0TAR

No. The example with the positive reactance is at a frequency below
parallel resonance, where the reactance goes the other way than it does
just below normal resonance.

As I pointed out in my most recent posting, the antenna reactance
becomes more negative at frequencies just below resonance, and the
transmission line reactance more positive. Beginning at resonance, the
net feedpoint reactance (the reactive part of the parallel combination
of the antenna and transmission line impedances) becomes more negative
as frequency decreases or the antenna gets shorter -- until parallel
resonance is reached. At parallel resonance, the reactance abruptly
jumps from a large negative value to a large positive value, then
decreases as frequency further decreases or the antenna shortens. The
example I gave in that posting showed the parallel resonance at a
frequency somewhat higher than where the antenna is an eighth wave high.
But the earlier example antenna with 46.1 + j1893 ohm feedpoint Z is
about an eighth wave high, shorter than self resonance. Don't forget
that the actual frequency of parallel resonance depends on the impedance
of the transmission line, so don't make generalizations about where
parallel resonance will occur for all antennas. But if you know that the
folded monopole or dipole is shorter than a resonant length and its
feedpoint reactance is positive, it's below parallel resonance and the
reactance will decrease as frequency drops or the antenna gets shorter.
If its feedpoint reactance is negative, it's above parallel resonance
and the reactance will become more negative as the frequency drops or
the antenna becomes shorter.

An unfolded monopole's impedance is monotonic below resonance. That is,
the resistance drops and the reactance becomes more negative as you go
lower in frequency, as far as you want to go. Not so with a folded
monopole -- it has one behavior down to the parallel resonant point,
then the magnitude of the reactance goes the other way below that. The
reason is that there are two separate mechanisms at work, rather than
the single one for an unfolded monopole. So if you want to make a rule
about which way the reactance goes, you've got to specify whether you're
above or below parallel resonance.

Roy Lewallen, W7EL

Richard Harrison wrote:
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

Those impedances seem pretty low for a folded monopole, unless the
conductor diameter is large.

When modeling two parallel wires like a folded monopole or dipole with
any NEC-2 based program, it's essential that the segment junctions be
aligned. For the folded dipole or monopole, simply make the wires the
same lengths and give them the same number of segments.

Roy Lewallen, W7EL

John wrote:

"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.



Okay! I did it!

I didn't separate out the transmission line from the antenna. Instead, I
just modeled the folded monopole. I plotted on a Smith chart the resultant
terminal impedance as the vertical element varied from .23 wavelengths to
.245 wavelengths. It went from (73.31 - J 40.81) through (85.71 + J 0.7908)
to (93.59 + J 22.56). Easy!

I found that a vertical element of .2325 wavelengths gave me 76.05 - J 30.27
ohms. That's a sweet spot for this particular antenna in that feeding it
with a .143 wavelength piece of 75 Ohm coax will match to a 50 Ohm source.

To further my education, I also checked the anti-resonance point you
mentioned.

Thanks!

John

(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

Roy, W7EL wrote:
"No, the example with the positive reactance is at a frequency below
parallel resonance, where the reactance goes the other way than it does
just below normal resonance."

Just as the usual folded dipole selected for a particular frequency is
1/2-wave, the usual folded monopole is a 1/4-wave. Shorter or longer
antennas are used and harmonically related frequencies may or may not
have convenient drivepoint impedances depending on which harmonic.

The folded monopole is usually tuned to be a short-circuit stub at some
particular frequency and it has the shield side of the coax also
connected to an elevated radial system to keep signal off the outside of
the coax, and it replaces the "missing half" of a dipole. The ground
plane leaves the radiation up to the vertical antenna element.

The folded monopole antenna is a resonant system with distributed
constants.

Terman says on page 893 of his 1955 edition:
"As a result, the impedance of an antenna behaves in much the same
manner as does the impedance of a transmission line (see Sec. 4-7)"

Sec. 4-7 is found on page 98. Here Terman says:
"The expression "Transmission-line impedance" applied to a point on a
transmission line signifies the vector ratio of line voltage to line
current at that particular point. This is the impedance that would be
obtained if the transmission line were cut at the point in question, and
the impedance looking toward the load were measured on a bridge."

In the case of a short-circuit 1/4-wave stub, Terman has Fig. 4-10(b) on
page 99 of the 1955 edition. At the load, the short at the antenna tip,
the power factor is shown lagging by 90-degrees which by my electronics
dictionary is:
"Laggibng load - A predominantly inductive load - i.e., one in which the
current lags the voltage."

What`s more, the 90-degree lag persists almost unchanged until a point
is reached nearly 1/4-wavelength back from the short.In the folded
monole, that would approach the drivepoint.

If the folded monopole were lengthened beyond 1/4-wave, an abrupt flip
to a leading power factor angle of nearly 90-degrees would be
experienced on passage through the 1/4-wave resonance point. The phase
variations become less abrupt at subsequent flip points as any added
90-degree points become farther removed from the antenna tip
short-circuit. The frequency is getting higher so that the antenna
appears longer in terms of wavelength or the antenna is gaining in
number of 1/4-wavelengths some other way to produce multiple phase
reversals.

Best regards, Richard Harrison, KB5WZI

Al, WA4GKO wrote:
"---when it`s too short for resonance, reactance is capacitive, and is
inductive if too long."

Thats exactly correct for an open circuit dipole or monopole, but folded
elements are backwards because they operate like loops and shorted
transmission lines.

Kraus says:
"Consider a two-wire folded dipole shown in Fig. 14-27a. The terminal
resistance is approximately 300 ohms. By modifying the dipole to the
general form shown in Fig. 14-27b, a wide range of terminal resistances
can be obtained, depending on the value of D. This arrangement is called
a T-match antenna."

The ARRL Antenna Book says:
See Fig 9. Each such T conductor and associated antenna conductor can be
looked upon as a section of transmission line shorted at the end. (This
is also true of the short folded monopole.) Because it is shorter than
1/4-wave it has inductive (Not Capacitive) reactance. As a consequence,
if the antenna itself is exactly resonant at the operating frequency,
the input impedance must be tuned out if a good match to the
transmission line is to be obtained. This can be done either by
shortening the antenna to obtain a value of capacitive reactance (The
T-match antenna itself is open-ended) at the input terminals, or by
inserting a capacitance of the proper value in series at the input
terminals as shown in Fig. 10A."

The too-short T-match has excess inductance to be cancelled just as does
a too-small loop or folded antenna. This is accomplished by adding
capacitive reactance. This is the opposite of your short mobile whip
which needs a coil.

Best regards, Richard Harrison, KB5WZI

alhearn wrote:
"That is, when it`s too short for resonance, reactance is capacitive,
and is inductive if too long."

Just look at Terman`s phase diagram for a shorted transmission line and
I think you will agree with Capt. Lee.

Capt. Raul H. Lee, USNR, K6TS on page 31 of "The Amateur Radio Vertical
Antenna Handbook" wrote:
"The folded unipole feed principle may be easily applied to the short,
top loaded vertical radiator. The transformer action of the folded
unipole is used to give a more favorable input resistance than can be
obtained with a series feed. Another thing that the folded unipole feed
does is to reverse the sign of the input reactance. The input reactance
of a series fed tower shorter than 1/4 wavelength is always capacitive.
This means a series loading coil (spoken of in high power as a helix)
must be used to resonate the tower. With folded unipole input the feed
point reactance is always positive. (Consider the tower and feed wire to
be a shorted transmission line less than 1/4 wave long. Its input
reactance is positive.) Thus, the folded unipole may be fed with a
low-loss capacitive feed network.

Best regards, Richard Harrison, KB5WZI

"Roy Lewallen" wrote in message
...
Those impedances seem pretty low for a folded monopole, unless the
conductor diameter is large.

When modeling two parallel wires like a folded monopole or dipole with
any NEC-2 based program, it's essential that the segment junctions be
aligned. For the folded dipole or monopole, simply make the wires the
same lengths and give them the same number of segments.

Roy Lewallen, W7EL


You are correct again. I had different numbers of segments of the parallel
wires. When the segments are about the same number, the terminal impedance
goes up. I am also trying to run as many segments as practical so the
accuracy is greatest.

John

alhearn wriote:
"That is, when it`s too short for resonance, reactance is capacitive and
is inductive if too long."

True that a too-short open-circuit vertical radiator is impeded by a
capacitive reactance and that a slightly too-long open-circuit radiator
is impeded by inductive reactance. We are aiming for a 1/4-wave antenna.
The switch from leading to lagging power factor or vice versa when
passing through the resonance point is abrupt. See Terman`s diagram. The
folded monopole is a short-circuit 1/4-wave transmission line stub and
it behaves like one.

My 1998 ARRL Handbook displays a group of too-short antennas in Fig
20.44 on page 20.22.
Item (E) of a group of 6 would be 1/4-wave antennas is called a tri-wire
unipole. It`s a vertical tower with a top support to suspend a parallel
wire on either side of the tower. The tower itself is #1 wire of the
tri-wire assembly and it is grounded, not insulated at the earth. Wire
#2 connects the top of the tower to the earth through a variable
capacitor. wire #3 drives the top of the tower and it is insulated from
the earth, and is driven against the earth.

The variable capacitor is used to tune out the too-short folded
monopole`s excess inductance and present 50 ohms to the feedline. The
text says:

"This technique will not be suitable for matching to 50 ohm line unless
the tower is less than an electrical quarter wavelength high."

Why? Over 1/4-wavelength high, the folded unipole is capacitive and
adding more capacitive reactance will detune it even more. The unipole
is synonymous with monopole.

It`s a fact that the sign of the reactance in the folded antenna is
inverted in the too-short folded antenna from that in the too-short
open-circuit antenna. That is a powerful advantage in that low-loss
capacitance can be used to match the too-short folded antenna and your
practical choice may have to be a lossy loading coil to match the
too-short open-circuit antenna.

What`s right is right and isn`t decided by a vote, but on the
probabilities from all the examples I`ve presented they are likely right
by the numbers.

I have a comment on the results of WA4GKQ`s modeling results of the
folded monopole. Models can be wrong for many reasons including garbage
in, garbage out. I`ve measured many folded monopoles in service, at
resonance, accepting measured full power and reflecting negligible
measured power and I`m sure these antennas are working well as evidenced
by expected performance every day over decades at many places here and
abroad. Seeing is believing for me.

Best regards, Richard Harrison, KB5WZI

"Richard Harrison" wrote in message
...
Al, WA4GKO wrote:
"---when it`s too short for resonance, reactance is capacitive, and is
inductive if too long."

Thats exactly correct for an open circuit dipole or monopole, but folded
elements are backwards because they operate like loops and shorted
transmission lines.

Kraus says:
"Consider a two-wire folded dipole shown in Fig. 14-27a. The terminal
resistance is approximately 300 ohms. By modifying the dipole to the
general form shown in Fig. 14-27b, a wide range of terminal resistances
can be obtained, depending on the value of D. This arrangement is called
a T-match antenna."


On the page before that, page 417, Kraus also gives the impedance of the
folded dipole. He says it is 4 times the impedance of a two-wire dipole.
This means that, if the 2 wire dipole is capacitive slightly below
resonance, so is the terminal impedance of the folded dipole. Multiplying a
complex number by a real number does not change the sign of the imaginary
part.

John