Antenna matching for GSM using nanoVNA - reference plane - Page 1
Forum section: electronics / rf-microwave

author: jan019
author_rank: Newbie
timestamp: September 24, 2024, 01:56:23 pm
content: Hi all,I have designed a PCB which uses the SIM800C module for communication on the GSM network. Currently I am dealing with matching the antenna and although I have done many hours of reaseach, I still am not completely clear on a few things and would be grateful for your help.I am attaching pictures of the antenna schematic, layout of top layer (antenna) and the reference ground along with the physical stackup of the PCB. The antenna I am using is an external antenna connected via a UFL connector. The traces from the SIM800C antenna pad to the UFL connector are impedance controlled to 50 ohms:coplanar waveguide with width 0.8mm, distance to reference plane 1.3104 mm and gap to coplanar plane 0.16 mm for a frequency of 1350 MHz (because GSM operates in frequency range 876-915/921-960 MHz and 1710-1785/1805-1880 MHz the transmission line was tuned to a the middle frequency of both these ranges.I am using the NanoVNA-F V2 as the measuring instrument. I have a UFL connector MM8130-2600RA2 very close the the ANT pad, to which the VNA measurement probe (MXHS83QE3000) is connected.I have 5 pcs of the PCB and my original idea was to use the board itself for calibration, because the calibration would include the measurement probe as well. See the attached schematics to see how I soldered componenets for open, short and load calibration. This approach seemed to work, but when I applied the calculated components (always a shutn capacitor and a series inductor), the smith chart changed in a totaly different way than the calculation expected (I checked the calculations at multiple sources, so the values were correct). The used capacitors and inductors are from Johanson Technology from the set L603DC and S603TS.I couldn't figure out the issue, but after some research it seems that the issue is with the calibration. So I tried a different appraoch - I calibrated the VNA with thesehttps://www.aliexpress.com/i/4000403088831.htmlcalibration standards that attach directly to the VNA SMA connectors. Then I connected the measurement probe to my PCB with componenets assembled for OPEN calibration and using port extension (electrical delay on the nanoVNA) I changed the delay so that the point in the smith chart is as close to the right side as possible. I then soldered the recommeded component values (again shunt capacitor and series inductor) and now the movement in the VNA chart was closer to what was expected based on the calculations.However, using the port extension, according to what I read, is not as exact as calibration directly on the PCB and also I have no way of verifying the calibraiton.So my questions are:1) How can I verify the calibration?2) What could be causing the incorrect calibration when using the PCB to calibrate?3) Is the reference plane supposed to be at the start of the matching network or at the ANT pad?4) From the schematic for my calibration PCB modification, where is the reference plane to which the calibration is done?Thank you very much for any help.Jan