Have a 100 foot roll(approx) of used coax cable which has no markings
printed on it.
Can anyone tell me how I can work out what it might be ? e.g. 75 ohm or
50 ohm or ?
Going by the size it looks similar to 50 ohm but I'm not sure.

Any suggestions would be appreciated.

Boozo wrote:
Have a 100 foot roll(approx) of used coax cable which has no markings
printed on it.
Can anyone tell me how I can work out what it might be ? e.g. 75 ohm or
50 ohm or ?
Going by the size it looks similar to 50 ohm but I'm not sure.

Any suggestions would be appreciated.

You didn't mention what you have to make measurements with. The
simplest way to make a quick estimate is probably to guess at the
velocity factor first: solid polyethylene is about 0.66, and foam
polyethylene is roughly 0.78. The impedance in ohms is 60*velocity
factor*ln(D/d), where ln is the natural logarithm, and D/d is the ratio
of inner to outer conductor diameters. So for solid polyethylene
dielectric, D/d is about 3.5 for 50 ohm line, and about 6.2 for 72-75
ohm line. Even for fairly small line, it's not difficult to tell by
just looking at it. It's also possible to measure the capacitance of
the lenth of line and the electrical length in nanoseconds, and
calculate the impedance from those two. If you can measure RF
impedance, measure the line with the far end open, and with it shorted,
and take the square root of the product of those two.

Cheers,
Tom

Boozo ha escrito:

Have a 100 foot roll(approx) of used coax cable which has no markings
printed on it.
Can anyone tell me how I can work out what it might be ? e.g. 75 ohm or
50 ohm or ?
Going by the size it looks similar to 50 ohm but I'm not sure.

Any suggestions would be appreciated.

About coax impedance,

When you just have a (VHF) transmitter with 50 Ohm VSWR meter and
external dummy load, you can do the following:

Check whether the insulation (between inner and outer conductor) is
solid or not (may be foam).
Check whether the insulation (between inner and outer conductor) melts
(changes color to transparent) at relative low temperature.

If solid, the velocity factor is about 0.66, when foam, it will be
about 0.79..0.84
If it melts at relative low temperature it is very likely Polyethyleen
(PE) insulation, if not, it is PTFE (Teflon).

Based on your transmission frequency and the velocity factor, cut an
electrically quarter wave. Terminate the cable with the 50 Ohms dummy
load and connect the cable to the VSWR meter. Measure the VSWR of the
cable/dummy load combination.

When VSWR is almost one, you can be sure it is 50 Ohms cable. When it
is about 2.2, it will be 75 Ohms cable. When it has VSWR close to 3.5,
it is probably 93 Ohms cable. This is based on: Zload*Zinput = Zcalbe^2
for a quarter wave line.

When these measurements matches the mechanical identification of Tom's
posting, you can be very sure about the impedance of the cable.

Best Regards,


Wim
PA3DJS

Can anyone tell me how I can work out what it might be ? e.g. 75 ohm or
50 ohm or ?


Sure, attach it to a dummy load and see if you get a perfect 1:1 swr.
There are lots of other ways but this is simplest.

Boozo wrote:
Can anyone tell me how I can work out what it might be ? e.g. 75 ohm or
50 ohm or ?

If it is aluminum, it may be 75 ohm cable TV coax.
--
73, Cecil http://www.w5dxp.com

"Boozo" wrote in message
ups.com...
Have a 100 foot roll(approx) of used coax cable which has no markings
printed on it.
Can anyone tell me how I can work out what it might be ? e.g. 75 ohm or
50 ohm or ?
Going by the size it looks similar to 50 ohm but I'm not sure.

Any suggestions would be appreciated.


If it has a solid center conductor, it is almost certainly 75 Ohm. The only
exception I can think of is some weird version of RG58.

Tam

Tam/WB2TT wrote:
"Boozo" wrote in message
ups.com...
Have a 100 foot roll(approx) of used coax cable which has no markings
printed on it.
Can anyone tell me how I can work out what it might be ? e.g. 75 ohm or
50 ohm or ?
Going by the size it looks similar to 50 ohm but I'm not sure.

Any suggestions would be appreciated.


If it has a solid center conductor, it is almost certainly 75 Ohm. The only
exception I can think of is some weird version of RG58.

Tam

I do have a bunch of older RG-58 with solid center conductor, and
pretty much all the HP BNC patch cables I've seen the insides of have a
solid center. I believe the various semi-official versions (RG-58/U,
RG-58A/U, RG-58B/U and RG-58C/U) tell whether the center is supposed to
be solid or stranded, but that seems to have become *******ized. A
quick Google search for RG58A yields cables with both solid and
stranded centers. How about Belden 9913? Of course, the construction
of that line is "different" enough that it would be hard to mistake it
for something else, and I assume the poster would have mentioned the
strange construction. Also, you're likely to find a solid center in
higher impedance line like RG62/U, and you're likely to find stranded
center on 75 ohm line (like RG-11/U, and others). And of course for
some surplus custom line, all bets are off about both stranding and
impedance!

Cheers,
Tom

If it has a solid center conductor, it is almost certainly 75 Ohm. The
only
exception I can think of is some weird version of RG58.

Tam

I do have a bunch of older RG-58 with solid center conductor, and
pretty much all the HP BNC patch cables I've seen the insides of have a
solid center. I believe the various semi-official versions (RG-58/U,
RG-58A/U, RG-58B/U and RG-58C/U) tell whether the center is supposed to
be solid or stranded, but that seems to have become *******ized. A
quick Google search for RG58A yields cables with both solid and
stranded centers. How about Belden 9913? Of course, the construction
of that line is "different" enough that it would be hard to mistake it
for something else, and I assume the poster would have mentioned the
strange construction. Also, you're likely to find a solid center in
higher impedance line like RG62/U, and you're likely to find stranded
center on 75 ohm line (like RG-11/U, and others). And of course for
some surplus custom line, all bets are off about both stranding and
impedance!

Cheers,
Tom

Well, I was going to avoid this, but when you chimed in Tom, couldn't help
myself. In turn, I have a large helping of a Times Wire and Cable RG-8 size
50 Ohm coax with a solid inner conductor. Don't remember their nomenclature
for it but it's a real bear getting "N" connectors onto it..

W4ZCB

In article . com,
"Boozo" wrote:

Have a 100 foot roll(approx) of used coax cable which has no markings
printed on it.
Can anyone tell me how I can work out what it might be ? e.g. 75 ohm or
50 ohm or ?
Going by the size it looks similar to 50 ohm but I'm not sure.

Any suggestions would be appreciated.

just a note, lots of cable have sold conductors at 50ohms times
microwave and andrews cinta are examples

"Harold E. Johnson" wrote in message
news:A1N1h.261969$1i1.61365@attbi_s72...

If it has a solid center conductor, it is almost certainly 75 Ohm. The
only
exception I can think of is some weird version of RG58.

Tam

I do have a bunch of older RG-58 with solid center conductor, and
pretty much all the HP BNC patch cables I've seen the insides of have a
solid center. I believe the various semi-official versions (RG-58/U,
RG-58A/U, RG-58B/U and RG-58C/U) tell whether the center is supposed to
be solid or stranded, but that seems to have become *******ized. A
quick Google search for RG58A yields cables with both solid and
stranded centers. How about Belden 9913? Of course, the construction
of that line is "different" enough that it would be hard to mistake it
for something else, and I assume the poster would have mentioned the
strange construction. Also, you're likely to find a solid center in
higher impedance line like RG62/U, and you're likely to find stranded
center on 75 ohm line (like RG-11/U, and others). And of course for
some surplus custom line, all bets are off about both stranding and
impedance!

Cheers,
Tom

Well, I was going to avoid this, but when you chimed in Tom, couldn't help
myself. In turn, I have a large helping of a Times Wire and Cable RG-8
size 50 Ohm coax with a solid inner conductor. Don't remember their
nomenclature for it but it's a real bear getting "N" connectors onto it..

W4ZCB


Solid could be 92 ohms too.