I wrote, "At their design frequencies, these elements produce the same
_measured_ field strength in their directions of peak gain as produced by a
matched, linear dipole in its directions of peak gain (over the same
path)."

To clarify this for those who didn't read through the slide show I referred
to in the original post, the FM helix would need twice the input power as
the linear dipole to produce the same field peak strength over the same
path. The FM helix is circularly polarized; therefore its peak, net gain
per polarization plane is 1/2 that of a dipole.

Most FM broadcasters in the US are licensed to radiate a certain amount of
horizontally polarized "effective radiated power" (ERP) in the horizontal
plane. Typically they can radiate any amount of vertically polarized power
they choose, not to exceed their h-pol ERP.

The station's ERP is the product of the tx output power, the efficiencies of
their transmission line, channel combiners, filters etc, and their antenna
gain, per polarization. Using a c-pol antenna means that the broadcast
station needs twice the tx output power to produce their licensed h-plane,
h-pol ERP.

RF

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