I was sort of including that in "Check your antenna", but that's a good point.Rhino, you forgot another critical item = “electrical bonding“ of the antenna to airframe or ground plane.
Ok, will check the type of coax I used. Removed, cleaned contact points, and reinstalled antenna. No change.Check your antenna, antenna cable and the connectors. If the cable isn't RG-400 or better, you should consider upgrading it. Too many builders make the mistake of using something cheap, like RG-58. If someone else has an antenna you can try temporarily, that might help you determine if your antenna is going bad. Also make sure the antenna is sealed well against the elements. Antenna sealant tends to break down over time, and that can be death for an antenna.
Thanks for the ELT hint. A local shop mentioned this may be an issue. Will Ryan and check this today.Do you have an ELT antenna on your aircraft?
I have seen reports of harmonics between the two antennas causing transmission problems on certain frequencies