I can't see how the mag mount could change the
pattern shape any, so your experience indicates a severe loss in the
coupling capacitor, maybe as high as 90% or more.....

A pretty large amount...I guess it is possible maybe my
mag mount performance was down a bit due to it being
knocked off the car so many times...Not sure why that
would effect it much though..Maybe the connections were
getting a bit corroded also...Hard to say really...
Probably would be a better test to compare against a
brand new one...The one I had was the standard rectangular
looking larson mag mount. Looks sorta like a small box with
magnets underneath..I do have to say though...When I switched
to the hole mount, the increase in performance was instantly
noticable, as I knew what all the local machines did as far
as strength before the switch.
BTW, my comparison was mainly just receive on
various repeaters...Being things are reciprical, I never
really worried about transmit tests...MK