thanks for the idea. Seems the headset must be specific type with the antenna not grounded by the connector. Trying 5 different headsets i have at home, all do not work. But if you pull a bit from the connection, you can find the point where reception start to work much better. So, the specific type connector most probably needed
Here's a TRS to TRRS adapter that works great! A lot cheaper than having to buy a stereo headset w/mic. Once I used this, I now get 7-bar reception on 16 stations and can use my skullcandy headphones. And for the record, this is actually an HTC hardware flaw. They have been soldering the contact pin for the antenna in the slot where the ground is on a TRS plug (sleeve) instead of either the R or L output slots (either first or second ring) which grounds the antenna and makes it useless when using a TRS plug. Hence, a TRRS plug is needed where the third ring connects to the antenna contact pin.