Re: RE:LNB's WAS : Cable TV queries

["Mark McCall" <mark@xxxxxxx>]

Keith Doxey

Font of all technical knowledge!

:0)

M.


----- Original Message -----
From: "Keith Doxey" <keith.doxey@xxxxxxx>
To: <ukha_d@xxxxxxx>
Sent: Thursday, December 07, 2000 3:48 PM
Subject: RE:LNB's WAS : [ukha_d] Cable TV queries


> Hi Guys......in I come with both feet.....
>
> Satellite transmissions are sent with different polarisations,
Horizontal
> and Vertical. By that I mean that for some channels the actual radio
waves
> move up and down (V) whereas others move side to side (H).
>
> Theoretically you could transmit two signals on the same frequency
without
> interference but in practice that isnt done. Channels are alternately
sent
> on different polarities. On analogue ASTRA all the ODD channels are
> Horizontal and all the EVEN channels are vertical. This allows the
spacings
> between channels to be reduced allowing more channels to be squeezed
into
> the band.
>
> To receive all the signals requires two aerials, in an LNB there are
two
> tiny aerials at 90 degrees to each other, one for vertical, one for
> horizontal. The LNB contains circuitry to switch between the two. By
> supplying the LNB with 13volts you get Vertical, 18 volts and you get
> Horizontal.
>
> You can connect many receivers to one LNB and receive multiple
channels
but
> all must want the same polarity. If all receivers were watching EVEN
> channels then the LNB would be getting a 13v supply. If One of the
> receivers was switched to an ODD channel is would output 18V and the
LNB
> would switch to H. that receiver would get what it wanted but all
other
> receivers would lose their pictures.
>
> To overcome that two solution were developed.
>
> the TWIN LNB which is a single LNB with 2 switching stages and either
or
> both outputs an receive H or V without interfering with each other. A
> single coax runs from each reciever to the dish. with this arrangement
you
> are limited to 2 receivers.
>
> the DUAL LNB is a single LNB with SEPARATE H & V outputs. there is
no
> switching circuitry. with a DUAL LNB you connect the 2 outputs to a
special
> switch that performs the switching function. Switches can range from 2
in
4
> out to 4 in 16 out.
>
> That is probably what Jeff was describing although he was unaware of
what
> lay behind it. To look at the dish you see only one LNB but if you
look
> carefully you will see 2 or more cables.
>
> I used to have a 4 in 4 out that allowed a single cable from the
switch to
> the receiver.
> By using the 22kHz Tone Switching feature of later satellite receivers
I
> could select from 1 of 4 signals.
>
> 13V No tone ASTRA Vertical
> 18V No tone ASTRA Horizontal
> 13V Tone ON HOT BIRD Vertical
> 18V Tone ON HOT BIRD Horizontal
>
> It worked very well until all the channels either went subscription or
> digital :-(
>
> Here endeth the lesson on Satellite Distribution :-))
>
> Keith
>
> --Original Message-----
> From: Stuart Grimshaw [SMTP:stuart@xxxxxxx]
> Sent: 07 December 2000 15:18
> To: ukha_d@xxxxxxx
> Subject: Re: [ukha_d] Cable TV queries
>
>
> On 2000.12.07 15:13:31 +0000 Jeff wrote:
> > Why do you need 2 LNB's to receive two different channels - can
the data
> > stream not be processed more that once ?
> >
> > I live in an apartment which feeds the satellite to all
apartments using
> > only 1 LNB.
>
> You might be able to then, I just always thought you had to have 2
LNB's
> for more 2 boxes...
>
> --
>                                 ,,,
>                                (o o)
> ===========================oOO==(_)==OOo==============================
> Stuart Grimshaw         www.schoolsnet.com          sg@xxxxxxx
> Special Projects Developer                            t: 07976 625221
> Schoolsnet LTD             .oooO  Oooo.               f: 0870 7060260
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>
>
>
>
>
>
>
>


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