Winfield Hill said:
martin griffith wrote... [snip]
I know relatively little about 2.4G stuff, so what sort of Rx antenna
wiil I require as the Tx antenna will revolve through at least 360
degree...see
http://uk.geocities.com/martingriffith/splashcam.htm
Why consider directional antennas when the source will be taking
on all angles (including underwater I assume? :>)
I don't see the point in not going for a directional Rx antenna of
some description, it's not like the windsurfer is going to end up on
the beach behind the Rx so it's more gain without having to carry more
kit on the windsurfer.
The TX spinning through all angles would affect polarization, but
aren't most (all?) omnis polarized too? I can see how an onmi would be
better for a moving target, but not for a rotating one. Perhaps I
don't know enough about polarization.
Most of these
designs only give you +6 to 9dB gain anyway, easily gotten from a
more powerful transmitter power. With omni antennas you're legal
up to 1W, which is 10 to 20x more power than the wimpy units you
normally see, arguably more effective than a directional antenna
anyway.
It's the Rx antenna he's asking about, I presume there are no legal
limits on gain at that end (I can't see any logical reason for it and
there's no restriction in the UK on Rx, only Tx).
Have you considered windage?
At first I read that as the ballistics definition: "The point or
degree at which the wind gauge or sight of a rifle or gun must be set
to compensate for the effect of the wind" and wondered just how much
you knew about photons
The 2.4GHz antennas I've built for video have been for a different
application - robots driving around on the ground. For that I went
with vertical polarization, a 1/4 wave whip on the Tx and a diacone
(omnidirectional, compact, good gain and they look cool too!) on the
Rx. Given this experience wasn't in a commercial environment I
wouldn't take the following advice as gospel, but here goes anyway.
For the OP's application I'd go for horizontal polarization (left to
right in the OP's images: perpendicular to the vertical and
longitudinal axes of the windsurfer) with a flexible whip or dipole
antenna tough enough to take the punishment. For the Rx end a
horizontally polarized Yagi with the right combination of
directionality and gain. The right combination is left as an exercise
for the reader
A parabola would probably give more gain but you'd have to be aiming
it all the time and on a windy day that wouldn't be fun (the "windage"
effect Win was getting at I presume). Horizontal polarization wouldn't
change as the windsurfer flipped over, whereas vertical would be
rotating with respect to the receiver so your gain would vary
(theoretically dropping to zero as the mast pointed towards the
receiver).
Tim