Extreme Budget VNA

[benwis]

Hi,
I've been building a few boards with bluetooth, wifi,and 900MHz mesh trace antennas on them. I'd really like to be able to tune them, to get optimal performance, and I'm told that I need a VNA to accomplish that.

Trouble is, a VNA is an expensive piece of equipment. I was wondering if you guys could tell me if I could match antennas with a cheaper VNA.

Can anyone tell me if these solutions are adequate for tuning PCB antennas fairly accurately.

I can build one of these:
http://hackaday.com/2016/08/13/a-vna-on-a-200-euro-budget/

Is the performance on this sufficient for my needs? What I am I looking for when characterizing the performance of a VNA?

or buy one of these:
PocketVNA
MiniVNA Tiny
Used HP 8753A,B,C

I think the HP needs a test set as well, and most of them on EBay seem to be outside of the price range. I'm hoping for(~500US). I also think I need a calibration kit, either from ebay or homebuilt. Anyone know any good cheap ones?

Is there a good online tutorial or book I can buy that explains how to use the VNA, and what materials I need?

Thank you!

[valentinc]

 Hi,

     In order to to accurate measurements with a VNA you will also need (as you expected) a OPEN, SHORT, LOAD calibration kit. Without one, you can only do relative measurements, not absolute, which are more or less useless. Also you need those in order to compensate the properties of the cables and connectors you use (the insertion loss, VSWR, etc), so that what everything is accounted for. I don't think you can do a homebuilt calibration kit , because in order to measure the calibration kit, you will also need a higher accuracy VNA than the one you are using (exactly like in a multimeter, where to check a voltage reference of a multimeter you need a multimeter with an order of magnitude higher accuracy).

     The pocket VNA has a cheap calibration kit that you can buy, that is to be considered. What you need to keep in mind is that most of the USB-type stuff will not equal a professional VNA (like the Agilent/Keysigt ones). If I were to choose, I'd choose a used HP 8753A/B/C, with a good calibration kit, like this one

http://www.ebay.com/itm/DC-3GHz-OPEN-SHORT-LOAD-N-male-SET-CALIBRATION-KIT-50-ohm-/301888181778?hash=item4649f01612:g:ZTYAAOSwQItUHLQ0

     It all depends on what level of performance you need: if you only need to tune bluetooth, wifi and 900 Mhz trace antennas the PocketVNA will probably be more than enough for that purpose.
     That 200 euro DIY one seems interesting too, it's an opportunity for you to learn new things.
   
http://www.keysight.com/upload/cmc_upload/All/BTB_Network_2005-1.pdf?&cc=RO&lc=eng    This is a book I can recommend to read to better understand a VNA.

[Neganur]

[...]
If I were to choose, I'd choose a used HP 8753A/B/C, with a good calibration kit, like this one

http://www.ebay.com/itm/DC-3GHz-OPEN-SHORT-LOAD-N-male-SET-CALIBRATION-KIT-50-ohm-/301888181778?hash=item4649f01612:g:ZTYAAOSwQItUHLQ0
[...]

Please don't, you won't have the correct coefficients in your VNA for this unknown kit (unless the seller provides them which I think he won't be able to.)

[Koen]

To tune PCB antennas, you'll open/short/load on the board anyway. You won't need the calibration kit.

[valentinc]

Neganur, you are right, but a HP calibration kit costs as much as the VNA itself, or even much more. This kit is certainly way better than a home-made one with 2 100 ohm resistors soldered to a connector (which can be reasonable to only 100 Mhz or so, I tried in the past to do this with SMA connectors and measured them with a FieldFox N9912A, and the conclusion was, they were reasonable only to 100 Mhz, above that useless)

Koen, how would you open/short/load on the board ? I don't get your point

[G0HZU]

You can make a fairly good SMA SOLT cal kit using SMA end launch connectors and a good quality barrel 'thru' connector. I did this many years ago and used it a lot with my old HP8714B VNA. It worked pretty well up to 3GHz. Not as good as an HP cal kit but close enough to be negligible for most work.

For something like 900MHz antenna work it would easily be good enough. Because I salvaged the connectors from a discarded PCB and used SMD resistors for the load the cal kit effectively cost me nothing but the time to make it and produce a decent cal file for it. I did have to produce my own USER cal kit to load into the VNA but I think that my cal kit would often prove to be about as good as a genuine HP cal kit once you start trying to de embed into the PCB.
Above 3GHz my cal kit isn't very good but I could probably make a better version today.

[G0HZU]

This kit is certainly way better than a home-made one with 2 100 ohm resistors soldered to a connector (which can be reasonable to only 100 Mhz or so, I tried in the past to do this with SMA connectors and measured them with a FieldFox N9912A, and the conclusion was, they were reasonable only to 100 Mhz, above that useless)

Only up to 100MHz? Rubbish

You can get decent return loss and a decent 50R accuracy up to 3GHz using a couple of SMD resistors costing less than a penny and a decent SMA connector.

[benwis]

This kit is certainly way better than a home-made one with 2 100 ohm resistors soldered to a connector (which can be reasonable to only 100 Mhz or so, I tried in the past to do this with SMA connectors and measured them with a FieldFox N9912A, and the conclusion was, they were reasonable only to 100 Mhz, above that useless)

Only up to 100MHz? Rubbish

You can get decent return loss and a decent 50R accuracy up to 3GHz using a couple of SMD resistors costing less than a penny and a decent SMA connector.

This page http://www.qsl.net/in3otd/electronics/VNA_calkit/SMA_female.html describes the performance of a homemade calkit. While definitely not amazing, I think it's  workable up to 3GHz.

What about cabling? I think I'd need some specific cables and connectors, and to place certain networks of components and test connects on the PCB. Is there a good basic guide for that part of it?

[valentinc]

G0HZU is right, I made a mistake, I didn't remember right from the past, they are certainly workable to 3Ghz.

As long as you use cable that is rated for the frequency you use (like RG316), you need to do the calibration at the end of the cable to compensate for the transmission loss of the cable. Other than that, use quality connectors, and you should be fine.

[G0HZU]

Yes, here's an old plot of my home made SMA load up to 3GHz. The VNA was calibrated using a decent Agilent Ecal module and then my load was measured. The data was then sent to the PC running Genesys and the results are shown below up to 3GHz. Note that this load is old and has seen a lot of use over the years.

[G0HZU]

For comparison, here is a set of plots of a 950MHz bandpass filter using my VNA here. These plots are taken with a full 2 port calibration. For one set of plots I used a $$$ 13.5GHz rated Ecal module from Agilent as below:

http://www.keysight.com/en/pd-491111-pn-N4431B/rf-electronic-calibration-module-9-khz-to-135-ghz-4-port?cc=GB&lc=eng


If you look closely at the plots there are two sets of plots drawn almost perfectly over each other. The second plots are from my cheapo homemade SMA calibration kit and I did a full 2 port SOLT calibration using this kit for S11 S12 S21 S22.

The plots measure S21 and S11 and also show real and imaginary parts of the input impedance and also the transmission phase across 700-1200MHz. You can see that for casual stuff like this you can get very good results with a homemade calibration kit provided you create a valid USER calibration file to go with it to characterise the short, the open and the load and the through connection.
The homemade calibration kit cost me NOTHING to make and I haven't updated the user calibration file for it for a long time to account for connector wear over time. But it is still very useful and it helps me reduce the amount of wear on the Ecal module because the results are often just as good with the zero cost homebrew cal kit.

[danadak]

One possibility -

http://sdr-kits.net/VNWA3_Description.html


Regards, Dana.

[casinada]

https://www.aliexpress.com/item/Free-shipping-MFJ-225-Graphical-Antenna-Analyzer-Impedance-Meter-VNA-MFJ-225-Graphical-Antenna-Analyzer/32609208063.html?spm=2114.01020208.3.2.hZf9kJ&ws_ab_test=searchweb0_0,searchweb201602_3_10065_10068_10501_10503_10000032_119_10000025_10000029_430_10000028_10060_10062_10056_10055_10000062_10054_301_10059_10099_10000022_10000012_10103_10000015_10102_10096_10000018_10000019_10000056_10000059_10052_10053_10107_10050_10106_10051_10000053_10000007_10000050_10084_10118_10083_10000047_10080_10082_10081_10110_10111_10112_10113_10114_10115_10000041_10000044_10078_10079_10000038_429_10073_10000035_10121-10503_10501,searchweb201603_9,afswitch_2,single_sort_1_default&btsid=b1460b0b-2fda-4d5a-b910-09c3dfa416f9

https://www.aliexpress.com/item/3G-miniVNA-Tiny-Vector-Network-Analyzer-Frequency-1-3000-Mhz-RF-Antenna-Analyzer-VNA-Signal-Generator/32717858518.html?spm=2114.01020208.3.98.1N6oka&ws_ab_test=searchweb0_0,searchweb201602_3_10065_10068_10501_10503_10000032_119_10000025_10000029_430_10000028_10060_10062_10056_10055_10000062_10054_301_10059_10099_10000022_10000012_10103_10000015_10102_10096_10000018_10000019_10000056_10000059_10052_10053_10107_10050_10106_10051_10000053_10000007_10000050_10084_10118_10083_10000047_10080_10082_10081_10110_10111_10112_10113_10114_10115_10000041_10000044_10078_10079_10000038_429_10073_10000035_10121-10503_10501,searchweb201603_9,afswitch_2,single_sort_1_default&btsid=b1460b0b-2fda-4d5a-b910-09c3dfa416f9
 

[benwis]

https://www.aliexpress.com/item/Free-shipping-MFJ-225-Graphical-Antenna-Analyzer-Impedance-Meter-VNA-MFJ-225-Graphical-Antenna-Analyzer/32609208063.html?spm=2114.01020208.3.2.hZf9kJ&ws_ab_test=searchweb0_0,searchweb201602_3_10065_10068_10501_10503_10000032_119_10000025_10000029_430_10000028_10060_10062_10056_10055_10000062_10054_301_10059_10099_10000022_10000012_10103_10000015_10102_10096_10000018_10000019_10000056_10000059_10052_10053_10107_10050_10106_10051_10000053_10000007_10000050_10084_10118_10083_10000047_10080_10082_10081_10110_10111_10112_10113_10114_10115_10000041_10000044_10078_10079_10000038_429_10073_10000035_10121-10503_10501,searchweb201603_9,afswitch_2,single_sort_1_default&btsid=b1460b0b-2fda-4d5a-b910-09c3dfa416f9

https://www.aliexpress.com/item/3G-miniVNA-Tiny-Vector-Network-Analyzer-Frequency-1-3000-Mhz-RF-Antenna-Analyzer-VNA-Signal-Generator/32717858518.html?spm=2114.01020208.3.98.1N6oka&ws_ab_test=searchweb0_0,searchweb201602_3_10065_10068_10501_10503_10000032_119_10000025_10000029_430_10000028_10060_10062_10056_10055_10000062_10054_301_10059_10099_10000022_10000012_10103_10000015_10102_10096_10000018_10000019_10000056_10000059_10052_10053_10107_10050_10106_10051_10000053_10000007_10000050_10084_10118_10083_10000047_10080_10082_10081_10110_10111_10112_10113_10114_10115_10000041_10000044_10078_10079_10000038_429_10073_10000035_10121-10503_10501,searchweb201603_9,afswitch_2,single_sort_1_default&btsid=b1460b0b-2fda-4d5a-b910-09c3dfa416f9
 

Dang, that's quite cheap. I hope the thing still works. Maybe I'll build the forsten and compare to the MiniVNA tiny. Thanks for the link