NICE AND EASY: How to estimate loss on coax cables

If you’re doing your own home wiring, you’ll need to know about loss. Anytime you’re looking at a cable over about three feet, you need to start thinking about the loss inherent in that cable. That doesn’t mean shorter cables don’t have loss, and if you use enough of them you’ll have to deal with it.

The internet will not avail you

When you’re on the job site, it’s a little hard to use the online resources out there. For example, I really like this web site, but it’s not mobile optimized. Strangely enough there isn’t an app to help you either. Now, you could be inspired to write one, but for now let me help you by breaking it down. I’ll make it so easy to remember that you can actually do stuff in your head.

Yes I’m simplifying

I’m going to teach you some pretty simple numbers. This automatically means that they aren’t 100% accurate. Every cable has its own loss characteristics. So does every connector. Everything that you put on a line generates some loss.

Loss is measured in dB, or decibels. While I’ve written some pretty good articles about what decibels are, that’s not important. A decibel is a unit of loss, and if you lose too many, your signal will drop. That’s all you need to know while you’re in the field.

The basics: cables

Here’s the stuff you need to remember:

100 feet of coax cable used for antenna or cable TV will lose 6dB of signal. Remember that number.

100 feet of coax cable used for satellite or cellular will lose 10dB of signal. That’s because cellular and satellite run at higher frequencies, but in the field you don’t care about that. Just remember the number.

Those numbers above include the connectors. However for every barrel you add, or everything like a barrel (a ground block, etc) you will lose 1dB.

The basics: splitters

When it comes to splitters, it’s easy math.

A two-port splitter will lose 3.5dB. Double the number of ports and double the loss. so…

4-port=7dB

8-port=14dB

…and so on.

Applying it in the field

In general — and I mean this is a really rough estimate — you have about 25dB to play with when you’re installing. In other words, you can safely lose 25dB of signal before you have to put in an amplifier or plan differently. So, in general this means that you can split a signal with a 1×8 splitter and run about 100 feet before worrying. Sometimes you can do a little more, sometimes a little less. But I’m trying to give you simple math you can do in your head.

If you’re doing this all day long, you’ll have a much better idea of the “input window” for any device you have. The input window is the highest and lowest signal that it will work with. Sometimes the spread is a little more than 25dB and sometimes less.

Still no substitute for a meter

Ideally, when you’re doing this kind of work you should have a signal meter, plan your installation, and test to make sure your plans match reality. But let’s be honest with each other here. Sometimes you just don’t have that stuff. Sometimes you just have to rely on rough calculations. That’s what you have in this article… rough calculations. They’ll get you out of a pickle and back to doing whatever you need to do.

On the other hand, when you are ready to use the professional tools that will help you figure out precise loss.. shop the great selection of tools and installation supplies at Solid Signal.

About the Author

Stuart Sweet
Stuart Sweet is the editor-in-chief of The Solid Signal Blog and a "master plumber" at Signal Group, LLC. He is the author of over 8,000 articles and longform tutorials including many posted here. Reach him by clicking on "Contact the Editor" at the bottom of this page.