Here is a description of this antenna by Peter Onnigian, P.E., who developed
and patented the design:

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(Onnigian) Shunt Fed Slanted Dipole Antennas
The slanted dipole antenna in its present configuration was developed and
patented in 1970. It consists of 2 half-wave dipoles bent 90°, slanted and
fed in phase.
The slant angle is critical as it is the factor which determines the ratio
of vertically and horizontally polarized radiated power. The phase point
center is at the feed insulator on the dipole support arm. When fed through
a vertical support pole on which the antenna was mounted during initial
development tests, the axial ratio varied less than 1 dB.
The commercial adoption uses a horizontal boom containing a step
transformer. This boom supports two half-wave dipoles in which the included
angle is 90°. The two sets of dipoles are rotated at 22.5° from the
horizontal plane. Two opposite arms of the dipoles are delta matched to
provide a 50 ohm impedance at the radiator input flange. All four dipole arm
lengths may be adjusted to resonance by mechanical adjustment of the end
fittings. Shunt feeding, when properly adjusted, provides equal currents in
all four arms resulting in excellent azimuth circularity.
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(RF) The 1 dB axial ratio of this configuration is reduced by the adjacent
transmission line(s), and the tower structure needed to support it (as with
all sidemounted radiators).

NEC-2 patterns for this design, and three others commonly used in FM
broadcast transmit antenna arrays are shown in paper 10 at the link below.
Paper 6 shows examples of sidemount antenna pattern distortions from the
line and tower.

RF

Visit http://rfry.org for FM broadcast RF system papers.

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"Eric Schumacher"
Can anyone give me a brief explanation of the theory of operation of a
commerical FM broadcast antenna. Specfically the Jampro JMPC ...