Tom,

It should be obvious when somebody asks how to find VF by using an HF
antenna analiser he is interested, like every other radio amateur, in the HF
value of VF.

If you agree with the table in the other posting, which gives VF vesus
frequency, and you are aware VF decreases with frequency, then how come you
didn't realise you had taken the value of 0.665 out of its HF context.

All my figures are correct.
----
Reg

'''''''''''''''''''''''''''''''''''''''''''''''''' ''''''''''''''''''''''''''
'''''''''''''''
"Tom Bruhns" wrote in message
m...
"Reg Edwards" wrote in message
...
"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?
...
Freq Ro jXo Angle VF
------ ------ ------ ------ ------
50 Hz 967 -965 -44.95 0.034
1 kHz 220 -213 -44 0.151
10 kHz 80 -58 -36 0.41
100 kHz 56 -9.3 -9.5 0.59
1 MHz 52.4 -2.4 -2.7 0.63
10 MHz 50.7 -0.76 -0.86 0.65
100 MHz 50.2 -0.23 -0.27 0.66

Smith Chart calculations begin to be inaccurate around 2 MHz and
below. So
do SWR meters.

Tom,

It should be obvious when somebody asks how to find VF by using an HF
antenna analiser he is interested, like every other radio amateur, in the HF
value of VF.

If you agree with the table in the other posting, which gives VF vesus
frequency, and you are aware VF decreases with frequency, then how come you
didn't realise you had taken the value of 0.665 out of its HF context.

All my figures are correct.
----
Reg

'''''''''''''''''''''''''''''''''''''''''''''''''' ''''''''''''''''''''''''''
'''''''''''''''
"Tom Bruhns" wrote in message
m...
"Reg Edwards" wrote in message
...
"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?
...
Freq Ro jXo Angle VF
------ ------ ------ ------ ------
50 Hz 967 -965 -44.95 0.034
1 kHz 220 -213 -44 0.151
10 kHz 80 -58 -36 0.41
100 kHz 56 -9.3 -9.5 0.59
1 MHz 52.4 -2.4 -2.7 0.63
10 MHz 50.7 -0.76 -0.86 0.65
100 MHz 50.2 -0.23 -0.27 0.66

Smith Chart calculations begin to be inaccurate around 2 MHz and
below. So
do SWR meters.

"Reg Edwards" wrote in message ...
"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?
....
===============================

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?
....

----
Reg.

Hi Reg...

Well, the Google archive says it was from you. Perhaps you DO have
someone else posting under your name.

See below. Or is the "VF" column not actually velocity factor? OTOH,
I do agree with the posting below, that at low frequencies, beta
becomes dependent more on R than on L, and thus the VF changes.

Cheers,
Tom


From: Reg Edwards )
Subject: Coax Cable vs Freqency

View this article only
Newsgroups: rec.radio.amateur.antenna
Date: 2003-08-12 17:41:24 PST

For anyone who may be interested.

Typical of RG-58 and RG-11 type cables.
Zo = Ro - jXo
Xo is always negative.
Angle of Zo in degrees. Always negative.
VF = relative velocity.

Freq Ro jXo Angle VF
------ ------ ------ ------ ------
50 Hz 967 -965 -44.95 0.034
1 kHz 220 -213 -44 0.151
10 kHz 80 -58 -36 0.41
100 kHz 56 -9.3 -9.5 0.59
1 MHz 52.4 -2.4 -2.7 0.63
10 MHz 50.7 -0.76 -0.86 0.65
100 MHz 50.2 -0.23 -0.27 0.66

Smith Chart calculations begin to be inaccurate around 2 MHz and
below. So
do SWR meters.

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

"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.

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

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!)

(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

(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