# Folded Dipole Calculator Help

A

#### amdx

Jan 1, 1970
0
Hi All,
I'm having a disagreement about the use of this folded dipole calculator
impedance calculator.
http://www.k7mem.150m.com/Electronic_Notebook/antennas/folded_dipole.html
We are using it to calculate the impedance of folded dipoles to drive a
yagi.
When entering data it asks for "Simple dipole feed impedance (ohms)"
Default is 72 ohms. This assumes a 1/2 wave dipole.
The fellow I'm disagreeing with say's,
"the reason that the variable in the calculator for the "dipole feed
impedance"
could be if you want to transform the z to something else, such as 50 ohms."
I don't even understand what he means.
I can only think of one reason to alter the 72 ohms, that would be if you
use a a different length folded dipole.
I need more convincing
Can anyone explain this to me.
Thanks, Mike

T

#### Tony Hwang

Jan 1, 1970
0
amdx said:
Hi All,
I'm having a disagreement about the use of this folded dipole calculator
impedance calculator.
http://www.k7mem.150m.com/Electronic_Notebook/antennas/folded_dipole.html
We are using it to calculate the impedance of folded dipoles to drive a
yagi.
When entering data it asks for "Simple dipole feed impedance (ohms)"
Default is 72 ohms. This assumes a 1/2 wave dipole.
The fellow I'm disagreeing with say's,
"the reason that the variable in the calculator for the "dipole feed
impedance"
could be if you want to transform the z to something else, such as 50 ohms."
I don't even understand what he means.
I can only think of one reason to alter the 72 ohms, that would be if you
use a a different length folded dipole.
I need more convincing
Can anyone explain this to me.
Thanks, Mike
Hi,
The eq. seems right. Folded dipole is basically a loop.
Usually we use 300 Ohm feeder.
Tony
VE6CGX

T

#### Tony Hwang

Jan 1, 1970
0
Tony said:
Hi,
The eq. seems right. Folded dipole is basically a loop.
Usually we use 300 Ohm feeder.
Tony
VE6CGX
Hi,
And you can use a tranformer 300 Ohm to 75 Ohm to use coax if need.
It becomes a from 300 Ohm balaanced to 75 Ohm unbalanced feed.

A

#### amdx

Jan 1, 1970
0
Tony Hwang said:
Hi,
The eq. seems right. Folded dipole is basically a loop.
Usually we use 300 Ohm feeder.
Tony
VE6CGX

In what case would I alter the 72 ohms?
Mike

A

#### amdx

Jan 1, 1970
0
Tony Hwang said:
Hi,
The eq. seems right. Folded dipole is basically a loop.
Usually we use 300 Ohm feeder.
Tony
VE6CGX

In what case would I alter the 72 ohms?
Mike

P

#### Paul Keinanen

Jan 1, 1970
0
Hi All,
I'm having a disagreement about the use of this folded dipole calculator
impedance calculator.
http://www.k7mem.150m.com/Electronic_Notebook/antennas/folded_dipole.html
We are using it to calculate the impedance of folded dipoles to drive a
yagi.

Excuse me, but what does such simple dipole calculator have to do with
the calculations for a Yagi feed element ?

Even assuming a single element feed element antenna, playing with the
tube diameter (upper and lower bars) or the number of bars in a folded
dipole will radically change the feed impedance.

Paul

A

#### amdx

Jan 1, 1970
0
Paul Keinanen said:
Excuse me, but what does such simple dipole calculator have to do with
the calculations for a Yagi feed element ?

I think it all started with the MFJ-1800 that uses a folded dipole, but
there
are many yagis that use a folded dipole as the driven element.
But, I think (now) I understand enough to know why you ask the question.
I think your point is the impedance of dipole feed on a yagi is highly
modified
by the reflector and directors.
Even assuming a single element feed element antenna, playing with the
tube diameter (upper and lower bars) or the number of bars in a folded
dipole will radically change the feed impedance.

Paul

I excerpted this from my latest email to my friend I'm having the
disagreement with. (Might
not even be a disagreement, it could be a miscommunication.)
Excerpt;
"The online calculator takes the (Impedance of a dipole x Ratio). This
gives you the impedance
after folding the dipole.
We still have no idea what the impedance of a dipole is when surrounded by
the reflector and
director. We do know how to change the transformation ratio once we know
what the impedance
of dipole would be in that surrounding."
Mike

B

#### Baron

Jan 1, 1970
0
amdx said:
I think it all started with the MFJ-1800 that uses a folded dipole,
but
there
are many yagis that use a folded dipole as the driven element.
But, I think (now) I understand enough to know why you ask the
question.
I think your point is the impedance of dipole feed on a yagi is highly
modified
by the reflector and directors.

I excerpted this from my latest email to my friend I'm having the
disagreement with. (Might
not even be a disagreement, it could be a miscommunication.)
Excerpt;
"The online calculator takes the (Impedance of a dipole x Ratio).
This
gives you the impedance
after folding the dipole.
We still have no idea what the impedance of a dipole is when
surrounded by
the reflector and
director. We do know how to change the transformation ratio once we
know what the impedance
of dipole would be in that surrounding."
Mike

The nominal impedance of a dipole in free space is approximately 72
ohms. If it is part of a system its impedance will be influenced by
that system causing it to change from nominal.

Folding the dipole allows it to behave as a transformer ! Thus the
nominal dipole impedance can be altered to a value that can more easily
be matched to the feed line.

Commonly the impedance of a dipole that is part of a system. ie "Yagi"
falls to a much lower value. Under these conditions the folded dipole
can be used to raise the impedance seen at the feed point to a more
usable value.

Someone mentioned "Gama match". Whilst symmetrical gamma matches can be
used, single ended is popular because it is inherently unbalanced and
can be easily matched to a co-axial feedline.

T

#### Tony Hwang

Jan 1, 1970
0
amdx said:
In what case would I alter the 72 ohms?
Mike
Hi,
Feed point impedance becomes near that when it is just a dipole(not
folded). On a haf wave element at center feed point the current and
voltage phase is such that the impedance is near that figure.
If it is vertical uater wave whip, the other half mirror image is
prjected into the ground forming dipole and in that case feed point
impedance at the bottom is around 50 Ohm. I am old dinosaur EE(class of
60), my brain is not as bright as it used to, LOL!

T

#### Tony Hwang

Jan 1, 1970
0
Tim said:
I've seen older Yagis that use folded dipoles, but the newer ones seem to
lean more toward gamma or T matches. You get more latitude for adjusting
impedance, in the gamma case you get a kinda sorta good match to coax
without a balun, and if you're employing plumber's delight construction
it's no more difficult to fabricate than a folded dipole.

So why a folded dipole, pray tell?
Hi,
Also folded dipole tends to have broader b/w compared to dipole.

T

#### Tony Hwang

Jan 1, 1970
0
amdx said:
I think it all started with the MFJ-1800 that uses a folded dipole, but
there
are many yagis that use a folded dipole as the driven element.
But, I think (now) I understand enough to know why you ask the question.
I think your point is the impedance of dipole feed on a yagi is highly
modified
by the reflector and directors.

I excerpted this from my latest email to my friend I'm having the
disagreement with. (Might
not even be a disagreement, it could be a miscommunication.)
Excerpt;
"The online calculator takes the (Impedance of a dipole x Ratio). This
gives you the impedance
after folding the dipole.
We still have no idea what the impedance of a dipole is when surrounded by
the reflector and
director. We do know how to change the transformation ratio once we know
what the impedance
of dipole would be in that surrounding."
Mike
Hi,
Even now Yagi-Uda antenna design is empirical business tinkering with it
in the antenna range based on theory plus actual field result.

B

#### Baron

Jan 1, 1970
0
Tony said:
Hi,
Also folded dipole tends to have broader b/w compared to dipole.

Due to the transformer effect of folding it.

F

Jan 1, 1970
0
A

#### amdx

Jan 1, 1970
0
Jeff Liebermann said:
Ummm... I'm looking at "Antennas" by John D. Kraus, first edition
1950, and find that the Yagi antenna entry in the index is just "Yagi"
with no mention of Yagi-Uda anywhere in the TOC or text. Perhaps in
later editions?

Extra credit. Which was the teacher and which was the student?

Ok guys, back on the online calculator.
http://www.k7mem.150m.com/Electronic_Notebook/antennas/folded_dipole.html

If I set d1 at 4mm and d2 at 2mm and S at 12.5 mm
Would I need to adjust the input data labeled "Simple Dipole Feed Impedance
(Ohms)"
to a value other than 72 ohms?
In other words does the "Simple Dipole Feed Impedance" change when the d1
and d2 are
altered by these amounts. (I'm asking about larger amounts not just 1 or 2
ohms)
Mike

B

#### Baron

Jan 1, 1970
0
amdx said:
Ok guys, back on the online calculator.
http://www.k7mem.150m.com/Electronic_Notebook/antennas/folded_dipole.html

If I set d1 at 4mm and d2 at 2mm and S at 12.5 mm
Would I need to adjust the input data labeled "Simple Dipole Feed
Impedance
(Ohms)"
to a value other than 72 ohms?
In other words does the "Simple Dipole Feed Impedance" change when the
d1 and d2 are
altered by these amounts. (I'm asking about larger amounts not just 1
or 2 ohms)
Mike

Fundamentally, No !

F

#### Fred Abse

Jan 1, 1970
0
Ummm... I'm looking at "Antennas" by John D. Kraus, first edition 1950,
and find that the Yagi antenna entry in the index is just "Yagi" with no
mention of Yagi-Uda anywhere in the TOC or text. Perhaps in later
editions?

Second edition, 1988
Extra credit. Which was the teacher and which was the student?

"Shintaro Uda, an assistant professor at Tohoku University"

"Hidetsugu Yagi, professor of electrical engineering at Tohoku University
and 10 years Uda's senior"

[Antennas, John D. Kraus, second edition, 1988, 11-9b ]

F

#### Fred Abse

Jan 1, 1970
0
Ok guys, back on the online calculator.
http://www.k7mem.150m.com/Electronic_Notebook/antennas/folded_dipole.html

If I set d1 at 4mm and d2 at 2mm and S at 12.5 mm
Would I need to adjust the input data labeled "Simple Dipole Feed
Impedance
(Ohms)"
to a value other than 72 ohms?
In other words does the "Simple Dipole Feed Impedance" change when the d1
and d2 are
altered by these amounts. (I'm asking about larger amounts not just 1 or 2
ohms)

From the above URL:

"The folded dipole multiplies the normal feed impedance of a simple
dipole. For a 1/2 wave dipole, in free space, this is approximately 72
Ohms. You may not be dealing with a dipole in free space, but 72 Ohms is
appropriate text box below."

The nub is "in free space". Coupling to other conducting objects
(parasitic elements, even support structures) alters the feedpoint
impedance. There are published curves on this, I'll have to see if I've
got them.

The figure of 72 ohms is the feedpoint impedance of a (theoretical),
infinitely thin, dipole. You don't need to alter this to take account of
element diameters, the function shown does this for you. You will have to
alter it if there is coupling to other conducting structures, for example,
parasitic elements. How much to alter it depends on other variables.

The (then) National Bureau of Standards did a lot of work on yagis in the
1980s. They published results that may be around on the Web.

A

#### amdx

Jan 1, 1970
0
Fred Abse said:
From the above URL:

"The folded dipole multiplies the normal feed impedance of a simple
dipole. For a 1/2 wave dipole, in free space, this is approximately 72
Ohms. You may not be dealing with a dipole in free space, but 72 Ohms is
You can adjust it to other impedances in the appropriate text box below."

Yes. but for a simple dipole in free space we can leave it at 72 ohms.
The nub is "in free space". Coupling to other conducting objects
(parasitic elements, even support structures) alters the feedpoint
impedance.

Yes, agreed, coupling structures will alter the impedance of a dipole and
also a folded dipole.
The figure of 72 ohms is the feedpoint impedance of a (theoretical),
infinitely thin, dipole. You don't need to alter this to take account of
element diameters, the function shown does this for you.

As I see it the changing element diameter alters the transforming ratio
NOT the 72 ohms that is feedpoint impedance of a (theoretical), infinitely
thin, dipole.

( this is an important point and may be the nub of the disagreement)

Using the calculator you will find altering d1 and d2 will NOT alter the
The 72 ohms in the data line "Simple Dipole Feed Impedance (ohms)".
(I don't know, maybe it should)

Ok, I think I getting close to being able argue my point, but I'm looking
for some agreement
here for the statements I made above.

P

#### Phil Allison

Jan 1, 1970
0
"amdx"
As I see it the changing element diameter alters the transforming ratio
NOT the 72 ohms that is feedpoint impedance of a (theoretical), infinitely
thin, dipole.

( this is an important point and may be the nub of the disagreement)

Using the calculator you will find altering d1 and d2 will NOT alter the
The 72 ohms in the data line "Simple Dipole Feed Impedance (ohms)".
(I don't know, maybe it should)

Ok, I think I getting close to being able argue my point, but I'm looking
for some agreement here for the statements I made above.

** You are so dumb you keep inventing non existent complications.

1. The calculator assumes the folded dipole is in free space.

2. It then calculates the impedance of the design for you, long as you use
72 ohms in the data.

3. IF you KNOW the impedance value for some dipole that is part of a more
complex antenna - then you use that value in the data instead, to find the
value for a folded version.

..... Phil

A

#### amdx

Jan 1, 1970
0
Phil Allison said:
"amdx"

** You are so dumb you keep inventing non existent complications.

Those "non existent compications" are what I'm trying to eliminate so
I can make a better arguement for my position.
1. The calculator assumes the folded dipole is in free space.

Yes, I understand that
2. It then calculates the impedance of the design for you, long as you use
72 ohms in the data.

Would 72 ohms still be the correct number if d1 is twice the diameter of d2?
3. IF you KNOW the impedance value for some dipole that is part of a
more complex antenna - then you use that value in the data instead, to
find the value for a folded version.

I would think that is the reason the author put that variable in the
program.
Thanks, Mike

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