RE:LNB's WAS : Cable TV queries

[Keith Doxey <keith.doxey@xxxxxxx>]

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...

--
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