Satellite antennas

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.

1.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 profi le 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 diff erence to its reflective ability with microwaves.

Other materials are used too. Plastic, resin and glass fi bre dishes are easy to make, light, will not corrode and will spring back when fl exed. These materials are largely transparent to microwaves so a refl ecting 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), amplifi es the tiny electrical signal and reduces the frequency so it can be sent down the cable to the receiver indoors.

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

2.Noise and dB

The 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 fi gure’ of the LNB in decibels (dB). The noise fi gure 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 fi gure 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- LNB 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 ‘off set’ 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 off set dish – but it also means that the LNB must have a special wide-dish feedhorn which cannot be used on ‘normal’ dishes.

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

4.Why size matters

Whatever the shape of your dish, its size is key to its operation. This determines how much of the weak signal from the satellite is gathered for the LNB. If insuffi cient 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 fi gure 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.

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.

5.Footprints

Diff erent satellites transmit signals of diff erent powers and they spread that power over diff erent 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 below:

6.More than dishes

Although 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 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 refl ector 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 effi cient 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 Geoff Bains

Glossary

Parabolic

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

LNB

Low 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.