Jim,
The myVNA software for the N2PK does the remote normalization automatically
(with loads placed at the point of Z measurement -- and to the best of my
knowledge, so does the most recent software supplied with the AIM/W5BIG
analyzers.
Paul, W9AC
----- Original Message -----
From: "Jim Brown" <jim@audiosystemsgroup.com>
To: <towertalk@contesting.com>
Sent: Tuesday, July 17, 2012 7:07 PM
Subject: Re: [TowerTalk] Antenna analyzer
> 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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