Are sat dishes too small?
 

2BSur2Bsur wrote:

Bring back the squarial.

Several round here never went away ... saw one looking in good nick
recently.

Are sat dishes too small?
 

2BSur2Bsur wrote:


Think about telescopes/binoculars/magnifying glasses. The bigger the
lens the more you are able to "zoom" and pick up more detail from your
intended target, but the more accurate you have to aim.


The analogy is flawed. The main reason that lens sizes increase with
magnification is to keep the exit beam as large as the eye's pupil. The
same effect as with radio dishes does occur, but it is related to
blurring of fine detail, rather than magnification.

The telescope lens scaling issue is a geometric optics one, whereas the
antenna on is a wave optics one.

Are sat dishes too small?
 

Eh?

As a physicist by training, I'm not aware of anything known as
'geometric optics' or 'wave optics', nor therefore of any difference
between them. Optics is a simply particular branch of the study of
waves, that which deals with electro-magnetic waves at or near the
visible part of the spectrum, particularly with regard to refraction
and reflection. All wave study involves both geometry and, er, waves!

On Thu, 07 Jun 2012 20:05:51 +0100, David Woolley
wrote:

The telescope lens scaling issue is a geometric optics one, whereas the
antenna on is a wave optics one.
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Are sat dishes too small?
 

In article ,
Java Jive wrote:

As a physicist by training, I'm not aware of anything known as
'geometric optics' or 'wave optics', nor therefore of any difference
between them. Optics is a simply particular branch of the study of
waves, that which deals with electro-magnetic waves at or near the
visible part of the spectrum, particularly with regard to refraction
and reflection. All wave study involves both geometry and, er, waves!

No doubt you are also unaware of classical physics, since everything
is really explained by quantum theory :-)

Some aspects of optics could be discovered and used to get correct
results (to an extremely accurate approximation) without knowing that
light was a wave - as is clear from the fact that Isaac Newton did
just that. Examples are the calculation of the reflection and
refraction of light.

Other aspects of optics, such as diffraction, and the correct
explanation of refraction, depend on light being a wave.

It seems reasonable to label these two aspects geometric and wave
optics.

-- Richard

Are sat dishes too small?
 

Richard Tobin wrote:

Other aspects of optics, such as diffraction, and the correct
explanation of refraction, depend on light being a wave.

It seems reasonable to label these two aspects geometric and wave
optics.

This being the well known conundrum of wave/particle duality.

Bill

Are sat dishes too small?
 

In article , 2BSur2Bsur
wrote:
On 06/06/12 09:34, Jim Lesurf wrote:
In , Bill Wright
wrote:
Jeff Layman wrote:

I have always understood that the 45 cm Sky disk was the minimum
size that Sky could get away with for (usually?) reliable reception.
When I looked into getting Freesat a couple of years - or maybe
more - ago, the minimum size of disk available relatively cheaply,
complete with receiver, was 65 cm. Apart from having to move the
dish because of a tree growing too high, I've never had reception
problems from 28.5° here in Sussex, no matter what the cloud cover.

We find it helpful to use larger dish sizes for communal systems
where there is a lot of amplification, because the increased s/n
ratio (less beamwidth means less sky noise)

Erm...

I'm puzzled by you saying a larger dish ("less beamwidth") means "less
sky noise". I can see that a larger dish will have a higher gain, so
should pick up more signal. But why do you say a larger dish will
reduce sky noise? I'd expect the improvement in CNR to be due to more
signal, not less sky noise.

Larger dishes are more directional, e.g. you need to aim them more
accurately but get a commensurately higher gain.

Smaller dishes are less directional so have a wider beamwidth.

Yes, I am aware of that (having spent years designing/using/teaching wrt
antenna systems and 'telescopes'). However what you say isn't the same as
saying that "less beamwidth means less sky noise". It can certainly mean
"more signal" if the antenna is aligned carefully. But that isn't the same
as a reduction in the sky noise power.

Slainte,

Jim

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Are sat dishes too small?
 

In article , Java Jive
wrote:
Eh?

As a physicist by training, I'm not aware of anything known as
'geometric optics' or 'wave optics', nor therefore of any difference
between them. Optics is a simply particular branch of the study of
waves, that which deals with electro-magnetic waves at or near the
visible part of the spectrum, particularly with regard to refraction and
reflection. All wave study involves both geometry and, er, waves!

Yes and no, m'lud. :-)

As a "physicist" I'm well used to 'geometric optics', 'wave optics', and
all kinds of sub methods/models like 'geometric theory of diffraction'
optics (GTD) and 'Gaussian Beam Mode optics'. People use these terms to
indicate the kind of modelling they are using to estimate what will happen.

Fortunately, light doesn't have to stop and solve Maxwell's Equations to
work out how it can get in though the window, though. 8-] ...or if it
does, it must be cleverer than any students I've taught to solve the
equations so quickly. ;-

Slainte,

Jim

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Are sat dishes too small?
 

In article ,
Bill Wright wrote:
Richard Tobin wrote:

Other aspects of optics, such as diffraction, and the correct
explanation of refraction, depend on light being a wave.

It seems reasonable to label these two aspects geometric and wave
optics.

This being the well known conundrum of wave/particle duality.

Its perhaps an example of what Feynman described when he said you can know
all the names for a bird in all the languages, but still not really know
anything about the actual bird. We can use 'wave' or 'partical' ideas to
help up describe aspects of behaviour, but that doesn't necessarily tell us
what light 'is'. Despite that, the models can be very useful provided you
apply them in appropiate circumstances.

Slainte,

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

Are sat dishes too small?
 

On 07/06/12 20:05, David Woolley wrote:
2BSur2Bsur wrote:


Think about telescopes/binoculars/magnifying glasses. The bigger the lens
the more you are able to "zoom" and pick up more detail from your intended
target, but the more accurate you have to aim.


The analogy is flawed. The main reason that lens sizes increase with
magnification is to keep the exit beam as large as the eye's pupil. The same
effect as with radio dishes does occur, but it is related to blurring of fine
detail, rather than magnification.

The telescope lens scaling issue is a geometric optics one, whereas the
antenna on is a wave optics one.

They are all electromagnetic waves, Daves....

Are sat dishes too small?
 

On 07/06/12 23:51, Bill Wright wrote:
Richard Tobin wrote:

Other aspects of optics, such as diffraction, and the correct
explanation of refraction, depend on light being a wave.

It seems reasonable to label these two aspects geometric and wave
optics.

This being the well known conundrum of wave/particle duality.


Where's my tea. And my infinite improbability drive.