"Reg Edwards" wrote in message ...
The velocity factor of ALL solid polyethylene coax cable, regardless of
impedance, is 0.665

And this comes from someone who I could swear posted not long ago a
table that had velocity factors for solid polyethylene cable that were
significantly different from this magic number?

But even if we just limit ourselves to HF and above, there's a
problem: most "solid poly" cable I've encountered has small gas
bubbles in the dielectric, and the v.f. does not measure exactly
0.665. Most of the time, the difference doesn't matter, but sometimes
it does, and then it's not safe to assume it's 0.665. And of course a
lot of cable these days uses foam dielectric, which can be noticably
different from batch to batch.

"Tom Bruhns" wrote "Reg Edwards"
The velocity factor of ALL solid polyethylene coax cable, regardless of
impedance, is 0.665

================================

And this comes from someone who I could swear posted not long ago a
table that had velocity factors for solid polyethylene cable that were
significantly different from this magic number?

But even if we just limit ourselves to HF and above, there's a
problem: most "solid poly" cable I've encountered has small gas
bubbles in the dielectric, and the v.f. does not measure exactly
0.665. Most of the time, the difference doesn't matter, but sometimes
it does, and then it's not safe to assume it's 0.665. And of course a
lot of cable these days uses foam dielectric, which can be noticably
different from batch to batch.

===============================

Your para. 1. You can swear till you're appoplectic black and blue in the
face - it wasn't me. I'm not THAT stupid. So who was it then?

Your para. 2. If there are little bubbles in it, it is not solid. If it
is foamed, it is not solid.

Just to add a little more useless information, did you know the stuff also
varies with pressure and temperature as at the bottoms of the oceans? Also,
under pressure, water disassociates and hydrogen slowly diffuses through it.
Rodents seem to take a fancy to it.

Velocity factor = (Permittivity)^(- 1/2). Permittivity of polyethylene =
2.26 but it does vary slighty from one book to the next.
----
Reg.

"Tom Bruhns" wrote "Reg Edwards"
The velocity factor of ALL solid polyethylene coax cable, regardless of
impedance, is 0.665

================================

And this comes from someone who I could swear posted not long ago a
table that had velocity factors for solid polyethylene cable that were
significantly different from this magic number?

But even if we just limit ourselves to HF and above, there's a
problem: most "solid poly" cable I've encountered has small gas
bubbles in the dielectric, and the v.f. does not measure exactly
0.665. Most of the time, the difference doesn't matter, but sometimes
it does, and then it's not safe to assume it's 0.665. And of course a
lot of cable these days uses foam dielectric, which can be noticably
different from batch to batch.

===============================

Your para. 1. You can swear till you're appoplectic black and blue in the
face - it wasn't me. I'm not THAT stupid. So who was it then?

Your para. 2. If there are little bubbles in it, it is not solid. If it
is foamed, it is not solid.

Just to add a little more useless information, did you know the stuff also
varies with pressure and temperature as at the bottoms of the oceans? Also,
under pressure, water disassociates and hydrogen slowly diffuses through it.
Rodents seem to take a fancy to it.

Velocity factor = (Permittivity)^(- 1/2). Permittivity of polyethylene =
2.26 but it does vary slighty from one book to the next.
----
Reg.

Asswipe,

The question I posed wasn't "What's the velocity factor of ALL solid
polyethylene coax cable". Next time read the question and answer it. If
you don't know the answer then DON'T POST A REPLY!

Too many ignorant people in these groups anymore!

"Reg Edwards" wrote in message
...
The velocity factor of ALL solid polyethylene coax cable, regardless of
impedance, is 0.665

Asswipe,

The question I posed wasn't "What's the velocity factor of ALL solid
polyethylene coax cable". Next time read the question and answer it. If
you don't know the answer then DON'T POST A REPLY!

Too many ignorant people in these groups anymore!

"Reg Edwards" wrote in message
...
The velocity factor of ALL solid polyethylene coax cable, regardless of
impedance, is 0.665

In article , (Tom
Bruhns) writes:

But even if we just limit ourselves to HF and above, there's a
problem: most "solid poly" cable I've encountered has small gas
bubbles in the dielectric, and the v.f. does not measure exactly
0.665. Most of the time, the difference doesn't matter, but sometimes
it does, and then it's not safe to assume it's 0.665. And of course a
lot of cable these days uses foam dielectric, which can be noticably
different from batch to batch.

I tend to agree, based on past experiences in finding VP of lots of
different RF-video cables. That includes some "economy" type
cables where the dielectric looked like polyethylene but a burn-and-
sniff test proved it was some other polymer.

There's some good formulas for finding out the VP of coax in
the May, 1978, issue of HAM RADIO magazine. The title is
"Antenna Bridge Calculations" with my byline and was for folks
using a calibrated Noise Bridge and had access to an HP pocket
calculator. A Noise Bridge nulls for admittance but this will convert
to impedance with a complex inversion...much handier on the newer
HP 32 SII calculator.

The formulas in the May '78 issue of HR will work with the MFJ
analyzer's impedance read-out values.

Len Anderson
retired (from regular hours) electronic engineer person

In article , (Tom
Bruhns) writes:

But even if we just limit ourselves to HF and above, there's a
problem: most "solid poly" cable I've encountered has small gas
bubbles in the dielectric, and the v.f. does not measure exactly
0.665. Most of the time, the difference doesn't matter, but sometimes
it does, and then it's not safe to assume it's 0.665. And of course a
lot of cable these days uses foam dielectric, which can be noticably
different from batch to batch.

I tend to agree, based on past experiences in finding VP of lots of
different RF-video cables. That includes some "economy" type
cables where the dielectric looked like polyethylene but a burn-and-
sniff test proved it was some other polymer.

There's some good formulas for finding out the VP of coax in
the May, 1978, issue of HAM RADIO magazine. The title is
"Antenna Bridge Calculations" with my byline and was for folks
using a calibrated Noise Bridge and had access to an HP pocket
calculator. A Noise Bridge nulls for admittance but this will convert
to impedance with a complex inversion...much handier on the newer
HP 32 SII calculator.

The formulas in the May '78 issue of HR will work with the MFJ
analyzer's impedance read-out values.

Len Anderson
retired (from regular hours) electronic engineer person

Jason Dugas wrote:

I'm interested in measuring the velocity factor of some coax I have (more of
an exercise than necessity). In order to do this, the MFJ-259 Operations
manual states that the "stub" to be measured should be attached with a
50-ohm noninductive resistor in series to that center conductor of the
Antenna connector on the analyzer.


You may think it says this:
MFJ-resistor-coax_center_conductor--
|
---------coax_shield------------

But maybe it means this:

MFJ----------------coax_center_conductor-resistor-
|
----------------coax_shield--------------------

I don't know what the MFJ documentation says - but if
they want a 50 ohm resistor in series from the center
conductor and the shield, put it at the far end of the
length of coax you are testing. Connect it as shown in
the bottom diagram by soldering the resistor directly
across the center conductor and shield at the far end.

The way I do it is to cut a physical 1/2 wavelength
coax for the frequency, install a pl259 on one end and
connect it to the MFJ. I install a 51 ohm resistor at
the far end of the coax. When that resistor is 1/2
wave away *electrically*, it's impedance will appear
across the PL259 plugged into the MFJ. You need to
shorten the coax by snipping off some of the length
at the far end, then reinstalling the resistor.
When you have the thing at an electrical 1/2 wave
you'll get Z=50 at the MFJ. Your VF is the length
of that piece of coax divides by the original length.

Maybe the MFJ documents a better way - I dunno - but
I've used my way successfully.



What would be a good way of making this connection? I've thought about it
quite a bit; the best idea I've come up with so far is to have a small metal
enclosure w/ two SO-239's mounted. The resistor would go from center
conductor to center conductor of each SO-239. But this requires too many
extra connections & lengths. Is there a better way to do this? Would love
to see anyone else's experimental setup, particularly if there are pictures
or details on the web.

Thanks & 73,

Jason
KB5URQ

Jason Dugas wrote:

I'm interested in measuring the velocity factor of some coax I have (more of
an exercise than necessity). In order to do this, the MFJ-259 Operations
manual states that the "stub" to be measured should be attached with a
50-ohm noninductive resistor in series to that center conductor of the
Antenna connector on the analyzer.


You may think it says this:
MFJ-resistor-coax_center_conductor--
|
---------coax_shield------------

But maybe it means this:

MFJ----------------coax_center_conductor-resistor-
|
----------------coax_shield--------------------

I don't know what the MFJ documentation says - but if
they want a 50 ohm resistor in series from the center
conductor and the shield, put it at the far end of the
length of coax you are testing. Connect it as shown in
the bottom diagram by soldering the resistor directly
across the center conductor and shield at the far end.

The way I do it is to cut a physical 1/2 wavelength
coax for the frequency, install a pl259 on one end and
connect it to the MFJ. I install a 51 ohm resistor at
the far end of the coax. When that resistor is 1/2
wave away *electrically*, it's impedance will appear
across the PL259 plugged into the MFJ. You need to
shorten the coax by snipping off some of the length
at the far end, then reinstalling the resistor.
When you have the thing at an electrical 1/2 wave
you'll get Z=50 at the MFJ. Your VF is the length
of that piece of coax divides by the original length.

Maybe the MFJ documents a better way - I dunno - but
I've used my way successfully.



What would be a good way of making this connection? I've thought about it
quite a bit; the best idea I've come up with so far is to have a small metal
enclosure w/ two SO-239's mounted. The resistor would go from center
conductor to center conductor of each SO-239. But this requires too many
extra connections & lengths. Is there a better way to do this? Would love
to see anyone else's experimental setup, particularly if there are pictures
or details on the web.

Thanks & 73,

Jason
KB5URQ