We all know that for an open-wire feedline, Zo = 276 log 2S/d, where S=
the center-to-center distance between the two conductors, and d= the
outer diameter of each conductor.

As far as I know, this formula applies to single-core wires. Is there
any significant difference to Zo where multi-strand wire is used
instead? I'm thinking of how the RF skin effect might affect things. Or
is it simply a question of treating the overall diameter of all the
strands as one solid conductor?

Further, if the multi-strand wire is plastic coated, what effect does
that have on Zo?

--
73
Ian, G3NRW

Ian Wade G3NRW wrote in news:F3UuvlXZYcxMFwB2
@ntlworld.com:

We all know that for an open-wire feedline, Zo = 276 log 2S/d, where S=
the center-to-center distance between the two conductors, and d= the
outer diameter of each conductor.

Well, that is an approximation that falls down at closer spacings.


As far as I know, this formula applies to single-core wires. Is there

It applies to circular conductors.

any significant difference to Zo where multi-strand wire is used
instead? I'm thinking of how the RF skin effect might affect things. Or
is it simply a question of treating the overall diameter of all the
strands as one solid conductor?

I have seen some proposed corrections for stranded bare wires, but the
factors are small in the scheme of things for seven strand and smaller
for 19 strand.


Further, if the multi-strand wire is plastic coated, what effect does
that have on Zo?

Assuming you mean bare wires twisted and then served with insulation...

Dielectric with rel permitivitty 1 will reduce Zo.

Have a play with TWLLC at http://www.vk1od.net/calc/tl/twllc.htm .

An effective way to estimate the effective permittivity is to measure the
velocity factor of a sample of line, and use permittivity=vf^-2.

Owen

From: Owen Duffy
Date: Mon, 25 Oct 2010 Time: 19:31:38

Ian Wade G3NRW wrote in news:F3UuvlXZYcxMFwB2
:

We all know that for an open-wire feedline, Zo = 276 log 2S/d, where S=
the center-to-center distance between the two conductors, and d= the
outer diameter of each conductor.

Well, that is an approximation that falls down at closer spacings.


As far as I know, this formula applies to single-core wires. Is there

It applies to circular conductors.

any significant difference to Zo where multi-strand wire is used
instead? I'm thinking of how the RF skin effect might affect things. Or
is it simply a question of treating the overall diameter of all the
strands as one solid conductor?

I have seen some proposed corrections for stranded bare wires, but the
factors are small in the scheme of things for seven strand and smaller
for 19 strand.


Further, if the multi-strand wire is plastic coated, what effect does
that have on Zo?

Assuming you mean bare wires twisted and then served with insulation...

Dielectric with rel permitivitty 1 will reduce Zo.

Have a play with TWLLC at http://www.vk1od.net/calc/tl/twllc.htm .

An effective way to estimate the effective permittivity is to measure the
velocity factor of a sample of line, and use permittivity=vf^-2.

Owen


Many thanks Owen.

--
73
Ian, G3NRW

On Oct 25, 3:31*pm, Owen Duffy wrote:
Ian Wade G3NRW wrote in news:F3UuvlXZYcxMFwB2
@ntlworld.com:

We all know that for an open-wire feedline, Zo = 276 log 2S/d, where S=
the center-to-center distance between the two conductors, and d= the
outer diameter of each conductor.

Well, that is an approximation that falls down at closer spacings.



As far as I know, this formula applies to single-core wires. Is there

It applies to circular conductors.

any significant difference to Zo where multi-strand wire is used
instead? I'm thinking of how the RF skin effect might affect things. Or
is it simply a question of treating the overall diameter of all the
strands as one solid conductor?

I have seen some proposed corrections for stranded bare wires, but the
factors are small in the scheme of things for seven strand and smaller
for 19 strand.



Further, if the multi-strand wire is plastic coated, what effect does
that have on Zo?

Assuming you mean bare wires twisted and then served with insulation...

Dielectric with rel permitivitty 1 will reduce Zo.

Have a play with TWLLC athttp://www.vk1od.net/calc/tl/twllc.htm.

An effective way to estimate the effective permittivity is to measure the
velocity factor of a sample of line, and use permittivity=vf^-2.

Owen



- Hide quoted text -

- Show quoted text -

Just wondering how a few mils of PVC insulation would effect the
permittivity of a couple of 12 guage wires spaced a couple of inches
apart.

Jimmie

JIMMIE wrote in
:

Just wondering how a few mils of PVC insulation would effect the
permittivity of a couple of 12 guage wires spaced a couple of inches
apart.

By "mils", do you mean thousands of an inch? (The rest of your dimensions
are imperial.)

I doubt it would make much difference

The easiest way I know of to assess the effect of the mixed dielectric
environment, is to measure the velocity factor, and use that to infer an
equivalent permittivity (as I noted in my earlier post).

Owen

Ian,

Here's how results from your approximation formula diverge from more-
accurate Zo figures:
http://www.karinya.net/g3txq/temp/ladderline_zo.png

73,
Steve G3TXQ

From: steveeh131047
Date: Wed, 27 Oct 2010 Time: 05:10:05

Ian,

Here's how results from your approximation formula diverge from more-
accurate Zo figures:
http://www.karinya.net/g3txq/temp/ladderline_zo.png

73,
Steve G3TXQ


But I take it that your picture still applies to single-core conductors,
and very fat conductors at that.

With the feedline I have in mind, S=55mm and d=1.2mm, so S/d=27.5, which
is way off the end of the scale in your picture.

--
73
Ian, G3NRW

But I take it that your picture still applies to single-core conductors,
and very fat conductors at that.

Correct.


With the feedline I have in mind, S=55mm and d=1.2mm, so S/d=27.5, which
is way off the end of the scale in your picture.


Then your approximation formula will be fine.

Steve G3TXQ