[TowerTalk] Antenna analyzer

K8RI K8RI-on-TowerTalk at tm.net
Wed Jul 18 14:44:03 PDT 2012


On 7/18/2012 4:49 AM, Ian White GM3SEK wrote:
> A few points from this and earlier postings....

Thank you Ian for this excellent summary.  It certainly helps me make my 
decision as to what I need and can use for my projects.

Like others, I think you should publish it as it's information that many 
in the ham fraternity could put to use

73 and again Thank You,

Roger (K8RI)




>
> 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.
>
>




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