What is slope compensation?

So a bunch of us techs were talking the other day about running satellite lines over 1,000 feet. One of the guys said, “well the problem there is slope compensation.” Slope compensation, quite honestly, sounds like one of those made up terms that a mechanic will throw at you when he wants more money. (“Yeah, I think the problem is your Johnson rod isn’t frammatized properly.”) But I assure you dear reader that slope compensation is a real thing and while it probably wasn’t the root of the problem with that long cable run, slope compensation is definitely something you need to be aware of.

Let’s start with the basics. As electricity travels down a wire, it loses strength. This is why you can’t have a 600-foot cable run. The signal gets weaker and weaker as it travels, and it will do so in a predictable way. There are even web sites that will estimate how much loss there will be in a generic cable over a given distance.

However, signals at higher frequencies will lose strength more quickly than ones at lower frequencies as the distances increase. Here’s an example: a signal traveling at 400MHz (like VHF television) will be about three times stronger than the same signal traveling at 2000MHz (like satellite TV) after only 100 feet. That’s a problem.

In order to help signals go further, amplifiers are put in line. An amplifier isn’t a perfect thing, though, and as they say “there ain’t no such thing as a free lunch.” Amplifiers add noise to a signal and so you want to use them as sparingly as possible. There’s also such a thing as amplifying too much. It’s called “overdriving” and if you increase the signal strength too much you will burn out the tuners that receive that signal.

So what do you do if you need 20dB of amplification to make sure that the satellite signal gets from here to there? Putting that much power into the line could mean that you’re overdriving some of those lower frequencies. What’s the answer?

Slope compensation. 

Slope compensation means that you amplify the high frequencies more than the low ones. In other words, as the amount of signal decreases (in other words, it slopes down if you plot it on a graph) you compensate by amplifying more. You might only put 5dB of amplification down at 400MHz but you put 20dB of amplification up at 2000MHz where it’s needed. When the signal gets to where it needs to be, it’s all at the same level again.

So, dear readers, slope compensation turns out to be a good thing. A slope-compensated amplifier is more flexible than one that isn’t, and it is part of making sure that you’re powering things properly and not burning anything out.

The Johnson rod, on the other hand… I’d have that looked into.