Products > Test Equipment
Sub $1000 VNA for antenna matching
<< < (3/8) > >>
joeqsmith:
Calibrating the LiteVNA with my home made FR4 standards (really bad) then measuring the open supplied with the V2Plus4 and with nothing attached to the cable. 
ogden:
You may find this useful to measure small RF stuff:

https://www.pasternack.com/sma-female-unterminated-lead-sexless-pe-020sr-cable-assembly-pe3ca1107-3-p.aspx

When cut straight - it becomes sexless ;)

Other option:
https://www.fairviewmicrowave.com/test-probe-pigtail-sma-female-straight-cut-lead-cable-.020-coax-fmca1187-3-p.aspx
joeqsmith:
If I place a mismatch (20dB RL) inline with the cable and then calibrate the LiteVNA using my FR4 standards.  Then measure with nothing attached and the V2Plus4 open standard,  we get the following.    Because we are looking at systematic errors, the SOL will take care of much of the error. 
samofab:
The OP question was basicly "what are the requirements for VNA used for tuning PCB antennas up to 2.4GHz".
Members, please correct me if anything below is incorrect or misguided.

The process for tuning the PCB antenna goes like this (with many adaptations possible):


* attach SMA connector before before the matching network. That can be done in a few ways: a.) if you planned for this you'll have a PCB connector availible, but this can be tricky becuase could be interrupting a transmission line (TL) b.) more likely you'll solder a micro-coax (rigid or semi-rigid pigtail) to the point where transciever would be or you'll cut the TL and attach the micro-coax to the first component in matching network or close by
* calibrate you VNA + cable
* with no matching network in place (or with short on first element) measure the electrical length of your micro-coax
* enter this number to VNA
* populate the matching network with reasonable initial components (e.g. from datasheet)
* measure S11 (in final mechanical environment) and display as return loss around your frequency of interest or wide enough to see a drop in response
* verify that your antenna is radiating (block it with hand to see a difference on RL)
* if the drop is lower than -10dB at your target frequency, (my) congratulations! Also, remember.. a lot of things closer than lambda to your antenna will move this a lot.
* more likely you'll need to modify the matching network. Display S11 as Smith chart at frequency of interest.
* iteratively modify components until the dot is in the center of the Smith chart. Absolutely keep lab notes on the the procedure; iterate and triple check everything. Don't move your cables. Wait for your VNA to heat up (=stabilize).
Based on this process, here are my thoughts on the VNA:

For measuring return loss you have two numbers that are important: frequency and drop in response (level / SWR). If your antenna is very mismatched, the drop in response can be far away, so it's useful to have VNA that covers larger frequency range than your interest (say x1.5 or x2). In my opinion: The accuracy of frequency will never be a problem. The accuracy of the level is also not very important unless you're trying to have an exact measurement how good your antenna is. In other words -10dB or -11dB will not make any difference.

For matching the antena (which you'll be doing on Smith chart) the most important thing is not absolute accuracy, but precision within one measurement/matching session. Also you can do matching on the very top of the frequency range of your VNA. If you fire up any Smith chart matching calculator https://www.will-kelsey.com/smith_chart/, you will see that errors accumulate very quickly. Even a 2% error in original impendance measurement followed by 2% in component tolerance (let's say for two steps) can move the impendance far far away from 50. Be especially careful when the measured antenna impendance is close to border between different matching strategies.

Once you get unexpected results you will doubt (in no particular order): your VNA, your cables, your calibration kit, your matching componentes, your PCB design, you pigtail, your knowledge and your freespace environment. If you pay more money for the things you can buy from the prevoius list, you'll reduce the amount of things that will be the source of your doubt. Not sure how all of the above helps in purchasing decision :-)


joeqsmith:
I've never looked into it but it does seem like the VNA/software could calculate the values based on what model you select for the network.   Maybe there is standalone software that does this today with Touchstone files. 


--- Quote ---with no matching network in place (or with short on first element) measure the electrical length of your micro-coax
--- End quote ---

Or just leave the cable open, you just need a high reflect.   You could just build it up and use gating to remove it?   

Good point about the micro-coax.  UT047 is about 1.2mm OD.   You could get something fairly cheap from Digikey assembled:
https://www.digikey.com/en/products/detail/microchip-technology/RN-UFL-SMA6/2094296
Or
https://www.digikey.com/en/products/detail/taoglas-limited/CAB-058/5287277
https://www.digikey.com/en/products/detail/amphenol-rf/095-902-462-003/7386033

Paper describing some of the basics.   
https://usb-vna.com/2020/08/26/wi-fi-bluetooth-pcb-tuning-and-antenna-testing/

Software
https://coppermountaintech.com/video-antune-antenna-impedance-matching-and-antenna-measurement-demonstration/
Or maybe look through some of these:
https://www.rfpage.com/free-tools-to-analyze-and-plot-s-parameters-file/
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod