[TowerTalk] Antenna analyzer

[Ian White GM3SEK]

A few points from this and earlier postings....

Warning: this has turned into something of a "Guide to Test
Instruments", so proceed at your own risk.


There are three completely different categories of instruments here. You
have to decide which category you are aiming for, and then pick the best
model (best for your own specific needs) within that category.

1. Simple "SWR analyzers" like the original MFJ-259 and Autek RF-1.
These are simply an RF signal source and an analog SWR bridge. (Other
facilities such as digital synthesizer and SWR graphics make no
difference to these basic RF capabilities.)

2. "Vector Lite" instruments like the MFJ-259B that can display
resistance and reactance (R +/- jX) separately, but categorically do NOT
have a facility for Open-Short-Load (OSL) calibration.

3. True Vector Network Analyzers that do offer - and indeed, require -
OSL calibration at the exact location where the measurement is being
made. The instrument then compensates for its own inaccuracies and
presents the user with a much truer result. This is a defining feature
of the true VNA that kicks it into a totally different league of
accuracy.

(For example, there is almost no difference in accuracy between the
home-built N2PK VNA and a high-end laboratory VNA. The only significant
differences are in extremely difficult measurements where it is highly
likely that both analyzers are wrong.)

Within each of these 3 categories there are many different instruments.
Each one offers a different combination of cost, versatility,
portability, operating convenience and accuracy. This is where consumer
choice comes in, to pick the best combination of features for your own
personal needs.

But even so, I know of no instrument that busts the boundaries between
those three major categories:

* SWR only, or some attempt at Vector measurements;

and within the Vector instruments

* OSL calibration and compensation, or only "Vector Lite".


Jim Brown wrote:
>On 7/17/2012 4:48 PM, Paul Christensen wrote:
>> 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.
>
"Normalization" is another word for a Category 3 instrument's ability to
compensate for its own inaccuracies, based on the results of an OSL
calibration.

OSL calibration requires a dialog between the user and the VNA control
software, like:
"Calibrate."
        "Please connect Open-circuit standard."
"OK, I did that."
        "Please wait...
        "Now please connect Short-circuit standard."
And so on...

The practical difficulty is when the OSL calibration is being carried
out remotely, perhaps several hundred feet away from the instrument
itself.

But "remote" normalization is only a problem for the operator - the VNA
sees nothing different about it. It doesn't even need to be told.

Wherever the Open, Short and Load (50 ohm) reference standards are
connected at the top of the tower, that location is then DEFINED to be
the "Reference Plane" for all future measurements. Anything connected
between the Reference Plane and the physical VNA has effectively been
"absorbed" into the instrument itself, so out at the Reference Plane we
still have a true Category 3 measurement.

True remote normalization - where the operator has to climb the tower
with the OSL standards in his pocket - must not be confused with a
different facility (which some VNAs have) to ESTIMATE the effects of a
connected cable. In this case you do the OSL calibration at the VNA
itself, and then you TELL the software that 200ft of LDF4-50 (or
whatever) is also connected. The software then makes its best attempt -
based on what YOU told it - to estimate the load impedance at the far
end. But this effectively demotes the instrument to Category 2; the
superb accuracy of a true OSL calibration at the tower top has gone.

However, there is another way around this. G3NRW pioneered the technique
of using a WiFi-enabled laptop out in the yard to operate the shack PC
remotely using LogMeIn as a virtual desktop. He can leave his AIM4170 in
the shack, connected to the feedline and with the control software
running, and from the laptop he can see and do everything that the shack
PC can. This allows him to make true Category 3 measurements from out in
the yard. Google for more details.


>Thanks, Paul.  I've heard nothing but high praise for the N2PK unit. I've
>looked several times for a way to buy one (or a kit) without success.
>The first time was at least three years ago, then a year or two later,
>and again this spring when I found the VNWA.
>
>BTW -- the VNWA also works as a Spectrum Analyzer and as a signal
>generator.
>
All analyzers work as a signal generator of some kind, because they all
contain an RF test source. At one extreme you have the Cat.1 sources
like the MFJ-259 and Autek which are fairly horrid. At the opposite
extreme, the N2PK contains two separate, fully programmable high-quality
DDS sources which can be either locked together or "broken out"
separately. (I'm not so familiar with the VNWA but believe it too has a
very wide range, high-quality, programmable signal source.)

All Cat.3 VNAs also contain a tracking receiver and precision level
measurement, which could be reconfigured as a spectrum analyzer. Again,
it depends mostly on the control software and the accessibility of the
various inputs and outputs.

Which brings us to...

>
>Thanks.  I was thinking of the AIM 4180, which, I believe, is a 2-port
>VNA.
>
"AIM4180"? VNA2180 perhaps - the AIM4170's bigger brother.


Again, all VNAs are 2-port instruments inside. Port 1 output the test
signal, and Port 2 receives the signal back from some Device Under Test.
The only difference in a "1-port" instrument is that a reflection bridge
is permanently connected to Ports 1 and 2, so it only allows impedance
(reflection) measurements.

The older N2PK VNAs were usually built with these two ports on the front
panel, so that "through transmission" measurements could be made between
these two ports. For reflection measurements, a separate bridge had to
be physically screwed onto those two connectors.

However, what is now called a "2-port" VNA is something slightly
different. As well as having a built-in reflection bridge permanently
connected to one port, it also contains a completely separate 2nd
measuring receiver with another input port of its own.  This allows you
to make reflection and "through" measurements simultaneously.

If you have any thought of measuring filters (eg setting up a bank of
W3NQN filters) then this kind of 2-port VNA is what you need. Modern
builds of the N2PK will do this, as will the VNWA and also the VNA2180.


Apologies for the length of this. I didn't have the time to make it
shorter :-)  ...or to make it even longer, because that was only part of
the story.


-- 

73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek