archive: Re: SETI Yet another hair-brained-antenna-idea?

Re: SETI Yet another hair-brained-antenna-idea?

David Woolley ( david@djwhome.demon.co.uk )
Tue, 8 Sep 1998 23:35:45 +0100 (BST)

> But I see no reason why salty water could not be a good reflector (pure water

That's certainly the assumption for an HF take off over the sea. I'm not
sure that it is true for microwaves though, because of the microwave oven
effect and because you can treat the sodium and chlorine ions in the
brine to some extent like the free electrons in the ionosphere, so
you would expect a maximum useable frequency in the same way.

It's a bit too long since I was taught the calculations for MUF to be sure
that the analogy holds well enough and to be able to work out the MUF
for brine, although it is clearly a lot less than that of red light.

> is an insulator). Is a rotating liquid truly parabolic? For all I know it is

I remember an article in a popular astromomy magazine about someone using
rotating mercury mirrors, but in any case, if you discount surface tension,
I believe it is, although it is a bit difficult to do the integrals in
plain text.

I basically integrated the excess pressure due to the centrifugal forces.
The model I actually used had manometer tubes, but as there should be no
flow across the tube boundaries in the steady state, you should be able
to ignore them and get the same surface shape.