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RE: Dish Question



You only get maximum gain if the feed is at the focus and properly
illumionating the dish.  In this case, the gain is proportional to dish
area and the beamwidth is inversly proportional.

For a poor arrangement, yest the beamwidth will suffer, but by the SAME
amount you will have fringing, so yes, the beam will be wider BUT it will
have HOLES in the pattern.  That is why a flashlight with poor focus does
not have as bright a spot, but the resulting broader spot has black places
in it..   Bob


 On Wed, 14 May 2003, 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.
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> Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
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de WB4APR@amsat.org, Bob

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