[time-nuts] Sun Outage

Brooke Clarke brooke at pacific.net
Thu Oct 9 20:26:41 EDT 2014


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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