WANTED: 72 ohm twinlead
Gary Stone
My brother-n-law (KI7XQ) is looking for about 3 feet of 72 ohm twinlead
for a balun on an antenna project. I know very little of the project,
but he does not have internet and I wanted to try and locate a source for
him.
Any ideas? Please e-mail response if possible.
Thanks and 73,
Gary
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Jim Devenport
3 feet? I could mail him some but the postage would exceed the
value of the twinlead. We have found 72 ohm twinlead to be
fairly difficult to locate and when we finally did we bought a
spool. For short HF uses many varieties of common 2-wire AC
"ZIP" cord closely approach 72-75 ohms impedance. Perhaps that
would work for his application.
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|Jim Devenport WB5AOX |
|All Standard Disclaimers Disclaimed |
|My views rarely (if ever) reflect those of my employers|
|-------------------------------------------------------|
Clifford Buttschardt
is also of use in baluns..73 Cliff Buttschardt W6HDO
On Tue, 23 Jan 1996, James P. Meyer wrote:
> Gary Stone <gary...@texoma.com> wrote:
>
> >My brother-n-law (KI7XQ) is looking for about 3 feet of 72 ohm twinlead
> >for a balun on an antenna project. I know very little of the project,
> >but he does not have internet and I wanted to try and locate a source for
> >him.
>
> For just 3 feet, he would be better off making the twinlead
> himself out of copper or aluminum tubing. The diameter and spacing should
> be easy to calculate or adjust from measurements.
>
> In the olden days, that's the way *real* hams did everything.
>
> 8-)
>
> Jim
>
>
>
>
Donald J. Miller
: My brother-n-law (KI7XQ) is looking for about 3 feet of 72 ohm twinlead
: for a balun on an antenna project. I know very little of the project,
: but he does not have internet and I wanted to try and locate a source for
: him.
You can't realize this transmission line with twinlead construction.
For an impedance that low, the diameter of the conductors is greater
than the spacing:
s = center to center spacing
d = wire diameter
Zo = 276 LOG(2s/d)
Usually transmission lines using twinlead type construction have Zo
values in the *hundreds* of ohms.
You can still make a 72-ish ohm balanced transmission line, however,
if you use a different type of construction:
Take four (4) sections of RG-59 (73 ohm coax) of equal length and
run them in parralell. Solder all the shields together on both
ends. Now, connect the center conductors together in pairs.
Cable A and B should have their center conductors connected
together on both ends, and cable C and D should have their
center conductors tied together. You now have a 73 ohm
shielded *balanced* transmission line. I will try to render it
in ascii:
------------------------------------------
--------------------------------------------
/ ------------------------------------------ \
-----------------------------------------------------
IN ------------------------------------------ OUT
-----------------------------------------------------
\ ------------------------------------------ /
--------------------------------------------
------------------------------------------
If you could find (or make) 37.5 ohm coax, you would need only
two parallel coax lines instead of four. It would not be
necessary to use pairs of coax's to get 37.5 ohms per side for
a net 75 ohm balanced line.
Have fun!
--
-------------------------------------------------
Don Miller
dmi...@crl.com
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James P. Meyer
>My brother-n-law (KI7XQ) is looking for about 3 feet of 72 ohm twinlead
>for a balun on an antenna project. I know very little of the project,
>but he does not have internet and I wanted to try and locate a source for
>him.
For just 3 feet, he would be better off making the twinlead
Robert Smits
>You can't realize this transmission line with twinlead construction.
>For an impedance that low, the diameter of the conductors is greater
>than the spacing:
>
> s = center to center spacing
> d = wire diameter
>
> Zo = 276 LOG(2s/d)
>
>Usually transmission lines using twinlead type construction have Zo
>values in the *hundreds* of ohms.
Your theory notwithstanding, 75 ohm twin lead HAS been available for
decades. Look in any ARRL antenna handbook for 214-023. Transmitting type
75-ohm Twinlead has two stranded 7/21 conductors about equal to solid #12
wire with quite close spacing.
Jeff DePolo
Donald J. Miller wrote:
>
> Gary Stone (gary...@texoma.com) wrote:
>
> : My brother-n-law (KI7XQ) is looking for about 3 feet of 72 ohm twinlead
> : for a balun on an antenna project. I know very little of the project,
> : but he does not have internet and I wanted to try and locate a source for
> : him.
>
> For an impedance that low, the diameter of the conductors is greater
> than the spacing:
>
> s = center to center spacing
> d = wire diameter
>
> Zo = 276 LOG(2s/d)
This is not always true. 72 ohm transmitting line is a common
item and is readily available from a number of sources. Either
The Wireman, RF Connection, or Radio Works should have it.
The above equation applies only directly to air-insulated parallel conductors.
A non-air dielectric will lower the characteristic impedance, which is what you
see with 72 ohm transmitting line, which uses something like 14 or 12 gauge
wire close-spaced in solid dielectric.
> You can still make a 72-ish ohm balanced transmission line, however,
> if you use a different type of construction:
>
> Take four (4) sections of RG-59 (73 ohm coax) of equal length and
> run them in parralell. Solder all the shields together on both
> ends. Now, connect the center conductors together in pairs.
> Cable A and B should have their center conductors connected
> together on both ends, and cable C and D should have their
> center conductors tied together. You now have a 73 ohm
> shielded *balanced* transmission line. I will try to render it
> in ascii:
Probably easier to take four sections of 300 ohm twinlead and connect
them in parallel. Just keep them safely separated.
--- Jeff
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Jeff DePolo WN3A Twisted Pair: H:610-337-7383 W:215-387-3059 x300
dep...@eniac.seas.upenn.edu RF: 442.1 442.2 442.4 443.45 443.8 444.15 linked
Claim to Fame: I got the first speeding ticket on the information superhighway
--
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
Jeff DePolo WN3A Twisted Pair: H:610-337-7383 W:215-387-3059 x300
dep...@eniac.seas.upenn.edu RF: 442.1 442.2 442.4 443.45 443.8 444.15 linked
Claim to Fame: I got the first speeding ticket on the information superhighway
Roy Lewallen
; Gary Stone (gary...@texoma.com) wrote:
; : My brother-n-law (KI7XQ) is looking for about 3 feet of 72 ohm twinlead
; : for a balun on an antenna project. I know very little of the project,
; : but he does not have internet and I wanted to try and locate a source for
; : him.
; You can't realize this transmission line with twinlead construction.
; For an impedance that low, the diameter of the conductors is greater
; than the spacing:
; s = center to center spacing
; d = wire diameter
; Zo = 276 LOG(2s/d)
The formula you quote is an approximation, which falls apart with close
spacing/large conductors, as you found. The correct formula is
Zo = 120 cosh^-1 (s/d)
where cosh^-1 is arc hyperbolic cosine.
This is for air dielectric. The presence of other material between the
conductors will lower the impedance.
72 ohm twinlead used to be common stuff, in two sizes, "receiving" and
"transmitting". I've got a bit of the transmitting stuff in my junk box,
but it's too large and stiff for practical use in a balun. In this day
and age, I'd use twisted pair with thinnish insulation. Unless it's being
used at a frequency where 3 feet is an appreciable fraction of a wavelength,
the Zo of the line probably won't be very important.
Roy Lewallen, W7EL