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Re: 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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