Let's say you had three vehicles:
(18" dish): small high-powered sports car with two seats.
(36" dish): large American sedan, eg Ford Crown Victoria
(8-12' BUD): large American pickup with towing package, maybe diesel
Each is suitable for a specific task.
You pick the one to do the job you need, be it blowing down the open highway, hauling a family to a wedding, or pulling your boat.
Back to satellite dishes:
They operate in two bands,
- Ku: around 12ghz (high frequency/short wave length)
- C: and 4ghz (lower frequency/longer wave length).
(18" dish): the very high powered satellites are about 9° apart in the sky.
So, this size will get the strong signal, and discriminate between birds 9° apart.
They are at the high end of the Ku band so don't interfere with the next group.
(36" dish): These lower-powered satellites are only 2° apart.
You may get sufficient signal for a -few- channels on smaller dishes, but 36" is recommended for everything.
To pick out the close satellites, the larger dish focuses on a smaller window in the sky.
(8-12' BUD): These satellites are C-band, around 4ghz.
They have varying power levels, so some can be received on smaller dishes.
Because the wavelength is longer, you need a larger dish to focus on just one bird in the sky and discriminate against the satellite next door.
This becomes more important when using advanced data transmission methods.
The clean signal is necessary to be able to recover all the data being broadcast.
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