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!)

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

Even if you just ignore the fact that at low frequencies, R dominates
over omega*L and affects the velocity factor, even if you just say
VF=c/sqrt(LC), I believe there are changes with frequency. That's
because with a good dielectric like polyethylene, C doesn't change
much with frequency (AFAIK), but L decreases as the skin depth
decreases. On large line, it's unlikely to be noticable, but on a
line with an inner conductor radius only a few skin depths at the low
end of HF, the change can be perceptable. Of course, the R term does
dominate over omega*L at low frequencies. If the velocity factor were
independent of frequency, I'd expect no dispersion.

BTW, I'd be a little surprised to find an inexpensive line which did
not have polyethylene dielectric...polyethylene isn't expensive, and
it's a good dielectric except for its low softening/melting
temperature. You should be able to get a pretty good idea what it is
from carefully measured VF, if it's a solid dielectric. There are
very few inexpensive and low-loss plastics with as low a relative
dielectric constant as polyethylene.

Cheers,
Tom

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!)

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

Even if you just ignore the fact that at low frequencies, R dominates
over omega*L and affects the velocity factor, even if you just say
VF=c/sqrt(LC), I believe there are changes with frequency. That's
because with a good dielectric like polyethylene, C doesn't change
much with frequency (AFAIK), but L decreases as the skin depth
decreases. On large line, it's unlikely to be noticable, but on a
line with an inner conductor radius only a few skin depths at the low
end of HF, the change can be perceptable. Of course, the R term does
dominate over omega*L at low frequencies. If the velocity factor were
independent of frequency, I'd expect no dispersion.

BTW, I'd be a little surprised to find an inexpensive line which did
not have polyethylene dielectric...polyethylene isn't expensive, and
it's a good dielectric except for its low softening/melting
temperature. You should be able to get a pretty good idea what it is
from carefully measured VF, if it's a solid dielectric. There are
very few inexpensive and low-loss plastics with as low a relative
dielectric constant as polyethylene.

Cheers,
Tom

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.