I hate to break it to you, but what you have is a
less-than-quarter-wave vertical with a loading coil
to bring the terminal impedance up to 50 ohms. Only
the straight parts of the antenna radiate and that
thing doesn't have very much. Your Moto guy was
right in recommending a quarter-wave whip; it would
out-perform this loaded version you have now.

To counteract hitting trees and other stuff, you
could substitute thin, strong 0.03"dia stainless
spring stock for the radiator. It has a lot of
"give" and could recover from close encounters.
I use that for my 1/4-WL whip that daily bangs
into my low-hanging garage door.

Jim, K7JEB

On Monday, June 24, 2013 5:57:05 AM UTC-7,
Kickin' Ass and Takin' Names wrote:

The current antenna is a vertical whip with a loading coil wound
along the length of the antenna.

The dimensions a
-- Overall height: 14.25 inches
-- 4 inches from the base the antenna is wound into a coil, about 3/8
inch diameter, 5 turns
-- the coil is 1.75 inches long
-- above the coil is 8.5 inches of antenna
-- NMO base

I suspect this antenna is an old model 5/8-wave VHF antenna, shortened
by winding a coil in the antenna.