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Re: Question regarding gain of Dish Antennas with different feeds

>From: Al Lawler <alawler@us.oracle.com>
>  John,
>  I think that just figures into the 'efficiency' of the
>  antenna.  Assume that the perfet dish of any given size 
>  is is capable of "n" db of gain.
>  The quality of the feed (I.e. how completely you illuminate
>  the dish without overilluminating it)   determines how close 
>  to the maximum efficiency you actually get.  (A higher gain 
>  helix would illuminate less dish area, resulting in a smaller 
>  overall effective antenna area, so it's essentially a zero-sum
>  game.)
>  -al
>John Wilcox wrote:
>> This is a question I had, as well. I am using a helical for the feed of a
>> Primestar 1.0m dish. The 5 turn helix has 11(?) dB gain. Is this added to
>> the 25 dBi gain of the dish? There must be some addittive value over just
>> putting a dipole at the focus...
>> > Question: Will the Gain of the patch antenna increase the overall gain of
>> the
>> > dish

Al gave the answer, but in case you didn't quite get it, I'll give the
plain answer.
The gain of the feed is not added to the gain of the dish...period.

The purpose of the feed is to "light-up" the dish...in other words provide
RF energy over the total surface of the dish.  If you could do this
uniformly then you would get the maximum gain of the dish
[efficiency=100%].  The gain of a dish is relative to the size of the dish,
and how well you illuminate it.

But feeds are not able to reach 100% in reality, so you get less than 100%
efficiency [which means you get less than 100% of the theoretical maximum
gain which is given by G = eff*4*pi*Area].  Most feeds will result in no
more than 55% efficiency.  That occurs when the edge illumination is about
-10 dB from the center of the dish.  

But for better listening > -15 dB is desired.  You will have slightly less
gain [maybe 45% efficiency], but that is more than offset by a lower
antenna temperature.  Antenna temperature is mostly established by the
level of the dish sidelobes.  If the feed beamwidth is too wide it looks
beyond the edge of the dish at the ground which is thermally very hot.
That adds lots of noise to your receiver and masks the weak signal you want
to receive.

So the only thing to care about regarding the feed is its beamwidth [at -10
dB or even better -15 dB from the maximum].  Be aware that normal beamwidth
formulas only give the width of the beam at -3 dB.


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