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Re: AO-40 transp. noise: hw big the dish ?



Claus Waldraff wrote:

> Hi board,
>
> Hope that's not to often discussed
>
> just want to be sure in recommending new A0-40 users to go for max 
> rec. capabilities and best being able hearing transp. noise.
> What I know you need is at least a 3ft. dish. Having a 90 cm/1 m solid 
> dish and 0.6 dB NF is ok ??
> Trading NF for diameter or vice versa ?
> This seems quite doable for me for many people, at least much easier 
> than 6 or 10 footer or even bigger monsters.
>
> Has anyone other recommendations ??
>
> Claus
>
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>
A larger dish will allow communications under more adverse conditions, 
such as low to the horizon, intervening vegetation, and higher squint 
angles. A 10 foot dish will have about 10 dB more gain than a 3 foot 
dish. At favorable squint angles in clear air, the 10 foot "BUD" will 
hear the transponder noise floor, and so be overkill to a certain 
extent. As conditions deteriorate (higher squint angles, trees, rain, 
etc) the larger dishes will hold on to a usable signal longer than the 
smaller ones will.

For a starter system, a 3 foot dish with a MMDS grade downconverter, 
such as an AIDC 3731 with a helical feed is a very common system (in 
fact it is what I have currently), and I typically can hear SSB signals 
about 4 S units above the noise under favorable conditions. A high 
quality downconverter, such as a DEM or the like will give maybe another 
1/2  S unit above the noise. A big dish can easily beat the disadvantage 
of a somewhat deafer downconverter, but a big dish is going to require 
much more in the way of steering hardware than a 3 footer.

A 3 footer and if you are careful, even a 4 foot dish can be set up on a 
rotor system such as a Yaesu G-5400B, and for limited uses even manual 
pointing.  A smaller dish is also less critical about offpointing 
errors, whereas a big dish needs to be accurate within a couple of 
degrees.  Whether it is worth it or not depends on your savvy, existing 
space, and budget. Unless you already have a big dish, I would start out 
with a 1 meter class dish, with an MMDS downconverter. Under reasonable 
squint angles and clear skies, this will be a quite usable system. That 
being said, if you happen to have one of those 10 footers in your back 
yard already or can put one there cheaply,  here is a good link to 
follow:   http://www.ultimatecharger.com/dish.html

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