MiniVNA or AIM4170 analyser?

[G8JNJ]

Hi Adam,

I appreciate what you are saying, which is why I stated 'Your mileage may vary'.

I use my VNA for general testing and development of antennas, matching networks, RF circuits and the usual stuff you would tend to do in a ham shack.

However if your background is from an RF development environment. Where you are used to using VNA's etc on a daily basis, and simply wish to have a low cost version of a professional instrument at home. Then your usage pattern is likely to be different.

I find that for quick, easy measurements the Aim user interface and general product attributes. Make it very quick and easy to make sufficiently accurate measurements for my purposes.

I do use the SDR Kits VNWA3 - just not as frequently as the Aim's.

I guess it's a bit like why some folks like particular brands of cars - they will all get you from A-B but you feel more comfortable in some models - but it's not always easy to say why - It's just down to personal preference.

Regards,

Martin - G8JNJ

www.g8jnj.webs.com

[G3RZP]

Do any of them offer much accuracy at high impedances - say around 5000 ohms? By my calculations, the return loss of a 5500 ohm load is 0.1579dB: If you increase it by 4% to 5700 ohms, the return loss increases by 0.006dB, which I would have put down as pretty difficult to measure even under lab conditions.

Back in the early 80's, HP  had a component tester which worked on the basis of return loss measurement, but they had a graph showing how the accuracy degraded as the you got further and further from 50 ohms. I presume all these devices must be similar if the work on  a return loss measurement.

[SP4SKR]

Measuring such a big impedance in reflection mode is not a good idea as it introduces big measurement error - as shown in your calculations. The better way is to do that in transmission mode where VNA port is used as a voltage source and what is actually measured is the current flowing through this big impedance. It is quite well described in http://sdr-kits.net/DG8SAQ/VNWA/VNWA_HELP.pdf starting from page 233. It is called "I-Method" there and it should give you the accuracy reaching approx. 1% for 5k impedance - depending on quality of your calibration. This method actually measures S21 (transmission) and from this one can calculate impedance. VNWA software provides t2s() function to do that conversion for you automatically.

There is also a simple example showing how to measure small value capacitor with "I-Method" starting on page 252 of http://sdr-kits.net/DG8SAQ/VNWA/VNWA_HELP.pdf Measuring impedance follows the same principle.

Please note that "I-Method" is not very accurate for low impedances. In that case one should use "V-Method". Please also note that in order to get best possible accuracy for very wide range of impedance measurement one should use RF-IV head for measurement. VNWA supports that with what is called "RF-IV Mode" employing external additional head - please refer to page 164 in http://sdr-kits.net/DG8SAQ/VNWA/VNWA_HELP.pdf

Hope that helps,
Adam -SP4SKR

[G8JNJ]

Hi Peter,

I concur with Adam.

The Aim series of VNA's are specified to measure impedances up to 5K. The original Mini VNA and many 'antenna analyzers' struggle above about 1K.

However the degree of accuracy you are looking for is difficult to achieve even in a lab environment. Just moving stuff around the bench or even breathing :-) can easily introduce fractional dB errors. Personally with semi-pro kit in the home. I'd be happy if I could achieve +/-0.5dB day to day repeatability. The SDR Kits VNWA3 and AIM's when calibrated, should be able to achieve significantly better tolerances than this value.

Regards,

Martin - G8JNJ

www.g8jnj.webs.com

[SP4SKR]

Just to clarify some things in naming. AIM series devices are not VNA (Vector Network Analyzer). They are Vector Impedance Analyzers. The naming may be quite confusing. There are two categories of parameters used to characterize RF/microwave impedance/scattering parameters analyzers:
1) If it is scalar (measures only impedance magnitude) or vector (measures both real and imaginary parts of the impedance)
2) If it is impedance analyzer (has a single port and measures only in reflection mode) or network analyzer (has two or more ports and measures transmission as well as reflection)

So in total one would have four possible kinds of these devices:
a) SIA (scalar impedance analyzer) - very simple device like SWR meter
b) VIA (vector impedance analyzer) - LCR bridges, AIM series antenna analyzers
c) SNA (scalar network analyzer) - for example HP been making this kind of stuff in 1960's
d) VNA (vector network analyzer) - currently most popular stuff at the professional level made by Agilent, HP, R&S, SDR-kits and many more

All of above devices perform their measurements in frequency domain.

It is also worth mentioning another category of impedance analyzers employing TDR (time domain reflectometry). They are working on principle of injecting very sharp pulses into measured object and measuring reflected pulses in time domain - kind of mix of pulse generator and fast oscilloscope. But this is another story...

Hope that helps,
Adam - SP4SKR

[G3RZP]

I've always used RF impedance bridges - but that's good up to about 100MHz. You can get quite good accuracy on a 4% change at 5kOhm at HF. For impedances like 2 - j2.5 at 400 MHz (small patch antenna), resonant line techniques work very well and are good on repeatability. You lightly couple to a quarter wave resonator with a short circuit at one end, measure bandwidth and frequency. Substitute the patch antenna for the short circuit, and measure the new bandwidth and frequency, and it's a fairly simple calculation.