Twisted Wire Z Formula
David Covick
wire based on the wire size/gage and the twists per inch.
I have a formula for balanced wire pairs with a diameter and distance, but I
don't think this applies when twisting is done.
Or, can someone tell me how to "test" the twisted wire for its impedance? I
have a HP impedance analyzer to do this, but don't know how to set up the
test.
Does anyone have clue on this one?
Thanks,
David
John Becich
to evaluate. If such xmsn line is TERMINATED on one end with a resistance
(real impedance) of the same ohmic value, then a signal traveling from the
other end to the terminated end will not be reflected at the termination.
Thus you can use trial and error to evaluate Zo. Start with 100 ohms. Make
xmsn line long enough to see reflections; try several meters, but if you
have a fast O-scope your xmsn line can be shorter. You'll need some kind of
pulse generator, the sharper the better.
This is the general idea for testing. It is not a formula, obviously, for
twists/meter.
Good luck.
"David Covick" <d...@west.net> wrote in message
news:WK345.12$FG1...@newsfeed.avtel.net...
David Covick
This gives me a start!!
I was hoping for a formula....but, I guess I'll just have to work for the
answer :)
Regards,
David
A transmission line has a characteristic impedance, Zo, which you are trying
to evaluate. If such xmsn line is TERMINATED on one end with a resistance
(real impedance) of the same ohmic value, then a signal traveling from the
other end to the terminated end will not be reflected at the termination.
Thus you can use trial and error to evaluate Zo. Start with 100 ohms. Make
xmsn line long enough to see reflections; try several meters, but if you
have a fast O-scope your xmsn line can be shorter. You'll need some kind of
pulse generator, the sharper the better.
This is the general idea for testing. It is not a formula, obviously, for
twists/meter.
Good luck.
Russ.Shaw <rus...@webaxs.net> wrote in message
news:39515A25...@webaxs.net...
> Get 20cm of the wire, and measure its inductance with one end shorted.
Then
> measure the capacitance with the end open. Do the test at a low frequency
> where the wavelength is 2m or more in air. Now Zo=sqrt(L/C). Next, with
> end shorted, measure input impedance and increase the frequency. When
> Z hits infinite (very high), that's a quarter wavelength. Then it will
> go down to zero (half wavelength). This gives the velocity factor.
>
>
> David Covick wrote:
> >
> > I'm looking for a formula or table for determining the impedance of
twisted
> > wire based on the wire size/gage and the twists per inch.
> >
> > I have a formula for balanced wire pairs with a diameter and distance,
but I
> > don't think this applies when twisting is done.
> >
> > Or, can someone tell me how to "test" the twisted wire for its
impedance? I
> > have a HP impedance analyzer to do this, but don't know how to set up
the
> > test.
> >
> > Does anyone have clue on this one?
> >
> > Thanks,
> > David
>
> --
> *******************************************
> * Russell Shaw, B.Eng, M.Eng(Research) *
> * Electronics Consultant *
> * email: rus...@webaxs.net *
> * Australia *
> *******************************************
Steve Dunbar
>
> I'm looking for a formula or table for determining the impedance of twisted
> wire based on the wire size/gage and the twists per inch.
Here's a formula from Johnson & Graham's _High-Speed Digital Design, A
Handbook of Black Magic_, pp. 186-187.
d = conductor diameter (in.) s = separation between wire centers
(s>d)
er = effective relative permittivity (use permittivity of wire
insulation for twisted pairs)
Z = ( 120/sqrt(er) ) * ln(2*s/d)
-- Steve
John Larkin
|I'm looking for a formula or table for determining the impedance of twisted
|wire based on the wire size/gage and the twists per inch.
|
|I have a formula for balanced wire pairs with a diameter and distance, but I
|don't think this applies when twisting is done.
|
|Or, can someone tell me how to "test" the twisted wire for its impedance? I
|have a HP impedance analyzer to do this, but don't know how to set up the
|test.
|
|Does anyone have clue on this one?
|
|Thanks,
|David
|
David,
Brian Wadell's book, Transmission Line Design Handbook, has the
equations for a twisted pair. They are fairly hairy so I won't retype
them here. But basicly the twisted pair is like a parallel
insulated-wire-conductor line except that the effective dielectric
constant is a fairly weak function of twist. It goes something like
Eeff = 1 + q * (Er-1)
where Er is dielectric constant of the insulation
q = 0.25 + 0.0004 * a^2
a = tan-1 (T*pi*D) = pitch angle of twist, radians
Whew! I could fax you the page (despam my email address!). The bottom
line is that for moderate twists, impedance doesn't change much.
John
Reg Edwards
a function of wire spacing over wire diameter and the permittivity of
the insulating material.
Get some twin, figure-of-eight, 18, 20 or 22 gauge speaker cable from
Tandy's. The impedance at HF will be about 140 ohms whether it is
twisted or not. It makes a good aerial downlead.
For accuracy of home-made stuff, measure at a relatively low frequency
like 2 MHz on a length about 1/4-wavelength. When the measured input
resistance is the same as the terminating resistance then that's the
correct answer. For terminating resistors use 1/8 or 1/4-watt metal
film, 5% tolerance resistors.
Why twist it anyway ?
---
Reg
--
=============================
Regards from Reg, G4FGQ
For free radio design software go to:-
http://www.btinternet.com/~g4fgq.regp
=============================
David Covick <d...@west.net> wrote in message
news:WK345.12$FG1...@newsfeed.avtel.net...
Russ.Shaw
measure the capacitance with the end open. Do the test at a low frequency
where the wavelength is 2m or more in air. Now Zo=sqrt(L/C). Next, with
end shorted, measure input impedance and increase the frequency. When
Z hits infinite (very high), that's a quarter wavelength. Then it will
go down to zero (half wavelength). This gives the velocity factor.
David Covick wrote:
>
> I'm looking for a formula or table for determining the impedance of twisted
> wire based on the wire size/gage and the twists per inch.
>
> I have a formula for balanced wire pairs with a diameter and distance, but I
> don't think this applies when twisting is done.
>
> Or, can someone tell me how to "test" the twisted wire for its impedance? I
> have a HP impedance analyzer to do this, but don't know how to set up the
> test.
>
> Does anyone have clue on this one?
>
> Thanks,
> David
--
James W. Swonger
>I'm looking for a formula or table for determining the impedance of twisted
>wire based on the wire size/gage and the twists per inch.
>
>I have a formula for balanced wire pairs with a diameter and distance, but I
>don't think this applies when twisting is done.
degree of twist to them. I've seen them from twists per inch to
twists per foot, but I've never seen any straight-parallel 2-wire
data cable. If you're looking at a table for common data standards
like RS-422, that's shielded twisted pair.
I think that until the "core diameter" is bigger than the twist
spacing, you don't have enough of an inductance increase over plain
wire to bother with recalculating.
--
##########################################################################
#Irresponsible rantings of the author alone. Any resemblance to persons #
#living or dead then yer bummin. May cause drowsiness. Alcohol may inten-#
#sify this effect. Pay no attention to the man behind the curtain. Billy!#
John Woodgate
<j...@space.eng.intersil.com> inimitably wrote:
>I think that until the "core diameter" is bigger than the twist
>spacing, you don't have enough of an inductance increase over plain
>wire to bother with recalculating.
Because twisting make the real conductor length a little longer than the
'cable length'. Therefore both the inductance and capacitance per unit
cable length increase. But the effects are normally very small.
--
Regards, John Woodgate, OOO - Own Opinions Only. Phone +44 (0)1268 747839
Fax +44 (0)1268 777124. http://www.jmwa.demon.co.uk I wanted to make a fully-
automated nuclear-powered trawler,but it went into spontaneous fishing.
PLEASE do not mail copies of newsgroup posts to me.
Jonathan Bromley
>David Covick wrote:
>>
>> I'm looking for a formula or table for determining the impedance of twisted
>> wire based on the wire size/gage and the twists per inch.
>
>
>Here's a formula from Johnson & Graham's _High-Speed Digital Design, A
>Handbook of Black Magic_, pp. 186-187.
>
>d = conductor diameter (in.)
-------------------------^^ doesn't matter as long as s is in
--------------------------- the same units
> s= separation between wire centers (s>d)
I hope s>d; if not you're in trouble!
>er = effective relative permittivity (use permittivity of wire
>insulation for twisted pairs)
>Z = ( 120/sqrt(er) ) * ln(2*s/d)
all of which verifies the well-known fact that it is *very*
difficult to make a wire-pair or other "open" transmission line
with an impedance outside the range of about 70 to 120 ohm.
So, for a wide range of purposes, 100 ohm is such a good guess
that you don't need to worry.
You can't get away with being so careless for serious RF work
or very fast logic, of course.
Jonathan Bromley
David Covick
I'm using twisted wire on. I want to match a very low impedance of the
MOSFETs to the 50 ohm load. Things are not exactly where I want them so I
decided that I need to understand the twisting aspect of the wire. I think
that I am seeing anywhere from 30 to 80 ohms on the twisted pairs. I
appreciate you all helping me to understand this as I did not have a clue.
Everything I could find on the Internet and my reference books talks about
parallel lines.
Actually, this seems like an interesting topic, but possibly not all that
complex with the small change in Z relative to twists.
Thanks Again!!!!
David
David Covick <d...@west.net> wrote in message
news:WK345.12$FG1...@newsfeed.avtel.net...
> I'm looking for a formula or table for determining the impedance of
twisted
> wire based on the wire size/gage and the twists per inch.
>
> I have a formula for balanced wire pairs with a diameter and distance, but
I
> don't think this applies when twisting is done.
>
Robert
"twists-per-inch" factor involved. The reason is that the twists are there to 1)
keep the dimensions that actually are relevant constant 2) reduce emissions and
3) reduce susceptibility to plane field coupling.
Leon Heller
"David Covick" <d...@west.net> wrote:
> Thanks all for the good information! I am playing with toroid baluns
that
> I'm using twisted wire on. I want to match a very low impedance of
the
> MOSFETs to the 50 ohm load. Things are not exactly where I want them
so I
> decided that I need to understand the twisting aspect of the wire. I
think
> that I am seeing anywhere from 30 to 80 ohms on the twisted pairs. I
> appreciate you all helping me to understand this as I did not have a
clue.
> Everything I could find on the Internet and my reference books talks
about
> parallel lines.
> Actually, this seems like an interesting topic, but possibly not all
that
> complex with the small change in Z relative to twists.
Twisted pairs are often used when winding broadband transmission line
RF transformers with thin wire, because of the difficulty of keeping
them parallel. Power transmission-line transformers are wound with
carefully spaced thick wire.
Leon
--
Leon Heller, G1HSM
Tel: (Mobile) 079 9098 1221 (Work) +44 1327 357824
Email: leon_...@hotmail.com
Web: http://www.geocities.com/SiliconValley/Code/1835
Sent via Deja.com http://www.deja.com/
Before you buy.
Tony Williams
> I think that I am seeing anywhere from 30 to 80 ohms on
> the twisted pairs.
I thought that twisted-pair was traditionally about
120 ohms characteristic impedance.
--
Tony Williams.
John Woodgate
David Covick
This is a broadband transmission line RF transformer....and for power. I've
also had some heating of the wire, so I may have to go with parallel as you
suggest. John Woodgate suggested that the wire insulation was thin....I
think he is right because I've has some arcing problems (1000 watts)
Possibly I need Teflon coated wire rather than "magnet wire".
Thanks for all the suggestions.
David
Leon Heller <leon_...@hotmail.com> wrote in message
news:8ivm52$t8b$1...@nnrp1.deja.com...
> In article <W0A45.48$FG1....@newsfeed.avtel.net>,
> "David Covick" <d...@west.net> wrote:
> > Thanks all for the good information! I am playing with toroid baluns
> that
> > I'm using twisted wire on. I want to match a very low impedance of
> the
> > MOSFETs to the 50 ohm load. Things are not exactly where I want them
> so I
> > decided that I need to understand the twisting aspect of the wire. I
> think
> > that I am seeing anywhere from 30 to 80 ohms on the twisted pairs. I
klock...@cix.compulink.co.uk
jona...@oxfordbromley.u-net.com (Jonathan Bromley) wrote:
> all of which verifies the well-known fact that it is *very*
> difficult to make a wire-pair or other "open" transmission line
> with an impedance outside the range of about 70 to 120 ohm.
>
> So, for a wide range of purposes, 100 ohm is such a good guess
> that you don't need to worry.
Unless it's screened, then it could be as low as half of these figures.