Electronics > RF, Microwave, Ham Radio

Antenna matching circuit - weird behaviour

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alexxs88:
Hi,

I have a 2.4GHz device with room for a Pi matching circuit. I cut J5 (to disconnect the MCU - brown line in layout picture), soldered the ground of the coaxial cable in the white region and the feedline from the coax going to the VNA where it's marked in light green.





Calibrated the VNA using C24 to install a short and the 50R as load calibration. With that done, I soldered 100pF caps as shorts in L1 and C23 and started measuring to get a baseline. With the help of a series capacitor (0.5pF) and a shunt inductor (6.8nH) I went from the green trace in the Smith chart to the red one and from a VSWR of of ~7 to 1.17. In theory the gains should've been massive. However, and this is what I'm puzzled about, doing back to back tests (transmission) with a device that only has the 100pF caps in series, the wireless performance is nearly identical.



I racked my brain over this and can't find any decent explanation. The only theory (and it's a pretty weak one) is that angling the feedline of the coax back towards the MCU somehow influenced my readings. I've used the same VNA and tools to match another device a few weeks back and it all worked perfectly, so I'm quite puzzled.

Thanks all!

alexxs88:

--- Quote ---Were you testing "in the far field" i.e. at a fairly long distance e.g. 10 wavelengths distance from the antenna?
--- End quote ---

Yes.


--- Quote ---Were you measuring the RSSI strength from a paired IC receiver, or were you using a spectrum analyzer or what to measure the received signal strength?  Or were you doing a receive test from an unmodified distant transmitter and looking at the incoming RSSI?
--- End quote ---

This is how I tested. Had two identical devices acting as the transmitter, one had shorts in place of the Pi network and the other one was tuned. They were both ~2m away from the receiver, in exactly the same orientation. The signal strength is called "link quality" and derived from RSSI on the receiver. I also did various other tests, with one of the transmitters disabled,  swapped positions, just to make sure it wasn't some freak occurrence.


--- Quote ---What about those VNA sweeps, is the frequency you tested the link loss at near the frequency where the feed was closely matched to 50 ohms?  I see the red trace that goes in sweeping all the way from a match not much better than the green trace to a pretty good match near the end of the frequency sweep but the test frequency / frequency range is not indicated with markers or comment.
--- End quote ---

The receiver is the one who controls the frequency, the network was formed on channel 11 on 802.15.4 . Below is the VNA sweep with the specific frequency markers before/after the matching circuit was implemented.


--- Quote ---Did you calibrate the VNA at the end of the coax cable over the sweep range before connecting the coax to the PCB?
--- End quote ---

Yes, I have. Soldered the coax cable to the PCB at the points marked on the layout, then used a short and a 50ohm resistor to calibrate it.

alexxs88:
Thanks for all the suggestions! I'll try a different "path" on the Smith chart. Just so I understand things a bit better:
1. When you say "not tight relative to capacitive parasitics", do you mean the unplanted stubs in the Pi circuit or something else?
2. If the problem were the cap, wouldn't it show on the VNA sweep as well?

David Hess:
Is the transmitter output actually 50 ohms?  Usually it is intended to match to 50 ohms but is itself much lower than 50 ohms so a VNA measurement in place of the transmitter would produce the wrong result.

alexxs88:
From everything I could find online and in their documentation it seems to be 50 Ohms, the balun is on-chip

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