J. Harvey wrote:
"Thanks Roy. It now makes perfect sense to me that CP can be generated
with two linear antennas (crossed dipoles, phased appropriately), it
must be true that one could make linear using two CP antennas - just as
you reminded us."

That`s true according to J.D. Kraus, inventor of the helical antenna. On
page 477 of his 1950 book "Antennas":
"Elliptical Polarization as Produced by Two Circularly Polarized Waves -
When the amplitudes are equal, the resultant wave is linearly polarized
(Fig.15-28a)."

Best regards, Richard Harrison, KB5WZI