"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