Here's a diplexer I built around 1977 (time flies when you're having fun). It allowed me to transmit on 146 MHz FM using about 10 Watt & at the same time listen to AM & (more surprisingly) FM radio. The AM/FM radio emitted a click as I hit the transmit button, but otherwise worked fine.

The "2 stub tuner" lengths are adjusted to minimise SWR. You should be able to find more about this tuner in the RSGB & ARRL Handbooks.

This diplexer has outlived two cars & co-existed peacefully with AM/FM radios in both. (It has also spent over 20 years cluttering up my shed ;-) It has allowed me to play ham radio without driving around under a mobile aerial farm.

The diplexer is guaranteed to be environmentally friendly and completely invisible to YLs & XYLs. (and it doesn't contain PCBs ;-)

The only disadvantage is that you will have to adjust it for:

1. Minimum feedthrough to the AM/FM radio &

2. Best SWR (better than 1.2:1 in my case) &

3. The AM antenna trimmer on the radio will need to be re-peaked

for best reception in the upper part of the AM broadcast band.

73, de Phil Rice VK3BHR

from the land of OZ (Australia)