[TowerTalk] Antenna analyzer

Jim Brown jim at audiosystemsgroup.com
Tue Jul 17 16:07:05 PDT 2012


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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