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RE: Dish Question



David,

To add to what Bob wrote:

Antenna gain is directly related to beamwidth...as the beamwidth narrows,
gain increases.  Gain is defined as a measure of the directivity of the
antenna (whether it is a dish or any other type of antenna).  Directivity
is how much you get radiation going in one direction and not in all the
other directions.  Gain is usually defined in comparison to either a dipole
or a theoretical antenna (one that radiates in all directions equally
called an isotropic antenna)...thus you have gain in dBd or dBi).  Zero dBd
= 2.1 dBi.  {a bare light bulb is pretty close to an isotropic radiator; a
flashlite is an example of a dish).

Now the effective use of a dish is affected by the feed antenna:  How
accurately it is placed at the focal point and how well it illuminates
(shines on) the dish.  Regarding positioning the feed somewhere other than
the focal point, Bob has done a good job of explaining.

If you place the feed properly at the focal point, increasing dish size
will cause the gain to increase (and beamwidth to narrow).  There is no
advantage in constructing it any other way.

Ed - AL7EB

At 09:18 AM 5/14/2003 +0930, 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."
>>
>>
>> ----
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