Yanis,
I would try the portable with its own power supply, its own antenna, and no intercom. this will totally isolate it from the plane. If you still get interference, it is RF interference generated by the ignition system. If it is ignition RF it will rise and fall in direct relation to the throttle setting. The RF chokes you installed are a good start but they are a little like black magic. Sometimes they work and sometimes they don't for no apparent reason. In my experience they usually help some.
If the portable works OK without the planes' antenna, intercom, and power supply, hook them up one at a time and see when the problem returns. If it is the antenna, make sure that both ends of the shielding in the coax are grounded. One end to the antenna base and the other end to the radio. The BNC connector on the radio usually grounds one end to the radio. the other end is sometimes grounded like this but sometimes the shielding has to be soldered or screwed to ground. You might consider the position of the antenna. If it is in direct line of sight with the ignition system, it can pick up small amounts of RF even from a good system. It is a good idea to keep as much as possible between the antenna and the ignition. Keeping your body, the airframe, the radiator, and anything else in between will help. If it is the intercom, you might check the headphone jacks for loose connections or bad grounding. If it is the power supply, this is where RF chokes will help the most.
Hope some of this helps.
Bill.
