"J. McLaughlin" wrote:
Mind, for LPFM stations, the FCC is concerned with the average (or
rms) antenna gain. That is little changed with large changes in the
shape of the pattern. The University wishes to know where the gain is
going. Remember the first law of antennas: all antennas work. The
second law is: some antennas work better in certain directions than
other antennas.
___________________

True, the FCC is concerned only with the RMS gain of an "omni" FM broadcast
antenna, no matter how bizarre its real patterns. The antenna OEM publishes
his antenna gains based on perfectly omni patterns in the horizontal plane,
and those values must be used when determining the ERP of an FM station --
no matter if the true patterns have multiple nulls in the horizontal plane
that may be only 10 or 15% of that gain. The problem is in knowing what
those true patterns are from an installed antenna.

Operating and installation details such as the tower width used, tower
construction details, FM frequency, antenna azimuth/elevation/offset on the
tower, presence and location of conduits and transmission lines and other
metallic structure in and near the antenna aperture, tower guys etc all can
have a significant, often unpredictable affect on the final radiation
patterns from the installed antenna.

The safest way to know what the patterns of the installed antenna are likely
to be is to have them measured on the OEM's test range, using a replica of
the tower or pole on which the antenna will be installed including all the
"stuff" in and near the antenna aperture, and the pick a mounting
arrangement that produces the most omni patterns. For c-pol (and v-pol
only) antennas, that may require adding vertical parasitics near each
element to fill in the natural v-pol null caused by the antenna supporting
structure, which is essentially vertical. ( N.B. The patterns from FM
transmit antennas mounted on towers of large cross-section are difficult to
impossible to fully optimize using parasitics.)

R. Fry

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