SWR reading changed after replacing components in this transceiver. Output impedance

From this thread https://www.edaboard.com/threads/242112/

I replaced D8, Q6, D13, D7, D1, R11.

Before replacing the components, the SWR read 1.13 but after replacing the components, the SWR read 1.4 using the same antenna.

The cause would be that I accidentally pulled out small circuit path on the board between R11 to Q26 and D8 to L15. I soldered the path to connect them already, but I am not sure if this would change the output impedance of the transmit signal, but I think it's unlikely.

I guess that the SWR goes up from 1.13 to 1.4 because the output impedance changed from 50ohm after I repaired the radio.

I hi-lighted the transmit signal path in the picture(red square). So, please give me a suggestion how to match the output impedance to come back to 50ohm. I have not put the metal cover over the transmit path as it was originally placed there. I'm not sure if it affects the output impedance.

I'm thinking about adding a capacitor to match the output impedance but not sure which point in the circuit I should put it to.

Hello,

The VSWR of an antenna only depends on that antenna, so if the transmitter's output impedance changes, VSWR should not change.

Sometimes VSWR becomes worse due to increased harmonic (or spurious) production of the final power stage.

Also many VSWR indicators show too optimistic VSWR values at low power, because of the diode voltage drop. When you increase the RF input power (to the antenna + VSWR indicator), VSWR indication will increase and will be more close to the real value of the antenna's VSWR.

So If you didn't change frequency, didn't change output power towards the vswr indicator and you are sure the antenna is still the same as before, your transceiver very likely produces too much harmonics or spurious.

As you changed PIN/band switching diodes in the output path, harmonic production may change.

I compared the SWR between two same radios. The other original one reads around 1.13. So, the change should come from my repaired radio.

If the radio after-repair produced harmonic, how could I solve the problem? Any suggestions?

Just to check for other issues:

What antenna do you use and how far is it away from your bench? If your antenna is close to the equipment, cable routing (even changing length) may change a present common mode component on your coaxial feeder. This may change VSWR of the antenna. If you grap the cable and or power supply lines, does this change the VSWR? How is the VSWR for both radios when using a dummy load?

Do you have some means to check the PIN diode current for both radio's? Did you use same diodes in both radioss? PIN diodes can introduce harmonics. However they are between the output and the matching and low pass filter section, so they shouldn't be that strong. Can you check for harmonics (spectrum analyser or receiver with some notch filter)?

I don't have any means to check for harmonics now. I'm going to build a 50ohm dummy load to check both the radios.

One thought of why the VSWR reading changes is the PA deterioration from changing D8 polarity and turning on the radio and pressing the transmit button while I was replacing it. I tried putting both polarities of D8 because the polarity in schematic and a pcb bottom view pdf file is in opposite. Then finally I put the D8 as cathode pointing toward L3.