I’m an absolute beginner using VNAs but have started playing around with measuring antennas. I’m using the PocketVNA. Now I’d like to understand the characteristic impedance of a transmission line on a pcb.
Measuring S11 on antennas have been working well for me but I’m a bit lost on measuring transmission lines on a pcb.
As I understand the method:
1. Make sure the VNA is warmed up.
2. Calibrate the VNA with open, short and load on port 1.
3. Connect DUT with one end of the PCB to port 1 and the other end terminated by a 50R load.
4. Measure S11 for desired frequency.
If the transmission line is working correctly (50R Z0) then shouldn’t the |Z| on a Smith chart be 50R? It is not for me.
The system I have setup is a U.FL contact soldered to one end of the transmission line and GND. In the other end there is an SMA contact. The SMA contact is directly connected to port 1 on the VNA. On the U.FL contact I have a U.FL to SMA coax. On the end of that SMA I have a 50R load. The VNA is calibrated directly at port 1. The connection is:
50R load -> sma to ufl coax -> ufl contact -> pcb transmission line -> sma contact -> VNA port 1
This is how the system looks connected up:
For 800-900 MHz I measure |Z| at 22R in this system. See the smith chart below that also shows the calibration. The calibration is not perfect and could be improved I guess but looks like a bad calibration alone doesn’t explain my results. Either my transmission line is way of (which it shouldn’t be) or I’m doing something wrong when measuring (more likely).
smithchart_50R_on_ufl.png
Another question, if I compare gain on the 50R load placed directly at the VNA compared to placing the 50R load at the U.FL coax after the transmission line I see a difference in S11 gain at 15dB (-10dB vs -25dB). What conclusions can I make from this? Does this mean that there is 15dB of loss in the system?
50R_gain.png
Also, if I look at the Smith chart for all the measurements I see a big difference in comparing measurements done directly at the VNA and measurements done with the DUT in between. How should I interpret this? If the transmission line was ideal the measurements would be similar right? I suspect the coax from U.FL to SMA has a big impact here but I can’t confirm since I don’t have any means to only measure that cable.
Smithchart.png
Right now my plan is to proceed with finding ways to measure the ufl to sma coax cable to understand its impact and also to calibrate with that cable attached. But anyway I would appreciate advice on how to proceed and what I’m missing
Are there any good guides on how to measure pcb transmission line characteristic impedance with a VNA? Is it meaningful at all to use a device as the pocketvna and my cheap cables and connectors or do I need professional grade stuff for this?
Thanks!