Re: Why don't they switch off analogue NOW ?!

The message <drajd9$8e9$1@xxxxxxxxxxxxxxxxxxxxx>
from "Bill Wright" <insertmybusinessname@xxxxxxx> contains these words:


> "charles" <charles@xxxxxxxxxxxxxxxxxxxxxxx> wrote in message
> news:4def4492eecharles@xxxxxxxxxxxxxxxxxxxxxxxxxx
> > In article <drad5e$pnj$1@xxxxxxxxxxxxxxxxx>,
> > Angus Rae <Angus.Rae@xxxxxxxx> wrote:
> >> Bill Wright wrote:
> >> > "Angus Rae" <Angus.Rae@xxxxxxxx> wrote in message
> >> > news:dr7ln7$ctb$1@xxxxxxxxxxxxxxxxxxxx
> >> >> In the case of a couple of villages I know quite well satellite
> >> >> reception would involve either placing a dish on top of a mountain
> >> >> (which, according to a bit of Google Earth measuring, would involve a
> >> >> 1.5 mile cable run and placing the dish at 2000 feet) or building a
> >> >> small artificial island in the middle of a loch. I haven't asked a
> >> >> fitter for an estimate for either of those options, but I'm guessing
> >> >> that it's not going to be cheap.
> >> >
> >> > I wonder if a passive reflector would work. On the face of it I would
> >> > have thought so.
> >
> >> I had thought of that, it would need to be positioned on the other side
> >> of the loch about 3 miles away (funnily enough, very close to the local
> >> relay). I'm guessing that the reflector would have to be reasonably
> >> large?
> >
> >> Of course the main problem with that would be getting access and
> >> permission to site the reflector...
> >
> > Long time ago, a colleague & I did the sums for a passive reflector for
> > the
> > Glenelg Self Help start point. ISTR that it would have needed a surface
> > area greater than 2 football pitches to give a useable signal at the
> > required destination. The higher frequency of the satellite service
> > might
> > will reduce the area needed, but it would still be very large to be
> > effective.

> Just thinking aloud. If you used a mirror to reflect the light from a point
> source to your eye it would only need to be big enough so that the
> reflection encompassed the width of the point -- an infinitely small
> dimension -- at every point on the lens of the eye. Making the mirror
> bigger
> would merely provide a view of the areas around the point source, and would
> not increase the brightness of the latter.

Presumably, use of a perfectly flat mirror in order to simply make an
otherwise obscurred point source of light visible via a dog legged path.

> Thinking about the passive satellite reflector, that makes me wonder if the
> reflector could be quite small. Increasing the size would merely reduce the
> pointing accuracy required. The reflector surface would presumably need to
> be perfectly flat, and the structure would need to be absolutely rigid.

In which case, it would also have to be perfectly angled to reflect the
light to the receptor, fine for a point to point link, useless for a
broadcast system.

What you're suggesting requires impractical to achieve tracking
accuracies on the part of the earth station transmitters and beam
confinement levels only practical using laser light collimation
techniques [1]. Also, the satellite reflector dish would, in the case of
a broadcast system, have to be of the convex variety to disperse the
beam over the required service area.

> If the reflecting efficiency was only 50% the dish would need to have twice
> the surface area to compensate.

It wouldn't compensate if the beam were narrow enough to be entirely
captured by the reflecting dish.

> Come on now someone, you know this can't be right. Prove me me wrong!

> Bill

In the case of an accurately aligned plane mirror, ignoring
diffraction effects for the moment, this might be close to true where
the distance between the reflector and the eye is 2 or more orders of
magnitude greater than the source to reflector distance.

Still ignoring diffraction effects, when the distances are equal, the
reflector would have to be half the diameter of the pupil and when the
seperation distances are reversed, equal to the pupil diameter.

Of course, diffraction effects will require the reflector dimensions to
be increased over and above what simple geometry dictates as the
minimum. Exactly by how much is determined by the wavelenghs involved
and how close to 100% 'reflection' (ignoring the inescapable losses due
to absorption and scattering) that you wish to attain in this 'point to
point' setup.

What you're basically suggesting (a passive reflector system in
geo-stationary orbit) is impractical due to several factors including
seismological and atmospheric disturbences to the alignment of the
up-link transmitter beam.

Passive reflectors can (and do) work (else ghosting of terrestial
analogue TV broadcasts wouldn't be a problem), but, at UHF, they usually
need to be about the size of a large building, such as a high rise block
of flats or offices.

[1] Essentially, a focussing system with an aperture of several thousand
wavelengths diameter.

--
Regards, John.

To reply directly, please remove "buttplug" .Mail via the
"Reply Direct" button and Spam-bots will be rejected.

.