Just about any 5/8 wave antenna mounted on top of a Ford pickup truck. The roof metal is so thin that frequent flexing from antenna sway, wind resistance, low tree branch contact can cause metal fatigue on the cab roof.  The antenna NMO mount was installed directly above the cab interior light. Nice install but the metal finally cracked about 1/2 inch circumference further out from the outer edge of the NMO mount. The singing problem was resolved by wrapping thin fishing line around the full antenna length at about 1 turn per 1 1/2 inches for its full length. The thinner the whip antenna the higher the audio frequency. Observe automobile antennas and you will see many factory manufactured AM/FM antennas wrapped as mentioned above. Go out and wiggle your mobile antenna. Look at the automobile metal near the base. Watch it flex. The longer the antenna the more flexing. I solved the problem by relocating the antenna using a homemade bracket mounted between the front fender and hood near corner of windshield.  
Gary  K2UQ
 








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