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RE: Dish Question - 2 cents from me -



Jon,
Just to add my 2 cents: Remember this; if it is not plugged in, there is
NO NEW gain. You only get new gain from consuming energy. A non active
device like an antenna can not make, or create, any gain. ALL gain
provided by a non active antenna is purely gathered from the air in
front of it. A big dish has more gain because it focuses the energy
found in front of it into a focal point where it accumulates in larger
quantities because of the larger size of the dish. That focusing of the
energy is stated as "GAIN". It is like the sun flowing through a
magnifying glass. The bigger the glass the faster you can cook the
hotdog. The magnifying glass is providing "GAIN" to the sun waves when
compared to a flat piece of glass. Move off of the focal point of the
magnifying glass, and the hot dog will cook more slowly or not at all.
73
Gunther
W8GSM


-----Original Message-----
From: owner-AMSAT-BB@AMSAT.Org [mailto:owner-AMSAT-BB@AMSAT.Org] On
Behalf Of Jon Ogden
Sent: Wednesday, May 14, 2003 1:01 AM
To: MERRETT, David; amsat-bb@AMSAT.Org
Subject: Re: [amsat-bb] Dish Question


on 5/13/03 6:48 PM, MERRETT, David wrote:

> I believe the beamwidth is a function of the physical arrangment of 
> the feed AND the dish, and if you 'de-focus' the position of the feed 
> (EG push it further 'in'), then the beamwidth should get bigger... 
> shouldnt it? As for how this affects gain - As long as your feed can 
> still collect all the power over the entire beamwidth, then the only 
> loss of gain (wrt to perfect focus) will be due to the 
> out-of-phasness.

Gain and beamwidth in any antenna are inversely proportional.  The more
gain, the tighter the beamwidth.  Moving outside of the focal point will
only cause your signal to scatter.  Yes, your beam will be wider but the
gain will certainly drop.  Your pattern will not be as directional.

> Maybe someone has studied in detail how much overall gain you can get 
> as you increase dish-size AND KEEP constant beamwidth.

Physically impossible.  Again, gain and beamwidth are inversely
proportional.  You increase gain by narrowing your beamwidth.

A perfect point source is an antenna with absolutely no gain.  This is
the isotropic radiator where we get our dBi numbers from.  Power is
emitted from the source equally in all directions.  Now if take an
antenna with gain, it means we are radiating a signal in less directions
than the point source. We've narrowed our beamwidth from a 360 degree
circular pattern to something else.  As we take our RF energy and
concentrate it in a pattern of narrower and narrower beamwidth, we get
increased gain.

A dish creates it's gain by focusing the reflected energy on the
feedpoint. This is why a larger dish has more gain because it has more
capture area
(ie: it ends up reflecting more of the received energy into the
feedpoint or vice versa from transmitting).  So the larger the dish the
larger the gain, the narrower the pattern (you get less sidelobes, etc).
And the more you increase frequency the more gain you have as well for a
given dish size. Since capture are is related to frequency.

73,

Jon
NA9D
-------------------------------------
Jon Ogden
NA9D (ex: KE9NA)

Citizen of the People's Democratic Republik of Illinois

Life Member: ARRL, NRA
Member:  AMSAT, DXCC

http://www.qsl.net/na9d   <- Updated on 1/22/03!!!

"A life lived in fear is a life half lived."


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