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RE: Dish Question



Im not convinced actual beamwidth is proportional to dish-size. (although
MINIMUM beamwidth is)
(Im only guessing here)
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. 

(imagining shining a torch into a ~parabolic saucepan lid - move it in and
out, the beamwidth changes accordingly).

Maybe someone has studied in detail how much overall gain you can get as you
increase dish-size AND KEEP constant beamwidth.
Dave - VK5DSM


-----Original Message-----
From: Rick Fletcher, KG6IAL [mailto:rick@teksharp.com]
Sent: Tuesday, 13 May 2003 07:27
To: Jon Ogden; George Humphrey; amsat-bb@AMSAT.Org
Subject: Re: [amsat-bb] Dish Question


As far as the downlink is concerned, you're absolutely correct; however,
there is a good reason to run a bigger dish and that's so that you can also
use it as an L-band uplink to AO-40.  With a 6 footer you can even use it
for U-band.  I'm currently running a 4 ft. (1.2m) dish with a dual-band
circular patch feed for L up and S down.  I'll soon be migrating to a 6 ft.
dish which will include a triband patch feed that adds U-band support.  It's
beamwidth is still within the accuracy of a Yaesu 5400/5500 and because it
consists of a lightweight aluminum dish frame, covered with 1/4" hardware
cloth, it is very lightweight and offers almost no windload.

73,

Rick - KG6IAL
www.teksharp.com

----- Original Message ----- 
From: "Jon Ogden" <na9d-2@speakeasy.net>
To: "George Humphrey" <kc5wbv@arrl.net>; <amsat-bb@AMSAT.Org>
Sent: Sunday, May 11, 2003 8:25 PM
Subject: Re: [amsat-bb] Dish Question


> on 5/9/03 9:53 PM, George Humphrey wrote:
>
> > This is probably a really stupid question, but does dish size have any
> > relationship to frequency, or is it just "bigger is better"?
>
> George,
>
> Ed had a great reply and I'd agree.
>
> Bigger is better, but for working satellites you can get too big.  The
> bigger the dish, the narrower the beamwidth.  This means your pointing
> accuracy has to be better.  For AO-40, if you have a dish big enough to
hear
> the noise floor of the transponder, having an even bigger dish won't help
> all that much except for marginal conditions (when the bird is at bad
> squint, etc).  The typical size that is recommended for AO-40 is a 3 foot
> dish for S-band.  However, there are people that successfully use smaller
> dishes and bigger dishes.
>
> Now, on AO-40 for K band, many people use an 18" dish.
>
> As frequency goes up the gain for a given size of dish moves up as well.
So
> size does relate to frequency and bigger is generally better.
>
> 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."
>
>
> ----
> Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
> Not an AMSAT member? Join now to support the amateur satellite program!
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>
----
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