Transmission line stuff 5
 

I seem to recall that ladder/open-wire feeders are relatively high
impedance ~500 ohms. Is it possible to construct 50 ohm open wire
feeder and if so, what would the spacing be?

Thanks!

Transmission line stuff 5
 

phaedrus wrote:
I seem to recall that ladder/open-wire feeders are relatively high
impedance ~500 ohms. Is it possible to construct 50 ohm open wire
feeder and if so, what would the spacing be?

Thanks!

Possible, perhaps, but not practical. Assuming air insulation, the
spacing between the wires would have to be just under 9% of the wire
diameter, and this spacing would have to be maintained very closely to
keep the impedance constant. I might still have some 72 ohm twinlead in
my junk box, which consisted of wires embedded in molded polyethylene.

With your interest in transmission lines, you'd benefit a great deal
from getting a copy of the _ARRL Antenna Book_.

Roy Lewallen, W7EL

Transmission line stuff 5
 

On Thu, 11 Feb 2010 08:02:55 -0800 (PST), phaedrus
wrote:

Is it possible to construct 50 ohm open wire
feeder and if so, what would the spacing be?

Look at your lamp zip cord - close enough.

73's
Richard Clark, KB7QHC

Transmission line stuff 5
 

phaedrus wrote in news:fe96541e-75fc-423a-8eb7-
:

I seem to recall that ladder/open-wire feeders are relatively high
impedance ~500 ohms. Is it possible to construct 50 ohm open wire
feeder and if so, what would the spacing be?

Such a low characteristic impedance requires very close spacing of round
conductors. Proximity effect which causes the current to mainly flow in a
small adjacent portion of the conductors at close spacing makes achivement
of very low Zo at low loss very difficult, and not very practical.

TWLLC at http://www.vk1od.net/calc/tl/twllc.htm will perform the
calculations for you, including calculating the matched line loss and loss
under mismatch conditions, but read the comments on the page re proximity
effect.

Owen

Transmission line stuff 5
 

Richard Clark wrote in
:

Look at your lamp zip cord - close enough.

There is an interesting example.

K8ZOA published a characterisation of ZIP line some years ago in this
groupd, and Zo is approximately 100 ohms.

Considering a half wave of ZIP cord with a load of 100+j0 at 14MHz, loss is
around 1dB.

Now, using N7WS's characterisation of Wireman 551 ladder line, again a half
wave at 14MHz, smaller conductors, wider spacing, longer, Zo=400, higher
VSWR, the loss with a load of 100+j0 at 14MHz, loss is around 0.14dB.

Owen

Transmission line stuff 5
 

On Feb 11, 8:02*am, phaedrus wrote:
I seem to recall that ladder/open-wire feeders are relatively high
impedance ~500 ohms. Is it possible to construct 50 ohm open wire
feeder and if so, what would the spacing be?

Thanks!

As hinted by others, especially Owen, it's quite possible to make
parallel-conductor line of very low impedance. There are two ways
that are easy to describe, and these may well bring others to your
mind:

First, consider a line made of broad, flat conductors: copper
ribbon. If the spacing is close compared with the conductor width,
the impedance will be low. This is not particularly practical for
"open-wire feeders," but is commonly used on printed circuit boards.
In the extreme, you can make transmission lines with impedances under
10 ohms this way, if you're of a mind to. You can use thin, flexible
Kapton tape, perhaps 0.1mm thick or less, and have conductors 10 or
more mm wide. According to a formula in Sams' "Reference Data for
Engineers," the impedance will be approximately 377*s/
(sqrt(epsilon)*w), where w is the conductor width and s is the
spacing. For air dielectric, where epsilon is 1, you can make a 10
ohm line if the width is about 38 times the spacing.

Second, consider a multi-wire line. One common construction is four
wires arranged as in the corners of a square, with diagonally opposite
wires connected together at the ends. That gets you a lower impedance
than open wire with the same spacing as the side of the square and of
course the same wire diameter...it's a bit more than half the
impdeance. You could also have more wires arranged around a circle,
with alternate wires fed in parallel. A variation on this theme has
been used on occasion by rabid stereophiles to feed (~8ohm) speakers:
use a 50-conductor ribbon cable. Mass-terminate with a standard two-
row connector. Tie all the pins on each side of the mating connector
together, so the transmission line is similar to 25 pairs that are
fairly closely spaced. (IMO, there are better ways to accomplish flat
frequency response in a stereo system...) But it does work to get a
low impedance line. (And yes, if you look closely, you'll realize the
transmission line impedance is NOT constant as you go to lower
frequencies...)

On the other hand, there's generally not much advantage to making a
low-impedance balanced line for RF work, so they are pretty uncommon
in practice.

Cheers,
Tom

Transmission line stuff 5
 

On Feb 11, 8:02*am, phaedrus wrote:
I seem to recall that ladder/open-wire feeders are relatively high
impedance ~500 ohms. Is it possible to construct 50 ohm open wire
feeder and if so, what would the spacing be?

Thanks!

Oh, and to more directly answer your question, the formula generally
given for accurate description of the impedance of two parallel, round
conductors of the same diameter, in air, is:

Zo = 120 * invcosh(D/d)

where D/d is the center-center distance and d is the conductor
diameter. So...

D/d for 50 ohms = cosh(50/120) = 1.088

For 1" diameter wires, you'd have to space them 0.088 inches apart,
which is tough to maintain unless you embed them in a solid
dielectric.

I suspect the formula above is in error for very close spacings
because of proximity effects...I suspect it assumes a uniform thin
cylinder of current...but I'm not sure about that.

Cheers,
Tom

Transmission line stuff 5
 

On Feb 11, 8:02*am, phaedrus wrote:
I seem to recall that ladder/open-wire feeders are relatively high
impedance ~500 ohms. Is it possible to construct 50 ohm open wire
feeder and if so, what would the spacing be?

Thanks!

And something else for you to ponder: Coax is generally available
commercially at about 50 ohms and about 75 ohms, with a few lines at
about 95 ohms and some specials of other low impedances. What would
~500 ohm coax look like (assuming a straight center conductor, not a
helix)?

Cheers,
Tom

Transmission line stuff 5
 

K7ITM wrote:
On Feb 11, 8:02 am, phaedrus wrote:
I seem to recall that ladder/open-wire feeders are relatively high
impedance ~500 ohms. Is it possible to construct 50 ohm open wire
feeder and if so, what would the spacing be?

Thanks!

As hinted by others, especially Owen, it's quite possible to make
parallel-conductor line of very low impedance. There are two ways
that are easy to describe, and these may well bring others to your
mind:

First, consider a line made of broad, flat conductors: copper
ribbon. If the spacing is close compared with the conductor width,
the impedance will be low. This is not particularly practical for
"open-wire feeders," but is commonly used on printed circuit boards.
In the extreme, you can make transmission lines with impedances under
10 ohms this way, if you're of a mind to. You can use thin, flexible
Kapton tape, perhaps 0.1mm thick or less, and have conductors 10 or
more mm wide. According to a formula in Sams' "Reference Data for
Engineers," the impedance will be approximately 377*s/
(sqrt(epsilon)*w), where w is the conductor width and s is the
spacing. For air dielectric, where epsilon is 1, you can make a 10
ohm line if the width is about 38 times the spacing.


You see this in microstrip construction on various substrates,
particularly where it's part of a matching network for transistor
amplifiers (which can have Zs of a few ohms)


On the other hand, there's generally not much advantage to making a
low-impedance balanced line for RF work, so they are pretty uncommon
in practice.

in RF communications, that's true.

In other high power RF applications, low Z can be desirable (because
you'd rather take the IR hit than deal with the high voltage).

In the pulsed nuclear research business you see lines with Z of a few
ohms in pulse forming networks :often using distilled water as the
dielectric.. that epsilon of 80 gets the C right up there so Sqrt(L/C)
is small.





Cheers,
Tom

Transmission line stuff 5
 

K7ITM wrote:
On Feb 11, 8:02 am, phaedrus wrote:
I seem to recall that ladder/open-wire feeders are relatively high
impedance ~500 ohms. Is it possible to construct 50 ohm open wire
feeder and if so, what would the spacing be?

Thanks!

Oh, and to more directly answer your question, the formula generally
given for accurate description of the impedance of two parallel, round
conductors of the same diameter, in air, is:

Zo = 120 * invcosh(D/d)

where D/d is the center-center distance and d is the conductor
diameter. So...

D/d for 50 ohms = cosh(50/120) = 1.088

For 1" diameter wires, you'd have to space them 0.088 inches apart,
which is tough to maintain unless you embed them in a solid
dielectric.


or you go away from round wires...