On Sat, 14 Nov 2009 12:05:21 -0600, amdx wrote:

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'll readily admit the other fellow knows more about this than me, but
I need more convincing :-)
Can anyone explain this to me.
Thanks, Mike

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?

--
www.wescottdesign.com

Tim Wescott wrote:
On Sat, 14 Nov 2009 12:05:21 -0600, amdx wrote:

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'll readily admit the other fellow knows more about this than me, but
I need more convincing :-)
Can anyone explain this to me.
Thanks, Mike

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.

Tony Hwang wrote:

Tim Wescott wrote:
On Sat, 14 Nov 2009 12:05:21 -0600, amdx wrote:

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'll readily admit the other fellow knows more about this than me,
but
I need more convincing :-)
Can anyone explain this to me.
Thanks, Mike

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.

Due to the transformer effect of folding it.

--
Best Regards:
Baron.

Also folded dipole tends to have broader b/w compared to dipole.

Due to the transformer effect of folding it.


Most of the bandwidth enhancement on a folded dipole comes from the
fact that it's a "fatter" radiator rather than because it's folded.
Section Bii) he

http://www.karinya.net/g3txq/folded_dipole/

Steve G3TXQ

"steveeh131047" wrote in message
...
Also folded dipole tends to have broader b/w compared to dipole.

Due to the transformer effect of folding it.


Most of the bandwidth enhancement on a folded dipole comes from the
fact that it's a "fatter" radiator rather than because it's folded.
Section Bii) he

http://www.karinya.net/g3txq/folded_dipole/

Steve G3TXQ


Of course, the 'transformer effect' is unlikely to increase the bandwidth by
itself, and when the common type of half-wave-line balun (and 4:1 impedance
transformer) is used to connect the dipole to co-axial cable the resonant
nature of this additional half wavelength is likely to impair the bandwidth
to some extent (by introducing additional reactance at frequencies for which
its length isn't a true half wavelength).

Certainly the additional fatness of the folded-dipole structure must
contribute to improved bandwidth by reducing the slopes of the terminal
resistance and reactance, but the treatment given in the link provided
above - suggesting that the susceptance added at the drive point by the
'parasitic' parallel wire or rod, joined to the driven element at its ends,
has insignificant effect - is a rather broad generalisation which may not be
true for some geometries (i.e. length/thickness ratios of the wires/rods and
their spacing). Generally, it's dangerous to generalise!

There's quite a lot going on in a folded dipole. Some further insight (on
allied, but not the same, topic) is offered in
http://www.physics.princeton.edu/~mc...cutantenna.pdf.

Chris