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dB & dBm Differences in Relation to Cellular Signal Boosters

dB vs dBm, what is the difference? In this article, we will explore this topic.

dBm is an abbreviation for the power ratio in decibels (dB) of the measured power referenced to one milliwatt. It is used in radio, microwave and fiber optic networks as a convenient measure of absolute power because of its capability to express both very large and very small values in a short form. DBm is used to define signal strength in electronic equipment such as signal boosters, wires and cables at radio and audio frequencies.

Let us now see how they are used in our cell phone signal booster industry. In a nutshell, you can think of dBm as the amount of power an amplifier or antenna's capable of producing. Or how much signal is at a site. While dB is a relative number of the amount of increase or decrease in signal. dBm is an absolute measure of signal strength. It is an abbreviation for Decimal Milliwatts, and is calculated as ten times the log of the signal strength in milliwats. dBm is an important figure of merit when measuring the outside signal strength. We typically think of an outside signal as being strong when it is negative 70 dBm or better, and poor, when it is negative 100dBm 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 negative 20 dBm can effectively cover about 3000 square feet. dB, on the other hand, 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 will be negative 20 dBm.

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.

This subject is obviously more complex and there's much more to know. But I hope this gives you a basic understanding of what these terms refer to. Understanding the difference in these terms will help you when it comes to installing cell phone signal amplifiers. Watch video below for illustrations.


What is minimum dBm strength needed outside for signal booster to work?

To measure signal strength in decibels, you can use a signal meter or put your phone in field test mode. Take a look at the chart below that shows signal strength in relation to measurement in Decibel Milliwatts (dBm):

Signal Strength Infographic Chart

Please note that above 3G signal strength chart is close, but not the same for 4G/ LTE. See the differences shown below based on whether measuring signal strength in dBm on a 3G network or 4G network due to comparatively better quality of the newer 4G LTE network signals in comparison to older 3G network signals:

3G or 4G Network Variances In Measuring Signal Strength

Since cell phone boosters require some signal in order to boost it or multiply it many times, there needs to be atleast "poor" or "weak" signal outside where donor antenna can be placed. Therefore, you can deduce from this that the minimal amount needed must be atleast within -101 to -109 dBm if on 3G network, and within -111 to -119 dBm on 4G / LTE network. However, the best estimate would be the median of those two which is approx. -105 dBm on 3G and -115 dBm on 4G LTE. This is absolute minimal signal strength in decibel milliwatts that is needed outdoor for any cell phone signal booster to help improve reception indoor to any minimum extent.

What if you do not have enough signal to use signal booster?

If you cannot garner minimal signal strength as stated using your directional antenna (survey kit), a cellular booster, bi-directional amplifier (BDA) or RF repeater will not work. In such instances, you may submit location details for a custom signal enhancing solution that would include a Small Cell, Microcell, Femtocell or active distributed antenna system (Active DAS).