I am using the 915 MHz RAK3172 on a 4 layer 61 mil FR4 PCB. I’m using the IPEX and not the RF pin (12)
The 4th layer (bottom layer) GND plane was not removed, the inner GND planes are removed. This has a dramatic affect on RF TX power and RX sensitivity. According to the capacitance calculator the stray capacitance is ~0.1pf. The pad size of 0.7mm x 2mm with a distance of 61 mil (1.6 mm) I’m seeing the RX sensitivity drop by 35 dB and the transmit power drop by 40 dB, this is very dramatic.
I resolved the issue by removing the part and not soldering the RF pin 12.
My question is why is the RF pin 12 so very sensitive to having a GND plane under it?
This is a warning in the RAK3172 specification: WARNING
When using RF pin for antenna connection and not the IPEX connector variant, make sure there is no ground plane (in all layers of the PCB) under the RF trace path to eliminate the possible effects of unwanted stray capacitance which can cause degradation of the RF signal levels.
This warning should also that any ground plane under the RF pin 12 should be removed. Whether using the RF pin 12 or not. Indeed the ground plane clearance should be bigger than pin 12.
The warning should be updated ASAP.
I would suggest looking at the design of the RF output to reduce the sensitivity to RF pin 12 to the GND plane. The stray capacitance being so small there must other effects going on?
Can you please share me on what section you find that warning in the documentation?
It was already updated since ground plane optimization for non-IPEX variant of RAK3172 is dependent on other factors aside on stray capacitance. For example, on monopole antenna, the ground plane is really needed.
In your situation (of soldering the RF pin), that pin is isolated to the RF path via 0201 resistor in RAK3172 IPEX variant so it is really strange that it has effect on the IPEX connected antenna performance.
If you can share me some image of your setup and other relevant info, I can share it to our RF engineer so he can have a look.
I have attached RAK3172 Sept. 2023 spec sheet with page 4 about the warning the ground plane underRF pin pin 12.
I’m not using the RF pin 12. My U.FL cable connects directly to the IPEX on the module.
Even thought I have removed the GND plane (4 layer board) on the inner layers beneath pin 12, I left a ground plane on the bottom layer 61 mil away beneath the pin. Yet I’m seeing 40 dB of RF attenuation. I solved the problem by removing the RAK3172 and soldered a new part and not soldering pin 12 to the PCB.
What I suggest when using the IPEX on the RAK3172 nodule, that all ground planes be totally removed on all layers around pin 12. Indeed i would also suggest that the ground plane be removed beneath the rF section of the module around the IPEX connector, an area of 300 by 200 mils. I my design I removed pin 12 from the pad stack.
See attached layout
WARNING
