Measuring Velocity Factor w/ MFJ-259
 

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.

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

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.

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.
===============
Since no power is involved the non-inductive resistor can be very small .
If accuracy is a point ,get a 50 Ohms , 1% metal film resistor or select a
near 50 Ohms resistor from a batch of standard 5% resistors with an accurate
Ohm meter .
Cut the leads as short as possible but adequate for insertion and soldering
into the SO 239 coaxial socket.
You now know the minimum distance between de 2 SO 239 sockets and can make a
suitable enclosure from bits of scrap plain printed circuit board ( in
Europe available at amateur radio fleamarkets and often sold by the
kilogramme)
With the enclosure completed and holes drilled for the SO 239 connectors
,fit one connector with the resistor soldered and subsequently fit the other
connector and solder it to the resistor.
The result is a resistor in between the 2 SO 239 connectors with hardly any
wire visible.
Finally finish the enclosure by soldering a cover of the same PCB material
or keep the cover removable by means of soldered nuts inside the enclosure
and matching screws.

Frank GM0CSZ / KN6WH

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.

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.
===============
Since no power is involved the non-inductive resistor can be very small .
If accuracy is a point ,get a 50 Ohms , 1% metal film resistor or select a
near 50 Ohms resistor from a batch of standard 5% resistors with an accurate
Ohm meter .
Cut the leads as short as possible but adequate for insertion and soldering
into the SO 239 coaxial socket.
You now know the minimum distance between de 2 SO 239 sockets and can make a
suitable enclosure from bits of scrap plain printed circuit board ( in
Europe available at amateur radio fleamarkets and often sold by the
kilogramme)
With the enclosure completed and holes drilled for the SO 239 connectors
,fit one connector with the resistor soldered and subsequently fit the other
connector and solder it to the resistor.
The result is a resistor in between the 2 SO 239 connectors with hardly any
wire visible.
Finally finish the enclosure by soldering a cover of the same PCB material
or keep the cover removable by means of soldered nuts inside the enclosure
and matching screws.

Frank GM0CSZ / KN6WH

The velocity factor of ALL solid polyethylene coax cable, regardless of
impedance, is 0.665

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.

"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

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.

"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.

"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.

"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.

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.

So, Reg, I note that (1) YOUR table shows more variation in VF over
the HF ham bands (especially so if you allow it to be extended down to
the 1.8MHz ham band, which I assume the MFJ analyzer would cover) than
the three significant digits of 0.665 would allow (the table showing
only 0.65 at 10MHz, and I suppose you'd go for about 0.64 at 2MHz,
maybe even 3.5MHz), (2) your posting in this thread of 0.665 did NOT
qualify the VF as being HF only, and (3) my posting DID say something
that SHOULD have indicated to you and anyone else who read it that I
was thinking beyond the bounds of HF with respect to large VF
variation. I'd STILL say there are instances where one should not
assume 0.665 for solid polyethylene at HF. There are instances where
0.64 and 0.665 are different enough to want to get the right value.
You're welcome to get all bent out of shape over that if you wish.

Cheers,
Tom

"Reg Edwards" wrote in message ...
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.
(As we noted at the time of the original posting, the numbers in the
table are rough approximations...certainly they're correct as
approximations...)
----
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.

So, Reg, I note that (1) YOUR table shows more variation in VF over
the HF ham bands (especially so if you allow it to be extended down to
the 1.8MHz ham band, which I assume the MFJ analyzer would cover) than
the three significant digits of 0.665 would allow (the table showing
only 0.65 at 10MHz, and I suppose you'd go for about 0.64 at 2MHz,
maybe even 3.5MHz), (2) your posting in this thread of 0.665 did NOT
qualify the VF as being HF only, and (3) my posting DID say something
that SHOULD have indicated to you and anyone else who read it that I
was thinking beyond the bounds of HF with respect to large VF
variation. I'd STILL say there are instances where one should not
assume 0.665 for solid polyethylene at HF. There are instances where
0.64 and 0.665 are different enough to want to get the right value.
You're welcome to get all bent out of shape over that if you wish.

Cheers,
Tom

"Reg Edwards" wrote in message ...
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.
(As we noted at the time of the original posting, the numbers in the
table are rough approximations...certainly they're correct as
approximations...)
----
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.

My figures are correct to a better accuracy than can be determind by a cheap
and nasty antenna analyser.

Now go away, leave the newsgroup in peace, and stop your childish
nitpicking.

My figures are correct to a better accuracy than can be determind by a cheap
and nasty antenna analyser.

Now go away, leave the newsgroup in peace, and stop your childish
nitpicking.

Clearly, if I had an o-scope and a waveform generator, I wouldn't be farting
with this analyzer!! :)


"Tim Shoppa" wrote in message
om...
"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.

Clearly, if I had an o-scope and a waveform generator, I wouldn't be farting
with this analyzer!! :)


"Tim Shoppa" wrote in message
om...
"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.

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

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