Path: utzoo!utgpu!water!watmath!clyde!cbosgd!mandrill!neoucom!wtm
From: wtm@neoucom.UUCP (Bill Mayhew)
Newsgroups: comp.sys.amiga
Subject: Re: dish formula
Summary: Satellite TV earthstation dishes
Message-ID: <981@neoucom.UUCP>
Date: 28 Jan 88 04:26:09 GMT
References: <2252@gryphon.CTS.COM> <5060@well.UUCP>
Organization: Northeastern Ohio Universities College of Medicine
Lines: 61


You can really make any sort of parabolic dish that you'd like.
The trick of the parabola is that any energy entering parallel to
the axis of the dish will be reflected to the focus of the dish.
Any energy that enters not parallel to the dish's axis will be
reflected someplace else.

The radiation from a transmitter several thousands of miles away
looks pretty much like a coherent wavefront -- for practical
purposes.  I know, if you wanted to be picky...  So the upshot of
this is that an antenna positioned at the focus of a parabolic dish
can only see a distant transmitter only when the dish's axis is
pointed right at that transmitter.  The accuracy of placing the
receiving antenna at the dish's focus is crucial to the selectivity
and sensitivity of the dish.

Some pragmatic aspects do govern dish size and shape.  Typical
earth based dishes have focus/diameter ratios around 0.3.  For
instance my 8 foot diameter TV dish has a 36 inch focal length (as
measured along its axis.  This puts the receiving antenna element
just about at the lip of the dish reflector.  A benefit of using a
deep dish is that the metal of the dish reflector tends to block
microwaves (terrestrial telephone for instance) that enter at an
angle 90 degrees to the dish's axis.

A shallow dish is easier to make than a deep dish.  A shallow dish
tends to have more susceptibility to off-axis interferrence.  A
shallow dish has a greater effective aperature for a given amount
of metalwork.  As you can see there are tade-offs of ease of
assembly versus sensitivity and interferrence rejection.

You can guestimate the effective field strengh from a distant
transmitter using standard propagation formulae from an
electromagnetic fields book.*  Knowing the field strength at your
locale, you can then figure out the watts/square-meter.  Knowing
dB power input sensitivity of your receiver you can convert to
power in watts watts needed and then you'll know the required
surface area of your dish.

*My fields book is at work; I know I'd get clobbered if I goofed
the formula, so I suggest looking it up at your local library.

It is amazing that an eight foot dish is sufficient for terrestrial
reception, considering that most satellite transponders put out
about 5-10 watts.  The satellites are in orbit approximately 23000
miles above the equator.  The path to us in North America is more
than 24000 miles from the satellites.

Space based receiving dishes can be somewhat smaller than earth dishes
since ground transmitter powers are typically several hundred watts, and
very large (10-20 meter) ground transmitter dishes are used.
Shallower dishes can be used in space too becase there is less
likelyhood of illumination from off-axis sources.

Dish size is not necessarily related to the frequcency being
transmitted unless the frequency has a wavelenght close to the size
of the dish.  It is true that the atmosphere does tend to more
readily absorb higher frequency microwaves, thus a bigger dish is
required for a given path when higher frequencies are used.

--Bill