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Posted by Andy Dingley on 01/25/07 16:39
On 25 Jan, 15:19, Ben C <spams...@spam.eggs> wrote:
> On 2007-01-25, Andy Dingley <ding...@codesmiths.com> wrote:
>
> > On 25 Jan, 12:34, Toby Inkster <usenet200...@tobyinkster.co.uk> wrote:
>
> >> However, the distance from the Earth to the Moon is *thirty* *times*
> >> *further* -- just think about that distance --
>
> > In a nice hard low-loss vacuum. With massive groundstations.
>
> > Power isn't the problem in long-range radio comms, it's noise. Although
> > the distance is high and so inverse square law losses are too
> Do you not somehow focus the wave? Using a dish for example on the
> transmitter, so it travels parallel like a spotlight beam, or perhaps
> you let it fan out a bit but not inverse-square law.
You can "focus the beam" in the "near field" but not in the "far
field". This is the same for spotlights or for radio, although as
antenna sizes are measured in terms of wavelength, it's generally
easier to make usefully "large" focussing devices for light than for
radio. Once you get to lunar distances though, it's the far field.
See http://en.wikipedia.org/wiki/Near_field
The big dish is just to capture more signal on reception.
> People overestimate the rate of development of technology because of
> computers.
It's not about the rate, so much as the prevalence. Something might
have been possible and demonstrated once in the 1930s but then didn't
become common until the late '60s. Whether it was in Apollo or not then
demands on whether NASA trusted it. In some ways Apollo was technically
adventurous, in others it was distinctly backward looking for
reliability's sake. Given the poor reliability of missile systems and
the US Navy space program, compared to NASA's good record, then it was
the right choice.
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