Understanding the basics of satellite signals – transmission frequency, what modulation is used, what compression method, encryption and the limitations on all these, helps us to see what problems you may encounter when installing a dish and receiver.
How does a dish work – What Frequency Does it Use?
A common satellite dish, seen on houses in any European country operates in the KU band ie. 10.95 to 12.75 GHz. This extremely high frequency (microwave) signal has to be downconverted to a lower frequency (IF Intermediate Frequency) to be useable for the mass satellite market using standard coaxial cable, otherwise wave guides would have to used ie metal pipes between the dish and the satellite receiver, which would be very impractical and expensive. See Wikipedia article
The dish is designed to focus the incoming satellite signals onto the downconverter (LNB) which is typically inside a waterproof box. The dish is typically made up of a parabolic shaped reflector with a metal reflective surface, which acts like a funnel to capture the signal from the satellite and direct it to the LNB. The entry has a feedhorn built into it, designed to present the signal to the two waveguides.
Signals are polarised, usually Vertical and Horizontal, and are in two bands. The lnb needs to switch between the polarities and the bands to be able to receive all signals. All this is done using only one simple coax cable by sending different voltages and switch signals to the lnb from the receiver.
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Dish Size Considerations
The dish size is the most important parameter when selecting a dish. Experience will guide you here, the size charts may give you an idea of the right size for the signal level expected, but you have to allow for bad weather reception, so err on the large size! Over the time I have been installing, several new satellites have been put into service, taking over from older ones. They are almost always more powerful than the old version and the beams ( coverage) are sometimes re-adjusted, resulting in a higher signal level.
Be aware that not all tp(transponders or frequencies) have the same signal strength, this is mainly dependent on the beam of the transponder, but power does also vary, so for example just because the Portuguese channels on Hispasat can be easily received with an 80cm or even a 65cm dish, it does not mean that the Spanish TDT from the same satellite will work with the same size dish, they are always weaker, so you’ll need a bigger dish.
Satellite Signals Noise Levels
The LNB is next in importance. All electronics have a level of noise, generated within its components, noise can cause the satellite signal to be distorted. The signal is just a string of 1s and 0s, but misinterpreting a 1 for a 0 due to noise can make a huge difference. The best commonly available LNBs today have a noise figure of 1.0dB, but sometimes a higher figure is acceptable, say 1.3dB when signal amplitude is more important. Changing the LNB for a lower noise version will help, but ultimately a bigger dish size has a lot more effect in getting a better signal.
Digital Satellite Signals
Understanding the types of modulation used for satellite signals helps to see why the LNB and dish size are so important.
With the old analogue signal, brightness and colour were represented by amplitude (am amplitude modulation) This was very expensive on the limited amount of bandwidth available, only one channel per TP (frequency)
With digital satellite signals, eg DVB-s or DVB-s2. It is all about efficiency. A higher spectral efficiency allows an operator to put more data into a leased transponder. With digital methods many channels can be sent on each TP, a look on any satellite data website will reveal that many Tps have over 20 channels, On Hotbird for example the RAI operated TP 125 has 28 channels. This will reduce operational expenses significantly for the operators.
However, the highest spectral efficiencies can only be achieved under certain conditions. Signals, that is trains of 1s and 0s are sent in frames. The trade-offs and overheads with security of data and cost are:
- Error Correction
- Modulation Type
- Pilot signals
- Headers
- Compression
1. Error Correction:
A frame might be 64k bits and may have 1 or 2 layers of FEC forward error correction encoding applied to the data in with the frame. A FEC of ½ is very secure but very expensive in terms of bits / frame. A FEC of 9/10 is less secure but (roughly) 9/10 of the frame will contain video , so its highly efficient.
2.Modulation:
The DVB s2 standard was designed to be used with Phase Shift Key modulation. QPSK (Quadrature Phase Shift Key) and 8PSK (8 phase..) modulation in which data is encoded onto 4 or 8 carriers that are out of phase with each other, then combined. The 8PSK allows higher data rates but is more subject to phase noise in the transmitter and receiver.
Digital transmission or data streaming uses logical units of data called symbols.
The higher order modulation allows each symbol to represent more data bits. If you map the bits and phases into a phase diagram or “constellation” it is easy to see why the higher order system is more prone to error and interference, the plotted points are closer together.
Some Digital satellite dish alignment meters show not only the signal strength but also these constellation diagrams, so that the satellite signals can be finely adjusted for the best (tightest) spots.
Slots
Slots are simply groups of 90 symbols, which make it possible to insert headers and pilots if used.
Down to a certain level of signal the errors can be corrected automatically, with the FEC methods. So a digital transmission is able to deal with low or varying signal levels. There are other overheads in each frame – pilots and headers
3. Pilots
Pilots are bursts of 36 successive symbols making a repeating pattern, to allow the receiver to easily lock onto the satellite signals. Pilots are not always used, it reduces efficiency by about 2%
4. Headers
Headers have the configuration details of the data, this allows receivers to automatically detect the various parameters of the transmission.
Digital DVB S2 is much more efficient and cost effective for the operators and with future improvements in receiver manufacture, as regards stable oscillators and phase noise, even higher order psk (more phases and more bits per symbol) methods will be employed.
When using a receiver, or alignment meter, tuning in, the number of errors is usually indicated by a quality rating, the signal strength by a strength rating, often shown as a bar display. Always aim for the best quality rating when aligning the dish.
With satellite bandwidth being so expensive, broadcasters use various methods to cut down on the amount each transmission takes, modulation type, compression standard and lately even using multistream technology.
Multi Stream
Often found on satellites and sometimes used as a “back transmission”, these can be seen with the right sort of equipment. For example most of the French and Italian channels can be found on the Eutelsat 5w satellite. Usually the tuners in these boxes are indicated with an X. eg one with DVB-s2 is not compatible, DVB-s2X is compatible with multistream. Tuning in requires some extra parameters:
- ISI Input Stream Identifier is simply the Stream number (a cardinal number)
- PLS Physical Layer Scrambling
PLS Physical Layer Scrambling
This is a low level encryption system, the key to decode it can be given as a Gold or Root processed number. (Its just a different mathematical algorithm). Some receivers can only enter one type, so if you need to convert between them there is a handy converter here:
https://www.promaxelectronics.com/tools/gold2root/
There is one more multistream type. T2-mi
This is a packet format used as a transport to transfer DVB-T2 data, over DVB-S2 to terrestrial transmitters. So effectively the satellite is used to service or back up terrestrial stations. You could see some Russian channels on 14°west, on the Russian operated AM8 satellite, currently many state owned Russian channels are blocked by European operated satellites. The T2-mi channels have a PLP parameter (usually 0)
The German Octagon series of receivers have had the multistream tuners for a good while now, some with totally automatic detection of all parameters some with an ISI menu addition. The Mira X from Amiko has some basic functions but not all the ms variants are possible. I noticed the Aston twin Fransat box has a ms tuner, as I tuned in the French channels for my customer, I noticed all the Italian ones were also present.
5.Compression of Satellite Signals
For a long time Mpeg2 was the only method used, but with HD, came Mpeg4 which is simply more efficient compression with lower losses in picture quality.
All these factors put new demands on LNBs. The stability of the frequency down converter is critical to produce clean signals at the receiver, where they are processed.
Dish Adjustment and Alignment
There are 3 main adjustments, elevation, azimuth and skew. All three need to be right, to get a good signal. A digital meter is essential to do a good job
See Russ Van Der Werff article https://www.tvtechnology.com/opinions/dvbs2-and-spectral-efficiency
Types of Dish Used in Europe
Almost all dishes are standard parabolic, offset type. They are taller than wide, so that when viewed from the elevation angle of the satellite, the dish is perfectly round, making the best of the signal available. The LNB casts no shadow over the dish, unlike with centre focus dishes.
Left – Typical Offset dish Height is greater than its width. Above – Lovell Telescope at Jodrell Bank – centre focus
The centre lnb arrangement makes the dish easy to manufacture, but the metal frame and lnb shade the dish, thus making the signal weaker. The offset arrangement needs only a single rigid arm to hold the LNB and casts no shadow, it is the dominant type of dish for all normal satellite signals (Ku band) up to about 1.3m although support arms are sometimes provided on some 1m+ size dishes.
Receiving 2 (or more) satellites on one dish
Sometimes, it is necessary to get signals from 2 (or more) satellites a wide angle apart, the choice is 2 separate parabolic dishes or a single wide angle dish
A multi lnb bracket can be attached to a parabolic dish, but the satellite signals get weaker the further the lnb is placed from the centre of the dish. This is due to only a proportion of the dish being used for each LNB. As long as the satellite is strong, up to 10° either side of the centre is acceptable.
Wide Angle Toroidal dishes
The solution is either a toroidal dish or a larger dish. One dish I use a lot is the Force frontier there are 2 models, T55 and T90 and these are designed to give an even strength signal across a 40° angle ! That is twice as wide as a parabolic dish. Plus, no signal drop off towards the edges, as with parabolic dishes.
The unique twin reflectors are well designed and having installed over 50 of these types of dish, over a 16 year period, they are very good, solidly made, no problems with wind. The powder coating is excellent no dishes that I have revisited, have ever been seen with powder coat peeling or rust unlike some notoriously bad dishes ( Orbital for example) . The dishes come with a template diagram for calculating the tilt angle required, simply enter the orbital degrees of the outside 2 satellites (widest spaced) . The set the dish tilt to the right angle. Minor skew differences can be set up on each individual lnb.
Due to the rather wide lnb clamps used, the closest distance apart 2 lnbs can be is about 4 degrees, although having the wide dish the effective min distance on the Toroidal dish is bigger than on a parabolic dish. This can be a big advantage when trying to receive 2 satellites only 2 or 3 degrees apart, like Eutelsat 16, which has the popular Hotbird and Astra 1, either side of it.
If necessary it is possible to strap 2 narrow head lnbs together using only one clamp, to get around this limitation, there are some universal multi lnb clamps available on Ebay etc.
Narrow Head LNBs
There are some good 40mm head lnbs available by AB.com, Inverto and Technomate (bullet)- that is the same diameter as the clamp, this makes it a lot easier to mount close up on a Parabolic dish, however on the Force dishes the clamps themselves are the limiting factors as the plastic is rather thick.
Examples of satellites that work with the T55 ; any 2 strong satellites up to 20° either side of the centre:
- Astra 1
- Astra 2
- Hotbird
- Astra 3
- Eutelsat 9
And with the T90: Any two strong or medium strength satellites up to 20° either side of the centre
- Turksat
- Hellas 2
- Eutelsat 5w
- Eutelsat 16
- All of the T55 group above
Cassegrain and Gregorian Dishes
Other types of dish that have been around are the Cassegrain and Gregorian which employ the LNB electronics in the body of the dish and have a small second reflector, where you would normally find the LNB. Lastly the Twist (based on the Cassegrain but for scanning use) as far as I know none of these are currently available. The Force Toroidal dishes T90 and T55 are still available, the T90 can be found in the UK, the T55 is still available from Germany.
Due to the odd size of the dish clamp (40 mm) if you intend to use a standard 1.5″ (38mm) elbow bracket to mount the dish, I suggest using a shim made from aluminium sheet , easily constructed and works well, bolt it into the clamp if necessary, in places where high wind is possible..
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