Short coax interconnections - phase implications??
 

Hello guys,

I recently picked up an old vector network analyser in working order
but minus the 3 interconnects between it and the
transmission/reflection bridge. These are specified in the manual to
be exactly 24" long, 50 ohms and terminated by N-type plugs.
I've been told the length of these patch leads is quite critical to
getting accurate measurements with this VNA, but am at a loss to work
out why 24" is specified when the frequency range of this device is
4Mhz to 1300Mhz. If it were only capable of measuring at one fixed
frequency, I could understand the need for a specifically cut length
of some fraction of a wavelength.
Can anyone explain the relevance of 24" in this context?
Also, will *any* 50 ohm coax suffice for this purpose or has it got to
be something special?
Thanks,
P.
--

"What is now proved was once only imagin'd" - William Blake

Short coax interconnections - phase implications??
 

it is likely the 'real' requirement is that they be exactly the same length.
though there may be some maximum length because of attenuation at the top
end. there is also a possibility that they figure in some length of coax
and calibrate that into the system, though i would think that is less
likely.

"Paul Burridge" k wrote
in message ...
Hello guys,

I recently picked up an old vector network analyser in working order
but minus the 3 interconnects between it and the
transmission/reflection bridge. These are specified in the manual to
be exactly 24" long, 50 ohms and terminated by N-type plugs.
I've been told the length of these patch leads is quite critical to
getting accurate measurements with this VNA, but am at a loss to work
out why 24" is specified when the frequency range of this device is
4Mhz to 1300Mhz. If it were only capable of measuring at one fixed
frequency, I could understand the need for a specifically cut length
of some fraction of a wavelength.
Can anyone explain the relevance of 24" in this context?
Also, will *any* 50 ohm coax suffice for this purpose or has it got to
be something special?
Thanks,
P.
--

"What is now proved was once only imagin'd" - William Blake

Short coax interconnections - phase implications??
 

The phase shift due to the 24 inch coax cables are factored into the
Network Analyzer for all frequencies and so any different lengths will cause
the correction factor which is built into the analyzer to be wrong and that
will make the reading displayed on the screen to be in error.

The phase velocity for most coax cables are similar, so I'd use RG-8 since
that accepts type N connectors reasonably well. The impedance bump caused
by a thinner cable being adapted to type N cables would be noticeable
especially at higher frequencies.

The only major question is whether the original cables had a different
velocity factor, and that I can't answer so I'd use the RG-8 and it should
be close enough.



Jim Pennell
N6BIU


--

11:50 Pacific Time Zone
Dec 3 2005

International Time
19:50 UTC
03.12.2005

Short coax interconnections - phase implications??
 

Did you also receive a calibration kit with the VNA? If you did so, then the
lengths of these interconnect cables are not crucial for calibrated
measurements. Only uncalibrated measurements will be affected.

It would help to know the manufacturer and model of the VNA.

Wayne
(KC8UIO)
"Paul Burridge" k wrote
in message ...
Hello guys,

I recently picked up an old vector network analyser in working order
but minus the 3 interconnects between it and the
transmission/reflection bridge. These are specified in the manual to
be exactly 24" long, 50 ohms and terminated by N-type plugs.
I've been told the length of these patch leads is quite critical to
getting accurate measurements with this VNA, but am at a loss to work
out why 24" is specified when the frequency range of this device is
4Mhz to 1300Mhz. If it were only capable of measuring at one fixed
frequency, I could understand the need for a specifically cut length
of some fraction of a wavelength.
Can anyone explain the relevance of 24" in this context?
Also, will *any* 50 ohm coax suffice for this purpose or has it got to
be something special?
Thanks,
P.
--

"What is now proved was once only imagin'd" - William Blake

Short coax interconnections - phase implications??
 

On Mon, 05 Dec 2005 02:37:47 GMT, "Wayne P. Muckleroy"
wrote:

Did you also receive a calibration kit with the VNA?

Nope.

If you did so, then the
lengths of these interconnect cables are not crucial for calibrated
measurements. Only uncalibrated measurements will be affected.

It would help to know the manufacturer and model of the VNA.

It's a good old HP 8754A
--

"What is now proved was once only imagin'd" - William Blake

Short coax interconnections - phase implications??
 

Paul Burridge wrote:

Hello guys,

I recently picked up an old vector network analyser in working order
but minus the 3 interconnects between it and the
transmission/reflection bridge. These are specified in the manual to
be exactly 24" long, 50 ohms and terminated by N-type plugs.
I've been told the length of these patch leads is quite critical to
getting accurate measurements with this VNA, but am at a loss to work
out why 24" is specified when the frequency range of this device is
4Mhz to 1300Mhz. If it were only capable of measuring at one fixed
frequency, I could understand the need for a specifically cut length
of some fraction of a wavelength.
Can anyone explain the relevance of 24" in this context?
Also, will *any* 50 ohm coax suffice for this purpose or has it got to
be something special?
Thanks,
P.
If they specify a physical length (24"), and nothing else will do, then
the type of coax is absolutely critical as its velocity factor will
determine the electrical length.

However I suspect the bridge output is not at signal frequency but DC
so the cables will not be critical. It is very difficult to see how
they could make it work to 4GHz with several wavelengths of coax in
the signal path, acting as an impedance transformer. Two feet is a
significant length even at HF, and will shift the phases considerably.

vy 73

Andy, M1EBV