Co-axial cables don't just carry signals from an antenna, they carry power and commands that control an LNB, motor or switch
DiSEqC commands are carried on the supply voltage in the co-ax cable to the switch just like the polarisation and frequency selection commands going to the LNBs
The co-axial cable that connects an LNB to the receiver does a lot more than carry the signals from the dish. Although the cable must be up to the job of carrying those signals without degradation, it is also constantly working ‘in reverse’, carrying power and commands from the receiver to the LNB.
Unlike most terrestrial aerials, a satellite dish is not a passive device and the LNB needs electrical power to operate. Since the beginnings of satellite TV this power has been supplied ‘up’ the same cable that carries the IF signal ‘down’ to the receiver. It’s a neat trick that saves on supplying and installing multiple cables for a reception system.
Also with its roots in history, the supply voltage required for an LNB is from about 12V to 21V. The wide range and high voltage was originally to cope with old LNB designs and potentially long cables.
The DC voltage on the co-ax cable means you must ensure that both the co-ax central core and its shield provide a proper connection all the way from the receiver to the LNB, with no breaks in the cable or at the connectors. Although the satellite signal can happily jump a small gap, the DC power supply cannot.
The same system of power ‘up’ the cable is sometimes used in terrestrial systems to supply active amplifi ers and distribution systems at the masthead. Many TVs now provide a 5V power supply from their aerial input, to power equipment close to the aerial where a mains outlet isn’t available. Not all UHF amplifi ers work on 5V and for those that require the more usual 12V supply an inline power supply can be added indoors to send power up the cable in the same way.
UHF co-ax cables inside the home can also carry power – remote control relay systems, like Sky’s TV-Link, require a 9V supply carried on the co-ax from the central distribution to each secondary TV set and the remote ‘eye’.
The very first satellite systems received signals of just one polarisation. When reception of both polarities was required, first, the whole LNB and feedhorn were turned by hand or a small motor, and then a polariser was used between the feedhorn and LNB, which mechanically or magnetically twisted the incoming signal to match the single-polarisation LNB.
With the launch of the first Astra satellite, the combined LNB-feed was introduced, with simultaneous reception of both polarisations and an internal switch to select which was passed to the receiver. The switch was triggered by the LNB supply voltage level: 13V for vertical and 18V for horizontal, although a couple of volts either way still selects the desired polarisation. The LNB is satisfactorily powered by either supply voltage so polarisation selection is achieved without an additional cable.
Before the launch of Astra 1E in 1995, only enthusiasts’ multi-satellite systems needed to access both the high and low frequency bands used to carry satellite TV signals (Astra 1A-1D used just the low band), and this was achieved either with two LNBs or a single ‘triple-band’ LNB that used the same supply voltage switching to select between bands.
Astra 1E used the high band, so dual-band ‘universal’ LNBs were required for mass-market systems, but another form of signalling was required because supply voltage had already ‘been taken’ for polarisation selection. The solution was for the receiver to superimpose a high frequency ‘tone’ on the supply voltage, so it rapidly oscillated by a small amount above and below the 13V or 18V sent up the co-ax cable. A tone of 22kHz was chosen because it was least likely to interfere with other equipment. No tone meant the new universal LNBs would receive the low band while a 22kHz tone switched them to high-band reception.
While the voltage supply level and tone took care of polarisation and frequency band selection, the increasing popularity of dual-feed antennas required a more sophisticated control system and DiSEqC (Digital Satellite Equipment Control) was introduced. This uses sequences of short pulses of the 22kHz tone on the voltage supply to represent a digital code. The pulses only occupy 1.5ms/bit and can coexist with both the voltage level and continuous tone commands already in use. DiSEqC level 1.0 allows for selection between up to four LNBs. DiSEqC 1.1 extends that to 16 LNBs and level 1.2 allows for the control of a motorised dish mount (a DiSEqC 1.2 motor) – all using the same signal cable between receiver and antenna.
All DiSEqC devices, such as switches and motors, are active and are designed to operate on the same voltage levels as LNBs, drawing their power from the co-ax cable like an LNB. They also pass on the voltage (with the level and tone commands intact) up the cable towards the LNB.
The supply voltage and level and tone commands present on a satellite downlead must be ‘let through’ by amplifiers, attenuators and splitters between the LNB and the receiver. The LNB is the last device on the cable from the receiver but it needs the supply voltage to power it and the level and tone to select the right polarisation and band. This means that any device fitted in between must be designed for satellite signals and include a DC pass to ‘let the voltage through’ unaffected.
What’s more, they must be used the right way round with the correct terminal connected to the LNB.
It’s unlikely that you will ever use a passive splitter on a satellite cable (as two receivers cannot properly control a single LNB) but even these must be used the right way round and with the DC pass output connected to the right receiver.
Fitting an in-line signal amplifier to a terrestrial system is simpler as the only power supply voltage present on the co-ax cable is probably for the amplifier itself, and DC-blocking splitters can be used on UHF systems provided you are not also using a cable-powered amplifier. Geoff Bains
By controlling the level and tone on the supply voltage carried by the co-ax cable, the receiver can select which sub-band the LNB receives
Co-axial cable contains a central conductor to carry the signal and a surrounding shield, insulated from the central core, to provide the ground connection and prevent radio interference picked up by or radiated from the central conductor.
Fixed satellite dish with two LNBs to receive signals from two satellite positions at once, originally Astra 19.2°E and Hot Bird 13°E.
The whole frequency spectrum used by European satellite TV transmissions is divided into low (10.70-11.70GHz) and high (11.70-12.75GHz) bands.
Satellite TV signals are broadcast on one of two polarities, horizontal and vertical to enable more transmissions to be fitted into the same frequency ‘space’.
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