For those of you who are not so familiar with the complexities surrounding SMATV (Satellite Master Antenna Television) distribution systems, Technical Engineer, Andy Bull, explains how it should be done properly.

SMATV systems are primarily used in applications which require the distribution of a selection of satellite and terrestrial channels to a number of reception points, but where it is not possible or practical to install an aerial or satellite dish on every dwelling, such as a hotel or a leisure complex.

This type of system begins with the reception antennas which will always include at least a satellite dish and a terrestrial television aerial for the local television channels. The nature of the system will also allow for the installation of FM and DAB antennas, as well as CCTV and other relevant channels such as a hotels own TV channel.

The amount of channels required by the customer will determine which type of LNB to use. If only four satellite channels are required, a Quad LNB can be installed, but if your customer wishes to add more channels in the future, this LNB will need to be replaced. If more than four
satellite channels are required then, a Quattro LNB should be used along with a suitable multiswitch. Top Tip: It sometimes makes more sense to use this option even if only four satellite channels are required because you then have a future upgrade path for the customer.

Before the signal reaches the multiswitch it may also need to be amplified which will be indicated by the signal level on your spectrum analyser.

The number of satellite receivers will need to correspond with the number of channels required. Once the satellite receivers have been tuned to the specified channels, the output of the receivers need to be equalised (balanced) and combined together with the terrestrial television signal. The output signals from the satellite receivers need to be allocated a UHF channel between 21 and 68. Each satellite receiver needs to be set to its own individual UHF channel, to avoid cross modulation. You must also avoid using the same channels as the terrestrial channels that you also want to distribute.

It is important to know what UHF channels you can use when you’re planning to distribute TV channels. In a perfect world we could put them anywhere, but in practice this is not the case.

There is a rule which must be followed when planning which UHF channels can be used. The simplest way to make sure you follow this rule is to remember that you cannot put channels 1, 5 and 9 above or below the channel you wish to use. This means that if you want to set the output on your Sky Digibox to channel 40, you could not put another signal on 39, 35, 31, 41, 45 and 49.

If you were to look at the output from a digibox on a spectrum analyser, you would see the main carrier on the UHF channel the box is set to and the weaker, duplicated signals on + & - 1,5, and 9 channels away from it. The graph below demonstrate the typical output of a basic modulator.

In order to avoid clashing with the side bands from the modulator of a receiver, it is important that you carefully plan which channels to use.

The channels available are 21-68, but to find out which ones you can use, the first thing you need to do is identify the terrestrial television channels you want to distribute because these can not be used. Remember that you also cannot use the + and - 1, 5 and 9 channels.

It is also important not to forget about DTT (Digital Terrestrial Television) multiplexes. With these you only need to leave out the channel they broadcast on, not the + and - 1,5, and 9 channels. Also take into consideration any unwanted channels from nearby transmitters. Assuming that their power is not enough to cause strong interference, leave out only the channel they broadcast on.

Once you have gone through this process of eliminating channels, a number of odd and even channels will be left. These are the channels that can be used, but you can only use all odd or all even, you cannot mix them up.

Below is a channel plan for the Sandy Heath transmitter showing which channels are available to use once the others have been eliminated. The analogue channels which can’t be used are in green and the digital channels which can’t be used are in blue.

Unwanted channels which may cause interference (in this case from Waltham) are in red and + and - 1, 5, 9 channels that can’t be used are in orange. (Channel 40 is also a digital channel).

ODD = 65, 63, 61, 59, 57, 55, 53, 51, 49, 47, 41, 37, 35.
EVEN = 64, 60, 58, 54, 52, 50.

If you were to use the odd channels for your signal distribution, the even channels will become unavailable to use, and visa versa.

If you were installing a SMATV system using the Sandy Heath transmitter and you wanted to distribute 6 Sky Digital channels, the table above indicates which channels you could use for the satellite channels (indicated in purple).

If you were to eliminate all the even numbers, there would still be room to add more channels in the future.

When you have decided which UHF channels you can use, you need to decide how you wish to extract these from the receiver. You can use the UHF output from the satellite receiver (TV output), but the stability of the modulator in the satellite receiver may be questionable and result in frequency drift. Alternatively you could take the AV output of the box, for example SCART, and then modulate it (convert it to a UHF channel), which should ensure that the channel does not drift.

Now you have all these channels, you can use either a wideband equaliser/combiner or a channelised filter/amplifier.

If you use a wideband equaliser/combiner then this will allow you to balance all the satellite channels to the same level. This will give you one output which you then need to combine again with the terrestrial television channels and then feed into a launch amplifier, which connects to your distribution system. The only concern with doing it this way is that the equaliser/combiner does not filter out any mess that comes out of the UHF output (modulator), so when the signal gets to the amplifier it amplifies the mess as well as the required channels which can result in poor pictures.

The other option is to use a channelised filter/amplifier. These will only pass through them the channels that are required ensuring that any unwanted mess is completely gone or kept to a minimum. The other benefit of channelising is that you can also use the filters/amplifiers for your terrestrial television channels as well. This type of system usually hasjust one output that combines all the channels together, keeping the amount of components in the system to a minimum. The less components the better.

As you can see from the diagrams on the previous page, the final stage of the headend is the amplification of the signals you have processed. From careful planning of the distribution system we should know what signal level is required out of the amplifiers so the headend can deliver good quality pictures.

Fig A shows the output of the system through a wideband equaliser. You can see how much mess can be caused by the unwanted sidebands. (Please note that the possible channel interference from other transmitters has not been shown but the channels have been avoided as you can see).

Fig B shows how a channelised headend can remove the unwanted sidebands and the benefits of using this type of system are clear to see.