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RE: Dish Question



Mostly...
With a single main lobe, beamwidth and gain are always proportional.
All variations from ideal gain (i.e., uniform amplitude and phase) increase 
the beamwidth, which reduces the gain.
->more amplitude taper increases beamwidth and reduces gain
->phase error across the aperture increases beamwidth and reduces gain
Gain is identical with your ability to collect incident energy.



At 16:48 2003-05-13, MERRETT, David wrote:
>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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> >
>----
>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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>----
>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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Scott Townley NX7U
Gilbert, AZ  DM43di
http://members.cox.net/nx7u

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