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ABC guide to...

Satellite antennas

Freesat Freeview euro tv skyThe best kit, The best programmes

2 What Satellite & Digital TV

Satellite antennasUnderstanding how your satellite dish works will help you to pick the best antenna for your needs and aim it correctly

Satellite TV microwave signals are incredibly weak. That we can pick them up at all is remarkable,

and that most dishes do a sterling job for just a few pounds is amazing. If you want to always get the best from your system, it’s important to have the correct antenna – the right type and the right size.

What’s in a dish?There are two operations to

receiving a satellite TV signal – collecting as much of the signal as possible, focusing it at one point and amplifying and processing it.

The most prominent feature of any satellite TV antenna is the means to collect the signals. It’s usually a dish reflector and this must be made to the precise parabolic profile and constructed so it remains that way through transport, installation and years of service. A dented, bent or distorted dish will not focus the microwaves correctly, ‘wasting’ some, so the received signal will be weaker and maybe insufficient for a picture.

The reflector is usually made of metal (steel is cheapest but dishes are also made from aluminium because that won’t corrode) which can be solid or perforated to make the dish less visible and less prone to wind loading – the holes are tiny and make (almost) no difference to its reflective ability with microwaves.

Other materials are used too. Plastic, resin and glass fibre dishes are easy to make, light, will not corrode and will spring back when flexed. These materials are largely transparent to microwaves so a reflecting metal foil or mesh layer is embedded inside.

Signal processing is performed by the low noise block-downconverter (LNB) at the antenna’s focus. This selects the signal polarity and frequency band (as we’ve seen previously), amplifies the tiny electrical signal and reduces the frequency so it can be sent down the cable to the receiver indoors.

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Noise and dBThe antenna’s LNB must introduce

almost no electrical noise into the signal. All electrical equipment adds noise into the signals passing through but because the satellite signals are so small, the levels of noise introduced by the LNB must be truly minute if the TV signal itself is not to be swamped by the unwanted noise.

The noise that an LNB introduces is expressed by the ‘noise figure’ of the LNB in decibels (dB). The noise figure is the ratio of the signal-to-noise ratio at the output to the signal-to-noise ration at the input. What’s key is that it’s not a linear measurement and every increase of 3dB means a doubling of the noise. LNB design Improvements have reduced the 1.8dB noise figure of a typical LNB 20 years ago to just 0.3dB today (ignoring a certain amount of marketing hype) – and the cost is down to about 20 per cent too.

For a straightforward Sky or Freesat system you are likely to get a dish-and-

02LNB package and any LNB with noise of 0.9dB or less will probably suffice, but if you are putting together an enthusiast’s system to hunt weak, obscure signals, get as low a noise LNB as you can afford.

Any dish will do?Dishes vary in both size and shape.

Dishes of old were circular with the LNB held on three arms at the centre (a ‘centre feed’ design), but most antennas now are ‘offset’ dishes – oval, with the LNB arm at the bottom, and ‘looking’ over the top of the LNB. This gets the LNB out of the way of the signal path and makes it less likely for rain or snow to collect on the reflector.

Old Sky analogue dishes (and many enthusiast dishes today) were taller than they were wide but Sky minidishes are wider than tall. This is to increase the size horizontally so there is better distinction between adjacent satellites (a larger dish has a smaller acceptance angle) and it is achieved by the minidish being just a slice of a normal offset dish – but it also means that the LNB must have a special wide-dish feedhorn which cannot be used on ‘normal’ dishes.

Why size mattersWhatever the shape of your dish, its

size is key to its operation. This determines how much of the weak signal

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ABC guiDe To

glossary

ParabolicProfile (cross section) of a dish reflector. Close to a semicircular shape but ‘more pointed’. An accurate parabolic shape for the reflector is needed for signals hitting the entire dish to focus at one point.

LNBLow noise block-downconverter. Device at the focus of the dish to select between signal wavebands and polarities, amplify the received signal, and lower its frequency for transmission to the receiver by co-ax cable.

A typical offset satellite dish and LNBs (left to right): without a feedhorn showing the polarity probes; dual output; quad output and single output

Signal strength and dish size

EIRP (dBW) Dish size (cm)

35 300

40 180

42 160

44 120

46 95

48 75

50 60

51 55

52 50

53 45

54 40

What Satellite & Digital TV 3

ABC guide to...

From centre to offset: The centre-feed dish (left) was the first type of dish antenna but offset dishes (middle) are more popular today. A Sky minidish (right) is really a section of a ‘standard’ offset dish, made wider for better adjacent satellite rejection

from the satellite is gathered for the LNB. If insufficient signal is collected then it can’t produce the picture on your screen.

Simply electronically amplifying the signal will just amplify the noise received too, so that doesn’t work. There’s no substitute for more signal from a bigger dish. Dishes are usually sized by their diameter – but they are rarely circular, so the figure can be confusing if you are not comparing dishes of the same shape.

Every centimetre of diameter counts; a dish need only be about 40 per cent wider to produce twice the signal; a 1.2m dish will give nearly 50 per cent more signal than a 1m dish. To be sure that your dish is big enough you need to know the strength of the signal at your home.

FootprintsDifferent satellites transmit signals

of different powers and they spread that power over different sized areas of the Earth’s surface. Where you live within that area will affect how much of the signal reaches you.

It’s impossible to predict exactly what signal strength you will receive from any particular satellite but we can get a good approximation with maps of the signal strength, called footprints. These show areas of (roughly) equal signal strength within the intended area of reception. Although the signal spills out from this area, it gets progressively weaker the farther you go from the beam centre.

Footprint maps show either the size of conventional dish needed or the equivalent isotropically radiated power (EIRP) in dBW, which is a measure of the signal strength. You can get an idea of the dish size likely to be needed using the table on the right:

More than dishesAlthough the simple dishes used by

Sky and Freesat are the most popular form of antenna, they are not the only kind available. Over the years many different types of antenna for satellite TV

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have been tried (and are still used).The closest variations use a second

reflector to bounce back the signals to an LNB facing forward. The second reflector can be either concave (a Gregorian dish), like the main one, or convex (a Cassegrain design). The two-reflector arrangement can increase the efficiency of a dish, produce less cross-polarisation interference, less noise from the ground, and reduce the overall length.

Very large uplink dishes almost always

use a dual-reflector design, but they’ve also found fans for domestic dishes.

Designs such as the horn antenna and flat plate array do away with a reflector altogether. In particular, the recent waveguide array antennas are more efficient for their size than conventional dishes and if the manufacturing cost can be reduced, they might become the domestic antenna of the future Meanwhile, dishes reign supreme n

Geoff Bains

Different dish types:Top row: Gregorian (left) and Cassegrain (right) dishes use a second reflector to increase the overall efficiency. Bottom row: the Innova ‘Horn’ (left) focuses the signal with a dielectric ‘lens’ while the flat plate antenna (middle) – as used in the BSB ‘Squarial’ – has an array of probes, each within a simple reception cell, all connected together to sum the tiny signals. The waveguide array antenna (right) – from Selfsat and Neovia – uses an array of feedhorns with connecting waveguides that add the signals and feed to a central LNB.