Hello,

i am a german student, so first of all i am sorry for my following english.

I have to develop a LoRa transciever. Therefore i have chosen the SX1272 (868 MHz) with connection to the PA Boost Pin.

I have chosen the reference design with RF switch from your website.

The transciever works fine and i have communication between two of them. The programmed output power is 17 dBm.

Now i have problems the match the output circuit to the PA boost impedance, to get the best possible output power and to increase the reach.

First of all i have matched the antenna to the output pin from the RF switch (RFC). Therefore i removed the switch and put an U.FL adapter to the output pin for VNA measurement. I matched these part to 50 Ohm with a Pi network.

To match the complete circuit to the PA boost pin i have removed the SX1272 and put an U.FL Adapter to the PA Boost pin. I have not removed the RF switch, but connected it to power supply, to have a connection to the antenna part. With an 50 Ohm to 50 Ohm LP filter and the L matching network behind the LP filter i matched the complete circuit to 50 Ohm. Afterwards i converted the L network to an 2. harmonic filter.
SWR: 1,1 ; Return Loss: -25 dB with matched circuit from PA Boost Pin to Antenna, measured with VNA.

Then i measured the output power at the antenna with an spectrum analycer for comparison measurement (not exact).

The problem:

With the antenna matching at the antenna part and the LP filter ONLY (no matching network after LP filter, complete circuit is not matched to 50 Ohm) i measured significant more output power (8dBm) in comparison to the complete machted circuit to 50 Ohm with L matching after LP filter (0.8 dBm).

But i have no idea why. Normally it should be more output power at the full matched circuit.

Do i have to match the PA boost pin to another impedance then 50 Ohm ? Which?

Or is there another problem i am not aware of.

Thanks a lot for helping.