What Is dB and dBm?

A Basic Description Of dB and dBM

You have probably heard and read the terms dB and dBm around and throughout our videos and website. But what exactly do these terms mean? Here is a little bit of information on just what these terms refer to and how they are used in the industry.

dBm - Decibel Milliwatts

You can think of the term dBm as the amount of power an amplifier or antenna is capable of producing or how much signal is at site. dBm is an absolute measure of signal strength. It is an abbreviation for decibel milliwatts and is calculated as 10 times the log of the signal strength in milliwatts. dBm is an important figure of merit when measuring the outside signal strength.

dBm - Strong Signal vs Weak Signal Measure

You will find dBm noted in negative numbers (-X dBm). We typically think of an outside signal as being strong when the signal is a -70 dBm or better and a poor signal when it is -100 dBm or worse.

dBm is also useful for estimating the indoor coverage area we might expect from an amplifier system. For example an indoor antenna that is radiating -20 dBm can effectively cover about 3,000 square feet.

dB - Decibel

dB on the other hand is a relative number of the amount of increase or decrease in signal. It is a relative figure and is simply an abbreviation for the word "decibel". We use dB to describe how much increase or decrease in signal strength we have in our system.

For example if we measure an outside rooftop signal of negative 90 dBm and our amplifier system has a total gain of 70 dB then our inside radiated signal would be negative 20 dBm. As stated above, a 20 dBm signal can cover about 3,000 square feet.

Magnitudes

Keep in mind that because dB and dBm are logarithmic the normal ways we are used to thinking about magnitudes no longer apply. For example a negative 30 dBm signal is twice as powerful as a negative 33 dBm signal and a negative 84 dBm signal is four times as powerful as a negative 90 dBm signal. The signal strength doubles every three DB.

-30dBm = 2 x -33dBm

-84dBm - 4 x -90dBm

The subject is obviously much more complex and there's much more to know but we hope this gives you a basic understanding of what these terms refer to understanding the difference in these terms will help you when it to installing cell phone signal amplifier.