On 7/17/2012 1:53 PM, Paul Christensen wrote:
> Forgot to mention that a very powerful attribute of the software-based VNAs
> is that in many instances, accurate Z measurements can be made at the rig
> end of a coaxial cable and you don't need to know anything about the type of
> coax, nor its length. During Open/Short/Load (OSL) calibration, the loads
> are moved to the antenna end of the line. The VNA software will then
> calculate loss, distance, and velocity factor. From that point forward,
> complex Z of the load can be measured at the rig end of the line.
Not quite -- to do an OSL calibration you must physically access the
point in the coax line where you want to place the unknown impedance.
This defines the "measurement plane," and, indeed, the coax to that
point are "tuned out." But that means that if you want your measurement
plane to be at the top of the tower, you must place the open, short, and
load at the top of the tower.
What DOES allow measurements made in the shack to be translated to the
antenna is a relatively simple procedure.
1) Take the impedance measurement from any convenient point at which you
are calibrated. Save that measurement as an s1p file.
2) Make a TDR measurerment to determine the length of the transmission
line. This will give you the electrical length of the line at VHF. You
will need to apply a small correction factor to account for the
variation of Vf with frequency to get the electrical length at the
frequency of interest.
3) Get a copy of Sim Smith, very nice free Smith Chart software that
runs in Java. Import the s1p file, add a negative length of transmission
line to the model equal to the length you have measured by TDR and your
computation. You should also enter the attenuation data for your
transmission line at the frequency of interest. Sim Smith will now
display the impedance at the top of the tower!
4) You can now use that impedance in Sim Smith to design matching
networks, including stubs.
Sim Smith can import s1p and s2p files from almost any source, including
any analyzers that write to that very standard format, and from antenna
modeling software like NEC.
Another point. While the AIM analyzers are popular, the VNWA is less
than half the cost, has excellent features, does NOT require power other
than from the computer, and the software written for it is quite
powerful. Why pay $1700 or more when you can buy something as good or
better, and with the useful feature of not needing local power to run,
for $750?
I use my MFJ 259B for quick resonance checks, and for the very useful
function of determining Vf at the frequency of interest by observing
nulls produced by viewing the line as a stub. It's the one you take up
the tower, and out on Field Day.
Another analyzer worth having is one that measures in 75 ohms. I have a
couple of vintage AEA graphing analzyers I bought cheap on fleabay quite
a few years ago, one for HF that has a switch for 50 or 75 ohms, and
another for VHF/UHF. Also sized to go up the tower.
73, Jim K9YC
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