Re: [amsat-bb] Offset Dish Antenna Aiming

["Edward R. Cole" <al7eb@xxxxxxxxxxxxx>]

>
> Even thought it looks bizarre, many dishes can be turned upside down (feed 
> arm at the top) to get them to operate down to 0 degrees. 
> ***snip**
> Fred W0FMS 


Hi gang,

Yes, you may see some unconventional mounting of offset dishes (upside-down or
sideways), but you may not want to do this because you are defeating one of
the
main advantages that offset feed dishes provide:  the feed looks toward the
sky, not the ground.  So any over-illumination (spill-over of the pattern)
comes from the "cold" sky.  If you invert the dish then the feed can be
subject
to picking up ground noise thus spoiling your noise figure.

Remember (not you Fred; I know you know this) that overall system noise is
determined by equivalent noise temperature:
Tsys = Tr + Tsky + Tgnd

where:  
Tr is the total receiving noise temp (about 80-100K for us with NF of 1.0 dB)
Tsky is the background temp for the sky at 2.4G (on avg about 10K)
Tgnd is the thermal ground pickup by the antenna from over illumination (big
sidelobes)

If feed illumination is 10 dB down at the dish edge then looking beyond the
edge at the ground (which is 290K) it picks up 29K.  If the feed looks at the
sky which is 10k then it picks up 1K.  {actually this is very simplistic
way to
look at this...the correct way is to perform a surface integration over all
4pi
radians around the antenna...but this is ham radio, not a doctoral thesis}

the difference in your Tsys is:
80+10+29= 119K vs 
80+10+1 = 91K

I know calling it Tgnd is a bit confusing.  Really we should call it
Tant-pickup.

OK so it will work inverted or sideways or with a patch or other feed that
over-illuminates...but just not optimally!

Ed, AL7EB






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