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Will an LNB amplifier improve my signal/picture?
This question is answered in our book "Installing Sky Digital TV". Briefly, if you are using more than 35 metres of the correct double-shielded cable, an amplifier may help. If your signal quality is no better when receiver is connected to your LNB with much less than 35 metres of cable, then an in-line amplifier will not help. The amplifier will give no benefit as it amplifies only what is there - signal plus noise - and adds more noise.
To spell it out: Connect your receiver as close to the dish as possible. If the signal quality is improved then an amplifier placed somewhere between the LNB and receiver should help. Try it after 5 metres from the LNB, 15 metres, 30 metres, to determine the best position. Bear in mind it requires a little power from the LNB supply.
Note that an amplifier can only amplify what's there. It's always better to use a larger dish instead.
The amplifier must always be near the input end of a long cable. If you put it at the output end, you are simply boosting the signal and the noise the cable has picked up, so you are hardly any better off!
If, when you do the test, you find that the receiver does not work better when connected close to the LNB then an amplifier will NOT help. In fact it will almost certainly make matters worse.
Even if the receiver does work marginally better when close to the LNB, you must bear in mind that all amplifiers add noise to the signal. So the addition of an amplifier could still make reception worse. And if you use an amplifier with too much gain, then that could also make reception worse.
Which way round does an LNB Amp go?
The arrow or triangle points towards the satellite receiver.
Calculating what length of cable can be compensated for by which amplifier is easy when you know how!
Now 2150MHz is a typical top-end frequency produced by an LNB.
You will see that 100 metres of WC100 attenuates a signal at 2150MHz by 30dB. That's nearly 0.3dB/metre. So you would need an amplifier of approximately 30dB gain to compensate for 100m of WC100 used for an LNB connection. Or an amplifier of about 18dB gain to compensate for 60m of cable. But read on...
Line Amplifiers (Cascade)
Line amplifiers are low power RF devices and satellite versions amplify frequencies from 950 MHz to 1750 Mhz. Later versions extend up to 2050 MHz making them suitable for Digital installations and some block conversion shared dish installations (SMATV). Versions (sometimes known as cascade amplifiers) are also available that cover 40 MHz to 860 MHz for use in FM radio and UHF television installations, typically in MATV projects. More recently we have seen wideband line amplifiers that cover most or all of the FM, UHF and satellite frequencies.
In a domestic satellite installation a DC voltage between 13 and 18 volts is present to power the LNB. This voltage is ideal for powering these products and, as the amplifiers require only a few tens of milliAmps, they shouldn't upset any other devices. However, in a UHF TV installation, DC is not usually available, so power has to be introduced with a DC line power inserter. Whichever frequency type is used the product ranges break down into two main groups: "Compensated" and "uncompensated". This is the area where caution is required.
Compensated or Equalised
These amplifiers are specially designed to improve signal levels that have been degraded by coax cable loss. Physically compensated line amplifiers may look identical to the uncompensated variety, but the important difference is that with compensated amplifiers, the gain of the device increases as the frequency increases. Often the device is not marked to identify if it is or is not compensated, so examine the specification before you buy! To understand why we need to compensate, we need to look at the effect coax cable has on high frequencies.
Coax Cable
All types of coax cable will lose signal. The extent of the loss will depend on the quality of the cable. Some cheap imported coax will lose a great deal more signal than the same length of a high quality cable such as WC100. However, all coax cable will lose more signal at higher frequencies (2000 MHz+) than at lower frequencies (950 MHz).
As an example, imagine we have a long length of cable connected to an LNB with the dish aimed at the Astra satellites. At the receiver, signals which appear from the LNB at, say, 1,627 MHz will be weaker than lower frequency signals at 994 MHz. The longer the cable length, the more of a problem this will become. If we use an uncompensated amplifier, higher frequencies will improve but lower frequencies can overload the line amplifier or the satellite receiver, or both! The overload may appear as a flickering or "blocky" picture over some or all channels dependent on how overloaded each device becomes. By using a compensated amplifier, the higher frequencies are increased the most, the lower frequencies the least. The result is the satellite receiver will have all frequencies at similar levels, thus providing the best quality pictures.
Uncompensated or Linear
These amplifiers have the same amount of amplification (gain) over their complete operating range and should be used only where signal loss is due to a cause other than cable losses. It can be used to increase a signal that is to be switched, split or divided. Because it is often the lower cost version of the line amplifier family, and technicians do not appreciate the differences between models, it is mistakenly used to improve cable attenuated signals - with unsatisfactory results.
Overload
To avoid overload insert compensated line amplifiers into the cable at a point where the signal has dropped approximately 15dB. The distance will depend upon the cable type. WC100 drops 15dB in 50m at a frequency of 2150MHz. It's worth noting that low quality amps will overload with signals at lower levels, particularly if there's a large number of transponders. The better models will handle larger signal levels without significantly distorting the signal and can be fitted closer to the LNB
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