[amsat-bb] Re: DirectTV Dish and 1.2 Patch

["Oppen, John D" <john.d.oppen@xxxxxxxxxx>]

Angelo,
The size of a Direct TV dish (probably 18-24") is too small to be
effective at 1.2 GHz.  An 18" dish is only two wavelengths in diameter
and the 24 is only 2.5.  Dishes should be more like 8 wavelengths in
diameter to be truly effective, although in typical ham fashion we saw
some usable examples of these dishes working on 2.4 GHz where they're
3.6 wavelengths in diameter.

A couple years ago AL7EB posted a very detailed note on this subject and
compared a dish working at too low a frequency as working more like a
dented pizza pan than a parabolic dish.  He also concluded that even if
it worked like a parabolic dish, the diameter would be too small to have
much gain.  Using an optimistic estimate of 50% efficiency an 18" dish
only makes 12.9 dBi gain at 1.2 GHz.  You could do better with a helix
or one of Kent's cheap yagis.  

John, KJ6HZ


____________________________________________________
   H Folks,

   I am looking for some advise when it come to a 1.2G antenna. I have a
   extra DirectTV dish

   around the house and was wondering how good this type of dish would
   work for an uplink for AO-51 with a 1.2 G Patch???

    I am a newbie when it come to dish construction and patch
   construction and what would work good with a 1.2 G Patch. Is this
dish
   to smaill???

   73 de Angelo
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