Are sat dishes too small?
 

In article , 2BSur2Bsur
wrote:
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.

A more recent duality which may have an impact here is that what we used to
call 'fairy cake' might now be called 'cup cake'. Can the drive tell the
difference?...

Slainte,

Jim

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Are sat dishes too small?
 

Jim Lesurf wrote:
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. ;-

At issue here is the extent to which geometrical optics can be applied
to an offset satellite dish and define the position of, say, the focal
point, when the wavelength of the satellite signal of around 3 cm is
significant in proportion to the size of the dish and the LNB. A while
back, we had a nice little discussion here which endeavored to calculate
the offset angle of a dish, assuming that the dish was a true paraboloid
and using standard geometry. I argued that the offset angle was a
property of the dish itself, and that the correct position of the LNB
could be calculated from just three dimensions - the width and height of
the dish and the maximum depth - assuming that the axis of rotation of
the paraboloid passed through the lower rim of the dish.

Since then, I have realized that the calculation can be done without
making any assumptions at all, since it is possible to use the three
dimensions to work out where the axis intersects the dish. For the
dishes I measured this proved to be about 15 mm from the lower edge.
It was all a bit theoretical, and in practice the placing of the LNB at
the exact (geometrical) focal point doesn't seem to make an enormous
difference to the efficiency of the average sized dish.

Are sat dishes too small?
 

On Fri, 08 Jun 2012 09:11:39 +0100, Jim Lesurf
wrote:

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. :-)

That sound ominous!

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.

Well, I have to bow to your superior knowledge and experience that the
terms "geometric optics" and "wave optics" actually do exist.

In fact I've found the former in Feyman, but used merely to
distinguish what we would simply have called "optics" from Fermat's
Principle Of Least Time, which we would also have called "optics"!

The latter term doesn't appear anywhere in Feynman, but I suspect it
just simply means what we would have called rather more accurately
"Wave Theory", the point being, for the benefit of others, but not,
I'm sure, yourself, that Wave Theory is applicable to a wide range of
wave phenomena, of which light is merely one. Examples of others
would be sound, waves on the ocean, how earthquake shock waves travel
through the earth, tsunamis, tidal waves, etc.

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. ;-

Heh! It would be bloody dark otherwise!
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Are sat dishes too small?
 

On Thu, 7 Jun 2012 22:11:46 +0000 (UTC),
(Richard Tobin) wrote:

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 :-)

Well, not really, because quantum theory doesn't really explain what
happens at levels above the atomic, we still need Classical Physics
and Relativity for that, and note that one of the fundamentals of
quantum theory is actually the Wave Equation.

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.

But that's just "Optics"! So, for example, I have on my shelf here a
book entitled "Fundamentals Of Optics", Jenkins & White.

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

And that's just "Wave Theory"! Hence another book entitled "Wave
Phenomena", Towne.

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

To call the latter "Wave Optics" is surely to miss the point that the
theory is applicable to many other types of phenomenon? And if you
use the more accurate term "Wave Theory", then the need for the
distinguishing word "geometric" also disappears.

It may not be wrong - I'd be interested to hear what Jim thinks
"Wave Optics" means - but I suspect that in the context that was
intended it's very sloppy terminology.
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Are sat dishes too small?
 

On Friday, June 8th, 2012, at 13:58:05h +0100, Jim Lesurf wrote:

A more recent duality which may have an impact here is that what
we used to call 'fairy cake' might now be called 'cup cake'.

Because marketing fairy cakes is an advertising agencies
worst nightmare!

Are sat dishes too small?
 

On Fri, 08 Jun 2012 16:02:44 +0000, J G Miller wrote:

Because marketing fairy cakes is an advertising agencies
worst nightmare!

Speeling alert! s/ies/y's/

Take II

Because marketing fairy cakes would be the worst nightmare
of any advertising agency.

Are sat dishes too small?
 

In article , Java Jive
wrote:
On Fri, 08 Jun 2012 09:11:39 +0100, Jim Lesurf
wrote:

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. :-)

That sound ominous!

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.

Well, I have to bow to your superior knowledge and experience that the
terms "geometric optics" and "wave optics" actually do exist.

They only exist as general groups of types of approaches/theories/models.
Hence my humorous sic first comment. :-)

When people I've worked with on "optical" systems (normally in the range
from IR down to microwave) used such terms they were talking about
approaches, but would then get into more specific methods. e.g. being GTD
(which comes in more than one flavour). Or 'Gaussian' approachs - which can
start from a 'wave diffraction' approach, or from complex ray theory (more
like 'geometric' but using complex numbers for lengths, etc) or even my own
favourite - Gaussian Beam Mode - which can use a mix of the geometric and
diffraction methods. (Hence my liking for it, since it pick-and-mixes for
situations where you are in situation with lenses, mirrors, that are just a
few wavelengths across, but big enough to allow for quasi collimated beams
to be possible.)


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. ;-

Heh! It would be bloody dark otherwise!

Night would last longer for some students than others. Come to think of it,
maybe that is why some were late for lectures... ;-

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?
 

2BSur2Bsur wrote:
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....

And or particals

Steve Terry
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Are sat dishes too small?
 

Allan wrote:
"Brian Gaff" wrote in message
...
snip
Most people who, like myself, spend much of their time living on a
canal boat use a 12" dish because it's beam width is slightly greater
and so the signal doesn't keep being interrupted when the boat moves
slightly. There isn't ever a problem with signal strength (unless
we're moored behind a tree!)

You must have a hot LNB!

Steve Terry
--
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Are sat dishes too small?
 

Geoff Pearson wrote:
"Bill Wright" wrote in message
...
R. Mark Clayton wrote:
"Bill Wright" wrote in message
...
R. Mark Clayton wrote:
snip
In Scotland:
2 antennas maximum (fronting and non-fronting (these terms are not
specifically used in the Order))
1 antenna: up to 100cm and 35 litres cubic capacity.
Other antennas: up to 60cm and 35 litres cubic capacity.
Chimney-mounted antenna: up to 60cm and 35 litres cubic capacity

I love the idea of a dish having cubic capacity.

I'll have to try that on the staff in Maplins
"Have you a 35 litre dish"?

Steve Terry
--
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