"J. McLaughlin" wrote
With reasonable offset distances from a 12 inch diameter
metal pole (the tower that is likely to be used), the driving
impedance changes dramatically.
_________________

The input impedance of any/all sidemount antennas (ring-stub or otherwise)
is affected by the electrical environment in which they operate. A common
input spec for a sidemount FM transmit antenna without on-site matching is
~1.5:1 VSWR. FM transmit antennas with input VSWRs greater than about
1.15:1 across the 200kHz bandwidth of the channel can noticeably reduce
stereo and SCA performance.

A transformer can be installed at the junction of the antenna input and main
transmission line to provide a way to optimize the match after antenna
system installation. But this takes some rather good test equipment, not
just the typical in-line wattmeter (Bird, etc) or the VSWR meter in the
transmitter.

Note that installing and adjusting an input matching transformer on an FM
transmit antenna does not affect antenna radiation patterns. Final patterns
are a function of the antenna design, and the electrical environment around
the antenna (within several wavelengths).

R. Fry