Are sat dishes too small?
 

On Wed, 6 Jun 2012 09:16:07 +0100, Geoff Pearson wrote:

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 teh idea of a dish having cubic capacity.

Should have been given in micro-olympicswimmingpool units - or possibly use
the Wok as a unit.
--
Peter.
The gods will stay away
whilst religions hold sway

Are sat dishes too small?
 

In article , David Woolley
wrote:
Jim Lesurf wrote:

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.

I'd agree with you. If there were no temperature variations, you would
expect the amount of noise to be independent of the dish diameter.

Indeed. Although there may be other effects like the extent to which the
feed sees itself, I assume. What puzzled me was the sweeping statement
about "less beamwidth means less sky noise", though.

A smaller dish may see more warm buildings than a large one. Warm is
relative; the sky is very cold.

Although that is true, I wonder if the feed sees rather more 'warm
ground/buildings' due to the feed's field-of-view not being filled by the
dish. Hence my question about illumination factor. FWIW I'm more used to
systems like Cassegrain, though, where the overspill is also mainly onto
the sky with most microwave designs I've known.

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?
 

Geoff Pearson wrote:

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 teh idea of a dish having cubic capacity.

This is measured in whisky of course. It's also important to remember
that that a dish can exceed the sizes given above by 50% if it is
painted using tartan paint.

Bill

Are sat dishes too small?
 

Jim Lesurf wrote:
In article , 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...
Got a bad throat Jim? That 'erm' that some people use, it means, "You're
talking ********." It's like, "With all due respect." Don't do it. It's
had its day, like ponytails on IT men and dungarees on lezzas.


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.
Less area of sky received. Same area of satellite tx dish visible.
Improved ratio.


Although I can see that reducing the dish overspill may reduce the noise
from what the feed sees around the rim of the dish (house, ground,
whatever). Are you assuming a larger dish will also give a changed
illumination factor?
No, I'm basing my remarks on various publications used in the industry.

Bill

Are sat dishes too small?
 

"PeterC" wrote in message
...
On Wed, 6 Jun 2012 09:16:07 +0100, Geoff Pearson wrote:

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 teh idea of a dish having cubic capacity.

Should have been given in micro-olympicswimmingpool units - or possibly
use
the Wok as a unit.

Can't you use a wok as a dish?

--
Max Demian

Are sat dishes too small?
 

In article , Bill Wright
wrote:
Jim Lesurf wrote:

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)


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.
Less area of sky received. Same area of satellite tx dish visible.
Improved ratio.

Which isn't the same as saying that a " smaller beamwidth means less *sky
noise* ". The actual level of sky noise may not change much when you change
the beam width. Depends on what is in the field(s) of view. The CNR may
well change for reasons that don't require any alteration in "sky noise"
when the "beamwidth" is altered. The most obvious being change of the
signal level.


Although I can see that reducing the dish overspill may reduce the
noise from what the feed sees around the rim of the dish (house,
ground, whatever). Are you assuming a larger dish will also give a
changed illumination factor?
No, I'm basing my remarks on various publications used in the industry.

That opens up the possibility that some "industry" "publications" are
either poorly worded, or that you haven't quoted them accurately.

As someone else has already agreed here, if the 'sky' has a fairly uniform
radiation temperature then changing the "beamwidth" may have little effect
on the "sky noise" level. What may well change is the signal power
collected from the source, of course. And you may get noise that is *not*
from the "sky".

Can you quote/cite one or two parts in the industry publications you have
in mind that I can read on the web? I'd be interested to see where they say
what you are telling us they do. Since textbooks often contain mistakes I
can understand other publications having misleading wordings, etc. So maybe
they are mis-ascribing changes in CNR to changes in "sky noise" when other
reasons are the cause.

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?
 

Jim Lesurf wrote:

That opens up the possibility that some "industry" "publications" are
either poorly worded, or that you haven't quoted them accurately.

As someone else has already agreed here, if the 'sky' has a fairly uniform
radiation temperature then changing the "beamwidth" may have little effect
on the "sky noise" level. What may well change is the signal power
collected from the source, of course. And you may get noise that is *not*
from the "sky".

Can you quote/cite one or two parts in the industry publications you have
in mind that I can read on the web?

Just leaving this aside while I find a minute to have a look through
some CAI publications, what's your view on my statement (might have only
implied it; can't remember) that on a system with a lot of amplification
a better c/n ratio has the (additional) benefit that with the full
bandwidth of noise reduced relative to the signal it should be possible
to obtain higher max signal output from the amps?

Bill

Are sat dishes too small?
 

In article , Bill Wright
wrote:
Jim Lesurf wrote:

That opens up the possibility that some "industry" "publications" are
either poorly worded, or that you haven't quoted them accurately.

As someone else has already agreed here, if the 'sky' has a fairly
uniform radiation temperature then changing the "beamwidth" may have
little effect on the "sky noise" level. What may well change is the
signal power collected from the source, of course. And you may get
noise that is *not* from the "sky".

Can you quote/cite one or two parts in the industry publications you
have in mind that I can read on the web?

Just leaving this aside while I find a minute to have a look through
some CAI publications, what's your view on my statement (might have only
implied it; can't remember) that on a system with a lot of amplification
a better c/n ratio has the (additional) benefit that with the full
bandwidth of noise reduced relative to the signal it should be possible
to obtain higher max signal output from the amps?

The above question is a bit long, and has many clauses, so I hesitate as
I'm not certain I've understood it. Are you asking if high CNR at input
probably means the noise power can be neglected when assessing how much
gain you can apply before the output reaches a given (clipping) level?

If the above is what you mean, then the answer is "yes" in principle,
on the basis that essentially all of the max output power and voltage
should be available for the signal. But you might need a fair CNR for the
effect to be significant. As usual it depends on the details.

If I've misunderstood the question then you'd have to clarify.

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 05/06/12 10:07, Andy Burns wrote:
David Woolley wrote:


Sky zone1 elliptical dishes are considerably smaller than the 1m dishes
usually allowed without planning permission.

Bring back the squarial. Wasn't that just an array of grid antennas? in a
white square box?

Are sat dishes too small?
 

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.

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.