MW Broadcast QRM on 160m

[VK4ADC]

 I had made up a series trap for 927kHz.

I had tried it on transmit previously, and it didn't affect the SWR.   When I instered the trap, the strongest station went up to R5, but the QRM was still there. I tried the attenuator. This gave a marginal improvement.

You didn't confirm whether you had placed the series trap across the coax inner to ground... which is what I would expect you did.

Another thing to try is to change those same two components into a parallel LC, still resonant at 927, but place it in series to the antenna port. If your L has a high enough Q, the impedance at resonance, and therefore signal rejection, will be magnitudes higher than you can achieve by placing a series LC across a 50 ohms line.

Use a 12V relay off the PTT line with a set of contacts to short the parallel LC circuit on transmit to minimise losses.