Fringe Reception

dishFringe reception is the term used to describe the edge of a satellites reception footprints.
Satellites do not broadcast in nice clean rings as shown on official reception maps, signals can vary greatly from area to area, or depending on whether you are east or west of the official reception map (footprint). Satellite signals do not however, stop at international borders and ‘overspill’ will occur even on the tightest of beams.

This is the situation overseas viewers will encounter with reception of the Astra 2E and Astra 2F satellites, these are used by all the main PSB broadcaster, the BBC, ITV, Channel 4 and Channel 5.

There can often be variations from transponder to transponder and from horizontal and vertical polarization.
All links in the reception chain are vital, the correct size dish, the best LNB, LNB skew, good quality cable and of course a good quality satellite receiver with the lowest possible threshold.
The threshold is the lowest level of signal required by the receiver to produce a watchable picture.
In fringe areas, reception is not guaranteed by the satellite operator, or the channels using the spotbeam, this is not the intended area of coverage.

The UK’s Public Service Broadcasters (PSBs), broadcast most of their services Free-To-Air (FTA), in the clear or unencrypted.
British broadcasters have the rights to transmit programming to the United Kingdom and Irish Republic. To prevent their services from being received outside the intended area of reception, they use the UK Spotbeams on the Astra 2E and Astra 2F satellites.



Dish Size

Dish size is paramount in the reception of weak signal outside intended reception areas.. This determines how much of the weak signal from the satellite is gathered for the LNB. If insufficient signal is collected enough signal for the receiver.
Although other elements are important in the reception chain, such as LNB type and the type of receiver used, there is no substitute for more signal from a larger dish. Dishes are usually sized by their diameter, so size can be confusing if you are not comparing dishes of the same shape.

A dish need only be about 40% wider to produce twice the signal; a 1.2 M dish will give nearly 50% more signal than a 1.0 M dish. To ensure that your dish is large enough you need to know the strength of the signal at your location.



LNBs

Theoretically the lowest noise figure obtainable from any device is limited by any components in the signal chain with the highest thermal noise. The first component in the chain would be the detector circuit and on a Universal LNB this would be a pin diode.

At Ku band the detectors are rated at manufacture to about 40K which converts to a figure of 0.5 dB. Presently the LNB market is active by those selling what appears to be an extraordinarily good device, some use the best of the component batch – and end up with good performance overall (rare).

Some modify existing LNBs by the use of fancy smoothing circuits to eliminate any further incoming noise from the power supply – there are definite improvements when used with cheaper receivers, especially those with switch mode supplies (few).

Some simply take the lowest noise figure during tests at a particular frequency, and then claim that this is the figure of the LNB (so you could have a 0.5dB LNB at 10.8GHz which has a 1.1dB noise figure at 11.6GHz, however it will be rated at 0.5 (depressingly common).



LNB Gain

The LNB gain tells us how much the incoming signal is amplified before being sent off down the coaxial cable to the receiver. The range of gain specified is between 40dB and 70dB (somewhere between 10,000 and 4,000,000 times the incoming signal power). At first sight, the highest gain you can get would be the obvious thing to look for; however that is not the only criterion when it comes to LNBs. When you have a large dish looking at high power satellites like Astra 2A and Astra 2B, the gain can be so high that the receiver is overloaded with signals.

These can ‘swamp’ the lower powered satellites signals. Even if the receiver itself can handle a massive amount of signal, there can be problems within the LNB itself when large amounts of amplification are employed. This leads to the generation of spurious signals and distortion. This distortion will interfere with the reception of your signals. To let the demodulator in the receiver work effectively, the gain at all frequencies, should be the same. This is not a very difficult requirement to meet, except perhaps at the edges of the band, as long as the LNB is constructed properly.



lnbscewLNB Skew

Skew refers to the angle of the LNB relative to the rest of the dish, you are maximising the gain of the LNB. This could be the difference between a good watchable picture or poor reception. All geostationary satellites are located above the equator (the Clark Belt). They are placed here to match the rotation of the earth. Inside a satellites footprint, LNB skew is not all that important, however as you approach the fringes of the signal footprint it becomes ever more crucial to getting good reception. The LNB is kept in place by either a screw or a nut. Loosen these and it will be possible to rotate the LNB left or right.

The degree of tilt varies depending on your location. Satellite appears to be tilted as viewed from Earth, this means the LNB has to be tilted to a similar angle so that it matches the geometry position of the satellite. The degree of tilt varies depending on both your location and on which satellite you want to receive. Skew varies from about 15º in the north of Scotland to around 22º in the south west of England, always rotate clockwise as viewed facing the front of the dish. Most commercial satellite dishes have a certain amount of skew built in, the LNB will appear to be lopsided. In areas where the signal is strong, skew is not such an issue, once outside the main reception area as with Astra 2D overseas, skew becomes important
The LNB is locked in place by either a screw or a nut. Loosen it and the LNB will rotate in its housing. Rotate it one way or the other until the signal quality is maximised.



fecForward Error Correction (FEC)

Forward Error Correction (FEC) is a technique used for controlling errors in data transmission, FEC is accomplished by adding redundancy to the transmitted information using a predetermined algorithm. Part of the data stream is used solely to correct errors in the downlink stream from the satellite. This prevents the picture breaking up.

A FEC of 2/3 means that every third bit of data is used to correct errors in the previous two bits. Here signal would be more robust and can be received easier, with less breakup in rain, for example. It is less efficient, with not so much data available to transmit the picture.

A FEC of 9/10 means that just one tenth of the data is used to correct errors, making the signal harder to receive and much less robust. It will break up more easily in weak signal areas, or when subjected to interference. For the broadcaster, 9/10 means that more channels can be squeezed in, at a higher data rate.
The difference between FEC 2/3 and FEC 9/10 is that an extra 4.5dB ES/No is required to stay above threshold. Currently, most UK PSBs use a FEC of 5/6, which is not very robust in fringe areas.



Symbol Rate

Symbol Rate (SR) is the number of symbol changes (waveform changes or signalling events) made to the transmission medium per second using a digitally modulated signal.
The Symbol rate is measured in baud (Bd) or symbols/second. ks/s is 1000 symbols / second. KHz is 1000 cycles / second.
The most common Symbol Rates used for British TV and radio are 22000 and 27500, BBC HD channels use a SR of 23000, while Sky HD tend to use 29500.



Bit Error Ratio (BER)

Bit Error Ratio is an objective measure of the quality of the digital TV signal after signal demodulation. The coding techniques, which are employed in the digital TV standards, are able to identify and correct a certain amount of errors. Consequently, bit errors can be tolerated up to a particular level without causing quality degradation of pictures or sound.



Carrier to Noise Ratio (CNR/C/N)

CNR (also C/N) is the carrier to noise ratio. This is the power ratio (usually expressed in dB) between the received RF power in a carrier and the thermal noise power (kTB) in the same bandwidth. It is a measure of how “good” the signal is and how “easy” it will be to demodulate the transmitted information. It is typically measured at the output of the antenna or the input to the demodulator.



Eb/No

Eb/No is the ratio (usually in dB) between the energy in 1 bit of a transmitted digital signal to the thermal noise power (kT) in 1 Hz. It is functionally equivalent to C/N and is related to C/N by the following formula: C/N = Eb/No + 10.log(R) + 10.log(B), where R is the transmitted information rate (bit/s) and B is the noise bandwidth (Hz).



Es/No

Es/No is the ratio (usually in dB) between the energy in 1 symbol of a transmitted digital signal to the thermal noise power (kT) in 1 Hz. In the case where the modulation scheme transmits 1 bit per symbol (e.g. BPSK) then Es/No and Eb/No are the same. Where there are multiple bits per symbol then the Es will increase (e.g. for QPSK there are 2 bits per symbol so Es/No = Eb/No + 10.log).



Effective Isotropic Radiated Power (EIRP)

eirp
Click to enlarge

Effective Isotropic Radiated Power (EIRP) is derived from the word isotropic which means equal in all directions. Effective Isotropic Radiated Power means, the power levels that would be received at any location if an antenna were radiating equally in all directions.

Therefore, a 37 dBw EIRP reading means that a perfect antenna would direct 37 dBw or 5012 watts per square meter in all directions.
The reason that a transponder having rather limited power, typically in the 8 to 150 watts range, can apparently have such a high EIRP stems from the fact that this power is not radiated equally in all directions and is concentrated in a narrow beam (Spotbeam) aimed at the earth below.

Ku-band transponders having a total power of 50 watts have EIRPs as high as 48 or 49 dBw when this power is directed into a tightly focused spotbeam. EIRP levels refer to the power of signals measured at the satellite downlink antenna.

Official satellite reception maps (footprints), have very conservative values, this is good if you are inside the official reception area, as it is easy to determine the dish size required for reliable reception. However, satellites do not transmit in the way they are displayed on official footprints, so outside the intended reception area, it is mostly guess work.


 

Quasi Error Free (QEF)

Quasi Error Free (QEF) The point where the signal quality is enough to give stable reception, a level of errors that is defined as the “threshold” of where the system is working and where it is not.
The “threshold of visibility” is another term sometimes used referring to a rate of errors that just becomes noticeable to a TV viewer.
A good quality satellite receiver should have a low threshold, this is an important factor in receiving weak signals.
DVB-S, QPSK, FEC: 2/3, C/N at QEF: 5,5
DVB-S, QPSK, FEC: 3/4, C/N at QEF: 6,5
DVB-S, QPSK, FEC: 5/6, C/N at QEF: 7,5
DVB-S, QPSK, FEC: 7/8, C/N at QEF: 8,2
DVB-S2, 8-psk, FEC: 3/5, C/N at QEF: 5,5
DVB-S2, 8-psk, FEC: 2/3, C/N at QEF: 6,7
DVB-S2, 8-psk, FEC: 4/5, C/N at QEF: 7,9
DVB-S2, 8-psk, FEC: 5/6, C/N at QEF: 9,4
DVB-S2, 8-psk, FEC: 8/9, C/N at QEF: 10,7
DVB-S2, 8-psk, FEC: 9/10, C/N at QEF: 11



Signal to Noise Ratio (SNR/SN)

SNR (also S/N) is the signal to noise ratio. This is related to CNR, but usually is only applied to analogue signals. It is the power ratio (again usually expressed in dB) between the received (and demodulated) wanted signal power in the baseband bandwidth and the thermal noise at the demodulator output in the same bandwidth. For example, in an analogue FDM/FM voice multiplex carrier the CNR would be measured in the bandwidth of the whole carrier but the SNR would be measured in the 4kHz bandwidth of one voice channel.

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