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

The other John Smith wrote:
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?

Unfortunately, a folded monopole goes the opposite direction to a
monopole, impedance-wise.
--
73, Cecil http://www.qsl.net/w5dxp



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The short answer to your question is no, with practical impedances. It
turns out you'd need a very low impedance transmission line (on the
order of a few ohms, if I recall correctly) to track very well.

The easiest way to analyze a folded dipole is as two separate circuits,
common-mode (or "antenna") and differential-mode (or "transmission
line"). Superposition applies, so the two can be analyzed separately.

First consider the "antenna". Its contribution to the feedpoint
impedance is the same as a conventional monopole made from the two wires
in parallel, but multiplied by 4 due to the transforming action of the
"folding" process. (Other ratios are possible -- 4 is what you get if
the conductors are the same diameter.) This "antenna" does all the
radiating.

In parallel with the "antenna" at the feedpoint is the "transmission
line". This is the short-circuited transmission line made from the two
conductors, with transmission line velocity factor taken into account.
The "transmission line" part does no radiating.

Now, if you shorten the antenna, two things will happen. The reactance
of the "antenna" will become more negative, and its resistance will drop
some. This will show up at the feedpoint just like it would for an
unfolded monopole, but multiplied by 4. But you're also shortening the
transmission line, whose impedance also appears at the feedpoint, in
parallel with the transformed "antenna's". Assuming negligible loss in
the feedline, this will cause a change only in parallel reactance at the
feedpoint. Because of the relatively high impedance of the transmission
line, and the relative sharpness of the impedance change of the
"antenna" compared to the transmission line, there's very little
compensation in the case of most practical antennas. Remember that the
two impedances are in parallel, not series, so the high Z contribution
of the transmission line has little overall effect. As I recall from
doing an analysis some time ago, you get more broadening compared to a
conventional monopole from having a fatter equivalent "antenna"
conductor than you do from the transmission line stub.

The reactance of the "antenna" can be a bit tricky to calculate
accurately, but a number of modern programs (such as the free EZNEC
demo) do a good job of it. The transmission line part of the effect is
easy, either with a scientific calculator or one of the transmission
line programs which are readily available. So overall, it's not
difficult to find the actual impedances you'll see in practice.

Roy Lewallen, W7EL


The other John Smith wrote:
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

John:

Andrew Corp. made a good living back in the dark ages (40s-60s)
manufacturing and selling 50 ohm folded monopoles. I can go measure the
element diameters on mine if you are interested. If I can recall where it
is stored. It uses a small inductance at the feed point for matching.

--
Crazy George
Remove N O and S P A M imbedded in return address
"The other John Smith" wrote in message
nk.net...
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

"Crazy George" wrote in message
...
John:

Andrew Corp. made a good living back in the dark ages (40s-60s)
manufacturing and selling 50 ohm folded monopoles. I can go measure the
element diameters on mine if you are interested. If I can recall where it
is stored. It uses a small inductance at the feed point for matching.

--
Crazy George


Thanks, George, but the inductor at the feed point is disqualified. The idea
was to learn if everything could be done by just clever design. It appears
not.

But, thanks for your offer.

John

Okay. Thanks, Roy. This is getting too difficult for me so I'll go back to
something more traditional.

John




"Roy Lewallen" wrote in message
...
The short answer to your question is no, with practical impedances. It
turns out you'd need a very low impedance transmission line (on the
order of a few ohms, if I recall correctly) to track very well.

The easiest way to analyze a folded dipole is as two separate circuits,
common-mode (or "antenna") and differential-mode (or "transmission
line"). Superposition applies, so the two can be analyzed separately.

First consider the "antenna". Its contribution to the feedpoint
impedance is the same as a conventional monopole made from the two wires
in parallel, but multiplied by 4 due to the transforming action of the
"folding" process. (Other ratios are possible -- 4 is what you get if
the conductors are the same diameter.) This "antenna" does all the
radiating.

In parallel with the "antenna" at the feedpoint is the "transmission
line". This is the short-circuited transmission line made from the two
conductors, with transmission line velocity factor taken into account.
The "transmission line" part does no radiating.

Now, if you shorten the antenna, two things will happen. The reactance
of the "antenna" will become more negative, and its resistance will drop
some. This will show up at the feedpoint just like it would for an
unfolded monopole, but multiplied by 4. But you're also shortening the
transmission line, whose impedance also appears at the feedpoint, in
parallel with the transformed "antenna's". Assuming negligible loss in
the feedline, this will cause a change only in parallel reactance at the
feedpoint. Because of the relatively high impedance of the transmission
line, and the relative sharpness of the impedance change of the
"antenna" compared to the transmission line, there's very little
compensation in the case of most practical antennas. Remember that the
two impedances are in parallel, not series, so the high Z contribution
of the transmission line has little overall effect. As I recall from
doing an analysis some time ago, you get more broadening compared to a
conventional monopole from having a fatter equivalent "antenna"
conductor than you do from the transmission line stub.

The reactance of the "antenna" can be a bit tricky to calculate
accurately, but a number of modern programs (such as the free EZNEC
demo) do a good job of it. The transmission line part of the effect is
easy, either with a scientific calculator or one of the transmission
line programs which are readily available. So overall, it's not
difficult to find the actual impedances you'll see in practice.

Roy Lewallen, W7EL


The other John Smith wrote:
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

Crazy George wrote:
"Andrew Corp. made a good living back in the dark ages (40-60s)
manufacturing and selling 50 ohm folded monopoles."

They could be had in stainless steel and they were tough. Like Andrew
cables and connectors, it was wrong to use anyything else.

I can attest, having used countless numbers of the above, that Andrew`s
folded monopole when ordered for the precise frequency and mounted atop
your tower gave a full forward power indication on your wattmeter with
nearly zero indicated reflected power which meant a good 50-ohm match.
All you had to do was install the antenna right and it would give
optimum performance for many years. Your signal reached as far as the
eye could see and then some. Your radio heard signals acutely.

Well grounded to the tower at the top and with proper grounding of the
tower and transmission line at tower top and bottom, lightning is routed
to ground and not to the inside of the attached radio. The antenna just
shook off the lightning strikes with small pits as calling cards. No
harm done.

Best regards, Richard Harrison, KB5WZI

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

"Crazy George" wrote in message
...
John:

Andrew Corp. made a good living back in the dark ages (40s-60s)
manufacturing and selling 50 ohm folded monopoles. I can go measure the
element diameters on mine if you are interested. If I can recall where
it
is stored. It uses a small inductance at the feed point for matching.

--
Crazy George


Thanks, George, but the inductor at the feed point is disqualified. The
idea
was to learn if everything could be done by just clever design. It appears
not.

But, thanks for your offer.

John

At the higher frequencies, one way to make a 50 Ohm antenna is to arrange it
as a full wave loop, but make it about twice as high as wide. That is, you
end up with an antenna that is 1/3 wavelength high, and 1/6 wavelength wide.
It's a balanced antenna, so you don't need ground, but the whole thing has
to be well up in the air.

Tam/WB2TT

"Tam/WB2TT" wrote in message
...

At the higher frequencies, one way to make a 50 Ohm antenna is to arrange
it
as a full wave loop, but make it about twice as high as wide. That is, you
end up with an antenna that is 1/3 wavelength high, and 1/6 wavelength
wide.
It's a balanced antenna, so you don't need ground, but the whole thing has
to be well up in the air.

Tam/WB2TT



That's an interesting one I'll remember.

However, the idea is to make an unbalanced antenna with all the elements
connected to ground. The folded monopole is an excellent example.

Thanks.

John

John

Since I'm a poor reader, I'm not confidant I fully understand your
objective. But, a folded monopole (1/4 wave) with a very fat "fed" element
and a very thin "grounded" element, with very close spacing will have an
input impedance lower that 150 ohms at *resonance*. It occurrs to me that
you already know that.

Jerry.



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