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A company calling itself "Digitalis" gives a more honest (if misleading) explanation of the "equalised slope" amplifier. This is designed specifically to compensate for a long cable run. It has no other purpose than to overcome losses in a long cable.
A company calling itself GMAX is selling a very cheap LNB amplifier that does NOT have slope equalisation. It states "This excellent device is perfect for boosting a signal on a long run of cable." In fact the reverse is true because it has no gain equalisation. This amplifier is intended for boosting the signal to compensate for losses in an LNB switch or a signal splitter. It has no other purpose.
Let me explain briefly: Coaxial cable loses signal along its length and adds "noise". The signal losses are higher at higher frequencies. An amplifier with "equalised slope gain" gives an additional boost to higher frequencies so that all frequencies coming out of the far end of the cable are roughly equal in amplitude and at a high level compared with the "noise" added by the cable. It uses the same principle as a hearing aid for older people.
So you might need an "equalised slope" amplifier to compensate for a long cable run.
An amplifier without this feature will result in low frequencies being boosted more than high, resulting in unequal amplitude signals, which is bad news.
A drawback of all amplifiers is that they also add "noise" and they can only amplify what they receive. If the dish is too small and provides a weak, noisy signal, that's what gets amplified. (If you make enough weak tea to fill a bucket, you'll still have weak tea.)