[time-nuts] Sun Outage

W4wj at aol.com W4wj at aol.com
Thu Oct 9 21:44:51 EDT 2014


Hi Brooke...
 
You are correct. My semantics were  confusing!
 
The offset feed certainly has an advantage because of no  shadowing,
but a lot of commercial Ku-Band antennas are complete  parabolic
reflectors with a sub-reflector and cassegrain  feed.
 
There obviously is some loss because of the sub reflector, but  these are 
larger antennas and the loss is  acceptable. 
 
TNX for you feedback Brooke!
 
 
73
Don
W4WJ
 
 
 
 
 
In a message dated 10/9/2014 7:26:55 P.M. Central Daylight Time,  
brooke at pacific.net writes:

Hi  Don:

It's my understanding that all satellite dishes have a parabolic  curve 
which focuses the signal on the feed.
The C-band dish has a round  outline and the feed is located along the dish 
center line.
Most commercial  Ku-band antennas have a parabolic curve, but have a 
elliptical or orange peal  outline.  These are off 
center fed so that the feed does not shadow  the antenna like it did on the 
C-band dishes.  This is the same problem  that 
the vast majority of reflecting astronomical telescopes have, i.e.  the 
secondary mirror area needs to be subtracted from 
the primary mirror  area to get the effective primary mirror area.

A very practical result  of that difference is that a C-band dish has it's 
main beam along the dish  center line, but a 
Ku-band dish does  not.

http://www.prc68.com/I/Images/SB_angw.jpg  - showing the beam  realtive to 
the dish and beam hitting gutter.
Better when dish mounted on  roof:
http://www.prc68.com/I/SBvsat.shtml
But the construction of the  older dish was better than the newer/cheaper 
dish.

The Free To Air  (FTA) Ku-band dishes also have a parabolic curve & round 
outline, but they  are offset fed, see:
http://www.prc68.com/I/FTA.shtml

Have  Fun,

Brooke Clarke,  N6GCE
http://www.PRC68.com
http://www.end2partygovernment.com/2012Issues.html
http://www.prc68.com/I/DietNutrition.html

Don  Murray via time-nuts wrote:
> Hello all...
>   
>  Not all satellite TV antennas are parabolic.  A  typical C-Band  antenna 
is
> parabolic and aligned for one satellite.  But, that  could  change if the
> feed was modified to receive  multi-satellites, while the  shape of the
> reflector remained  parabolic.  Or the antenna could be an  off-center
> fed  elliptical version.
>   
> Satellite antennas for Dish  and DirecTV are not  parabolic, but they are
> off-center fed and  either circular or  elliptical. The elliptical version
> usually  supports a feed that will cover multiple  satellites.
>    
> C-Band satellites in the U.S. Domestic arc are normally   spaced
> two degrees apart, with some at 4 degrees   spacing.
>   
> DBS (Direct Broadcast Service) i.e. Dish  and DirecTV,  satellites
> are spaced 9 degrees apart.   Clusters of satellites can  be parked
> at one location to supply  additional capacity for spot beam  coverage.
> DBS service is  located in the Ku-Band.
>   
> More info  at:
>   
>  
https://docs.google.com/presentation/d/1eNMYmcNIxRFpK1PY0GqbvOfvNfzRra4fHxs8
>  A4hSy7o/preview#slide=id.p18
>   
>   
>  73
> Don
> W4WJ
>   
>   
>  In a message dated 10/9/2014 4:17:20 P.M. Central Daylight Time,
>  andrew at cleverdomain.org writes:
>
> You pick  up satellite TV  with a parabolic dish that points at one spot
> in the sky  where  the geostationary satellite lives. A sun outage
> happens when the  sun  wanders into the focus and overloads the receiver
> with noise  that drowns  out the satellite signal (at least, it raises
> the  noise floor enough that  you can't receive the high bitrates  needed
> for a TV picture).
>
> You  pick up GPS with a  whole-sky antenna that receives signals from
> the   constantly-moving swarm of GPS satellites. It undoubtedly receives
>  some  noise from the sun, but the only factor in how much of that  you
> get is the  sun's elevation above the horizon. It's not  really relevant
> whether the sun  is "aligned with a satellite" or  not. Even if it was,
> the satellite would  be somewhere else a  minute later. :)
>
> Andrew
>
> On Thu, Oct 9,  2014  at 1:40 PM, Bob Stewart <bob at evoria.net> wrote:
>>  Two days this  week, there was a 3 or 4 minute outage on DirecTV as  the
> sun aligned with the  satellite and my dish.  So I was  wondering what 
kind of
> effect this has  on the GPS system and  especially timing receivers.
>>
>> Bob  -  AE6RV
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