In article , Bob Chilcoat
wrote:

My question is this: Does it seem reasonable that a very small amount of
oil on the antenna can effect transmission that much? Isn't even dirty
motor oil pretty non-conductive?

Maybe! The filter traps all the big pieces, but the microscopic stuff
that slips through acts like metal plating when it coats the antenna
and reflects those transponder signals back down the coax to the
transmitter final. ;-))