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

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

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

"JGBOYLES" 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
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

Gary, the manual indicates to 'Set up the line to measure 1/4 wave stubs as
in the section on "Testing and Tuning Stubs"'... In that section, it
requires a 50-ohm resistor in series to the center conductor. Perhaps, the
resistor isn't needed to measure VF? This is really a poorly written
manual-- while it acknowledges possibilities, it really isn't clear in its
procedures :-/

Let me know what you think. Maybe you're right and the resistor isn't
needed for VF.

Thanks,

Jason

"JGBOYLES" 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
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

Gary, the manual indicates to 'Set up the line to measure 1/4 wave stubs as
in the section on "Testing and Tuning Stubs"'... In that section, it
requires a 50-ohm resistor in series to the center conductor. Perhaps, the
resistor isn't needed to measure VF? This is really a poorly written
manual-- while it acknowledges possibilities, it really isn't clear in its
procedures :-/

Let me know what you think. Maybe you're right and the resistor isn't
needed for VF.

Thanks,

Jason

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.