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

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

"Marco S Hyman" wrote in message
...
writes:

Jason Dugas wrote:

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--------------------

How does the 259 differ from the 259B? Does the 259 have a distance
to fault mode?

I ask because the instructions for the 'B say that coax lines must be
*open* or *shorted*. Anything in between is verbotten.

With the 'B you use the "distance to fault" mode to figure
out the electrical length of the line then devide by the
physical length to caluclate the velocity factor.


// marc (KC7JL)

The MFJ-259 does not have a "distance to fault" mode. It has an "SWR" meter
and a "Resistance" meter.

jason

"Marco S Hyman" wrote in message
...
writes:

Jason Dugas wrote:

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--------------------

How does the 259 differ from the 259B? Does the 259 have a distance
to fault mode?

I ask because the instructions for the 'B say that coax lines must be
*open* or *shorted*. Anything in between is verbotten.

With the 'B you use the "distance to fault" mode to figure
out the electrical length of the line then devide by the
physical length to caluclate the velocity factor.


// marc (KC7JL)

The MFJ-259 does not have a "distance to fault" mode. It has an "SWR" meter
and a "Resistance" meter.

jason

On 31 Oct 2003 13:43:32 -0800, (Tom Bruhns) wrote:

"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.

It's quite accurate figure, but the cables are not so accurate.
Remember trying to make two RG213/U halfwave stubs for 100MHz, and was
surprised to learn that the 'halfwave' length varied about an inch

Suppose it wasn't only for the bubbles...

73
Jan-Martin, LA8AK
http://home.online.no/~la8ak/
--
remove ,xnd to reply (Spam precaution!)

On 31 Oct 2003 13:43:32 -0800, (Tom Bruhns) wrote:

"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.

It's quite accurate figure, but the cables are not so accurate.
Remember trying to make two RG213/U halfwave stubs for 100MHz, and was
surprised to learn that the 'halfwave' length varied about an inch

Suppose it wasn't only for the bubbles...

73
Jan-Martin, LA8AK
http://home.online.no/~la8ak/
--
remove ,xnd to reply (Spam precaution!)

"Jason Dugas" wrote in message ...
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
...
What would be a good way of making this connection?

Do you have a scope and a pulse generator? If so, just make a simple
TDR (Time Domain Reflectometry) setup to measure the reflection from a
un-terminated or shorted far end of the cable. You will get much more
accurate results with substantially less effort.

Don't get me wrong, the MFJ-259 is a good instrument. It's just that TDR
is so quick and easy and unambiguous for propogation delay measurements.

Tim.