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Author Topic: Why is my Satellite Dish full of holes?  (Read 7722 times)

Offline turnipsock

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Why is my Satellite Dish full of holes?
« on: 29/07/2008 00:03:35 »


See this, nice eh? Being delivered next Tuesday [/neilep]

The satellite dish, that my house is currently connected to, is full of holes. Wouldn't it be easier to make it smaller and fill in the holes?

Could I supe it up my current dish by covering it with some of my mums used tinfoil collection?


 

Offline chris

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Why is my Satellite Dish full of holes?
« Reply #1 on: 29/07/2008 00:18:30 »
Nice observation.

The reason that your dish is full of holes is for purposes of weight and reduced wind-resistance (aka likelihood of being blown away).

The dish works fine as a reflector, despite being full of holes, because it is picking up long-wavelength radiowaves / microwaves. Because these are long wavelengths they are easily reflected by the mesh of the dish because, put simply, the waves cannot fit through the grille, so what they see is effectively a smooth flat parabolic surface which focuses the incident waves onto a point where the detector is positioned. In this way by gathering lots of waves over a large area and focusing them to a point a much larger signal can be achieved where the receiver is located, producing a nice picture on your telly. That's why size is important when it come to satellite dishes. The bigger your dish the more radio waves you can collect and focus on your receiver, so the more powerful your dish and the weaker the sources can be that you are picking up.

You can see the same "holey" trick at work elsewhere in the home - by looking at the door of the microwave oven. The glass is covered by mesh. You can see light coming from inside the oven when it's on, but you can't pick up any microwaves outside the oven? Why, because the gauge of the grille on the glass is sufficiently large to let through visible light wavelengths (of say 600nm), but far too small to let through the longer wavelength microwaves (12cm).

Chris
« Last Edit: 29/07/2008 00:20:47 by chris »
 

Offline that mad man

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Why is my Satellite Dish full of holes?
« Reply #2 on: 29/07/2008 01:03:22 »
Rust?
 

Offline Karen W.

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Why is my Satellite Dish full of holes?
« Reply #3 on: 29/07/2008 01:46:23 »
I always wondered that myself about my Satellite dish! So that is interesting about the grid size as far as letting in and keeping out light wavelengths.. both keeping you safe as well as cooking your food... hummmm!
 

lyner

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Why is my Satellite Dish full of holes?
« Reply #4 on: 29/07/2008 10:37:54 »
Talking of holes - a single dipole antenna has a really small physical cross section but, around the frequency range it operates, it has an effective cross section in the order of a wavelength so it intercepts a lot of energy. Appearances can be deceptive.
 

Offline Karen W.

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Why is my Satellite Dish full of holes?
« Reply #5 on: 30/07/2008 05:46:57 »
What do you mean?? I don't understand...???
 

lyner

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Why is my Satellite Dish full of holes?
« Reply #6 on: 30/07/2008 10:17:03 »
Karen.
A radio antenna takes energy from the passing radio wave. That's what the radio receiver amplifies and detects.
The transmitter has a certain power and the receiver gets a certain fraction of that power (tiny fractions of a microWatt, of course). The transmitter power is spread over a hemisphere (roughly) so you can compare the transmitted and received powers and relate this to an effective collecting area for your receiving antenna as a fraction of the total area of the large hemisphere. This effective area is much greater than the area of a thin piece of wire.

There are various theories about how this actually works. The em fields which are induced around the wire, somehow cause the power to be directed in towards the wire. The Yagi type antennas - with a row of extra elements bolted to the boom (ordinary TV antennae) have an even bigger effective collecting area - or Gain - in the direction of their main beam.
 

Offline Karen W.

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Why is my Satellite Dish full of holes?
« Reply #7 on: 31/07/2008 18:59:26 »
Oh That is interesting.. Thanks for explaining to me... I have some difficulty understanding things sometimes... That helped... Thank you!
 

Offline Make it Lady

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Why is my Satellite Dish full of holes?
« Reply #8 on: 31/07/2008 21:26:41 »
I used to live next door to Jodrell Bank and didn't notice any holes.
 

lyner

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Why is my Satellite Dish full of holes?
« Reply #9 on: 31/07/2008 23:14:36 »
How close did you get?
As long as the holes are a bit less than a quarter wavelength of the minimum wavelength the  reflector is needed for.
I imagine that, then the Dish was built, they had no idea of a minimum wavelength it could be used for. It was a very expensive construction - money no object (not quite but you know what I mean).
The 1cm (ish) wavelength used for DBS was part of the design criteria and so was low cost - ergo, some of them are made of mesh of the appropriate size.
 

Offline Pumblechook

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Why is my Satellite Dish full of holes?
« Reply #10 on: 07/08/2008 11:41:17 »
A dish could be made of square mesh for low wind loading and as long as the size of the squares of the mesh is less than 1/4-ish of a wavelength at the lowest working frequency (7 mm) then there will be no advantage in signal terms of using a solid refelector.   The dish shape has to be within 1/10 of wavelength of a perfect parabola othwise gain (the signal) will be reduced.
 

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Why is my Satellite Dish full of holes?
« Reply #10 on: 07/08/2008 11:41:17 »

 

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