I am based in the Seattle area. Ten miles or so east of KSEA is Tiger Mt, a 3000 foot peak with numerous very high power commercial transmitters. Since purchasing my 1970 C model some 15 years ago I have been experiencing significant radio interference when flying within 5 miles of Tiger Mt. I experimented with placing ferrite beads on the outside of the cosxial cables connecting the radios to the antennas and heard some reduction in interference. During the recent annual my A and P and I measured the resistance between the antennas and the fuselage skin and found them to be within spec. We decided to replace the 50 year old coaxial cables with new RG400 shielded coax. Wow! The interference is gone. For those owners out there who are having interference with high power transmitters, I suggest swapping out the old cables for new ones. You will not be sorry.