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

Leading eventually to the suggestion that perhaps your MFJ analyzer
isn't worth the powder to blow it up...

But I submit that you can make VF measurements on your cable with it,
which may be limited by the accuracy with which you can measure the
length of line you are looking at and not by the fact that you're
using a relatively inexpensive instrument. Here's one way. If you
look at a shorted stub 90 electrical degrees long in parallel with a
50 ohm resistor, you'll have to move the frequency by some value to
resolve a difference. The amount you have to move the frequency
probably determines the resolution you can get in VF. But if you make
the line 270 degrees long, you'll have three times the sensitivity.
So a hundred foot length of line, measured near 10 meters frequency,
would be long enough to get very good resolution--it'd be on the order
of 1530 electrical degrees. I'd be surprised if you had trouble
seeing the difference between, say, 0.664 and 0.665 VF. But beware
that the VF _does_ change with frequency, even over HF. There are
times when _I_ care about that, even if some others don't.

Cheers,
Tom

"Jason Dugas" wrote in message ...
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

Hmmm, I would have sworn the manual covers this, or is that just on the B
model?....

I'll have to try this, but how about just connecting a length of coax,
open at the other end and looking for the min Z. The longer it is, the less
effect of any "end effect".

TO get around end effect, go to the shorted stub method...but...
I don't understand the 50 ohm in parallel. I'd put some large R in
parallel when trying the shorted-end method such that there will be a
reading. If I recall, mine stops above 1500 ohms. So, I'd put, say, 1 K in
parallel then find the two freqs where I see some specific value lower than
the resistor alone, then take the geometric mean for the stub's resonant
freq.

Steve
No U's in my email addr...


"Tom Bruhns" wrote in message
m...
Leading eventually to the suggestion that perhaps your MFJ analyzer
isn't worth the powder to blow it up...

But I submit that you can make VF measurements on your cable with it,
which may be limited by the accuracy with which you can measure the
length of line you are looking at and not by the fact that you're
using a relatively inexpensive instrument. Here's one way. If you
look at a shorted stub 90 electrical degrees long in parallel with a
50 ohm resistor, you'll have to move the frequency by some value to
resolve a difference. The amount you have to move the frequency
probably determines the resolution you can get in VF. But if you make
the line 270 degrees long, you'll have three times the sensitivity.
So a hundred foot length of line, measured near 10 meters frequency,
would be long enough to get very good resolution--it'd be on the order
of 1530 electrical degrees. I'd be surprised if you had trouble
seeing the difference between, say, 0.664 and 0.665 VF. But beware
that the VF _does_ change with frequency, even over HF. There are
times when _I_ care about that, even if some others don't.

Cheers,
Tom

"Jason Dugas" wrote in message
...
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

Hmmm, I would have sworn the manual covers this, or is that just on the B
model?....

I'll have to try this, but how about just connecting a length of coax,
open at the other end and looking for the min Z. The longer it is, the less
effect of any "end effect".

TO get around end effect, go to the shorted stub method...but...
I don't understand the 50 ohm in parallel. I'd put some large R in
parallel when trying the shorted-end method such that there will be a
reading. If I recall, mine stops above 1500 ohms. So, I'd put, say, 1 K in
parallel then find the two freqs where I see some specific value lower than
the resistor alone, then take the geometric mean for the stub's resonant
freq.

Steve
No U's in my email addr...


"Tom Bruhns" wrote in message
m...
Leading eventually to the suggestion that perhaps your MFJ analyzer
isn't worth the powder to blow it up...

But I submit that you can make VF measurements on your cable with it,
which may be limited by the accuracy with which you can measure the
length of line you are looking at and not by the fact that you're
using a relatively inexpensive instrument. Here's one way. If you
look at a shorted stub 90 electrical degrees long in parallel with a
50 ohm resistor, you'll have to move the frequency by some value to
resolve a difference. The amount you have to move the frequency
probably determines the resolution you can get in VF. But if you make
the line 270 degrees long, you'll have three times the sensitivity.
So a hundred foot length of line, measured near 10 meters frequency,
would be long enough to get very good resolution--it'd be on the order
of 1530 electrical degrees. I'd be surprised if you had trouble
seeing the difference between, say, 0.664 and 0.665 VF. But beware
that the VF _does_ change with frequency, even over HF. There are
times when _I_ care about that, even if some others don't.

Cheers,
Tom

"Jason Dugas" wrote in message
...
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

Leading eventually to the suggestion that perhaps your MFJ analyzer
isn't worth the powder to blow it up...

But I submit that you can make VF measurements on your cable with it,
which may be limited by the accuracy with which you can measure the
length of line you are looking at and not by the fact that you're
using a relatively inexpensive instrument. Here's one way. If you
look at a shorted stub 90 electrical degrees long in parallel with a
50 ohm resistor, you'll have to move the frequency by some value to
resolve a difference. The amount you have to move the frequency
probably determines the resolution you can get in VF. But if you make
the line 270 degrees long, you'll have three times the sensitivity.
So a hundred foot length of line, measured near 10 meters frequency,
would be long enough to get very good resolution--it'd be on the order
of 1530 electrical degrees. I'd be surprised if you had trouble
seeing the difference between, say, 0.664 and 0.665 VF. But beware
that the VF _does_ change with frequency, even over HF. There are
times when _I_ care about that, even if some others don't.

Cheers,
Tom

"Jason Dugas" wrote in message ...
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

"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.

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

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

Hi Jason, I just looked at the manual for the 259 and the 259B and it did not
mention a 50 ohm resistor in the velocity factor section. With both
instruments you measure the electrical length of the coax and compare to the
physical length to obtain velocity factor. It is a bit easier with a 259B
using the "distance to fault" function. The coax can be open or shorted (far
end) during measurement. No resistor needed as far as I can tell.
73 Gary N4AST

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