The main advantage of a helix is that it "gives" you circular
polarization for "free"...you don't have to phase or split anything.
The disadvantage is generally that they're relatively heavy and they do
require something of a reflector screen behind them, although there are
variants that use a ring reflector similar to what you'd see in a quad
antenna.

Helices have excellent impedance bandwidth, but really the point of
Cebik's article is that the pattern bandwidth is not nearly as large as
the impedance bandwidth. So really it's best to think of them as
single-band antennas like a yagi.

As to sidelobes specifically, you don't really care about that on
transmit for satellite work. But they are important (and to be
avoided) in receive and especially at microwave frequencies. As an
example, in the initial stages of AO-40 (2.4GHz earth receive) many
started with long (5') helices, but quickly realized that small TVRO
dishes were quite superior even though the gain of the two antennas was
about the same.

One great advantage that phased yagis have is that you can switch the
sense of circular polarization--very hard to do that with a helix.
Sometimes satellite links can be improved by switching either your
transmit or receive sense, depending on the relative satellite
orientation and motion.


ml wrote:
for sat work, wondering what antenna type might be better or if they
are close

i've seen those corkscrew like, helical's and say the M2 beam, with
switchable,& cir/pol performance wise are they in the ball park, does one have a clear adv
over the other in a particular circumstance??


thanks