Cell Phone Bars and dBm
You’ve got a great cell phone signal – 4 bars at 4G! That’s the best you can get, right? We are under the false assumption that the more bars you have, the better off you are. While we have come to think the more bars that are illuminated the better, the truth is that the number of bars on your phone may not be the best indicator of signal strength. dBm is a better way to measure signal. We are going to explain the concept and math behind dB and dBm, how this relates to cell phone signal, how to truly measure signal strength using the settings or a simple app on your phone, and finally how this relates to cell signal booster technology.
What is dB?
Decibel or dB is a standard used to measure the intensity of sound or the power level of an electrical signal by comparing it with a given level on a scale. It is also used to show an increase or decrease in signal strength. The decibel scale goes up in powers of ten; every increase of 10 dB on the scale is equivalent to a 10-fold increase in intensity. There are calculations to show the ratio of power and voltage: dB = 10 x log10 (power output/power input). We are going to keep things fairly simple here. Amplifiers cause gains in power measured in dB and is a positive number. Cables cause a loss of power measured in dB and is a negative number. One other thing to note is that decibels are used to handle very large or very small numbers. So, for example 1,000,000 is translated to 60 dB and .000001 = -60 dB. This will make sense in just a minute. The most important this to remember is that dB is a ratio and not a concrete measurement.
What is dBm?
dBm stands for decibels per milliwatt. While dB is the expression of a gain and is dimensionless, dBm meaning is a concrete measurement of the power level. It is used to define signal strength in wire and cables at radio and audio frequencies. dBm is measured is very small values. The formula for calculating dBm measurement is 10 x log (signal strength per milliwatts). Measurement of signal strength is in dBm and is the most accurate measurement available.
To recap, we use dB when expressing a ratio between two values and dBm when determining the exact power. We can use both in calculations such as this: if your signal outside uses -90dBm and you have an amplifier system that is rated at 70 dB, then you will have a signal radiated at -20dBm, or enough strength for about 3000 square feet. This number is important when choosing a signal booster as you will find out in a bit.
If you need an even more concrete showing of what a decibel sounds like, here is a chart showing common sounds measured in dB.
What is a good signal?
The cell phone tower emits a signal. As the signal travels away from the tower, it degrades or weakens. Signal is also affected by any obstacles in the way, such as buildings or trees. When you are close to the tower, the signal is quite strong. By the time it reaches your home, it has a lower dBm. Measured in dBm, a signal of greater than -70 dBm is considered an excellent signal in all networks. A poor signal will be -100 dBm or worse in 3G networks and -110 dBm or worse in 4G networks. It is important to take measurements in several areas to determine where you have the strongest signal strength. When placing a Wi-Fi router or modem, test signal strength and place the device in that particular space.
How does signal strength relate to bars on my cell phone?
Here’s where it gets a little tricky to believe the cell phone bars. Cell phone or signal bars are subjective and generally indicate the relative strength of the available signal. The type of cell phone you have as well as the manufacturer can affect how many bars are illuminated as well. You and someone else can be sitting in the same room, you have 3 bars and they have 4. You are using the same cell provider, but, for example, one of you has an iPhone and one of you has an Android. If you look at the dBm, it might be the same. Apple, in particular, is notorious for "inflating" the worth of the bars on their cell phones - hence why it's important to know what dBm is.
Here’s another example: Let’s say that your cell phone has 5 bars of service at 3G or 1X. What that really means is that your signal strength is excellent at about -70 dBm. Now you have 5 bars at 4G/LTE. Seems much better, right? Not necessarily. Excellent signal strength for 4G is around -90 dBm. In this case, you may have a stronger signal with the 3G! Check the dBm to see what your strength really is.
How can I determine the signal strength dBm of my phone?
On Android phones, the signal strength can be found in the Settings menu. Click About Phone, the select Status (or SIM Card Status).
The signal is listed in dBm signal strength and ASU. ASU stands for Arbitrary Strength Unit and is a value proportional to the received signal strength measured by the mobile phone. It can be translated to dBm, but there are different formulas depending on whether you are using 2G, 3G or 4G networks.
The iPhone has a Field Test Application to access and will give you the same information as above.
It’s fairly easy to get a measurement of signal strength with an app on your phone. We recommend the Network Cell Info Lite app for Android phones.
This App will give you a ton of information. You will notice that the signal strength changes constantly. I find the Map page fun because I can drive around and see where the signal is strongest. If I’m in the mountains and need to make a phone call, I can wait for a pull-off with a great signal strength. This is also a great App for deciding where to place your router or signal booster in your home to start with the strongest signal.
So, in an office building, you might see you have 4G/LTE with 3-4 bars of service. The App shows two readings – the serving cell tower shows -94 dBm, and the next strongest signal is -102 dBm. Say your office is concrete with no windows and lots of cubicles. It may have a bunch of amplifiers in the building as well, or other objects that create electrical interference. As a result, signal is not always great in here unless you're attached to Wi-Fi. I also tested this at my home in a rural area, it shows -95 dBm with about 2 bars. My house gets ok service, but it is not consistent and fades in and out. My house is a great candidate for a signal booster.
Cell phone signal boosters related to dBm
By design, cell phone signal boosters take the signal that is reaching your device or home and amplifies it up to 32 times. You need to start with some signal coming into your home, even if it is weak. If you test the signal and is comes up as -95 dBm, that’s a pretty weak signal. dBm is determined by factors of 3 and 10. Signal strength doubles every 3 dBm. For example, a -30 dBm signal is twice as powerful as a -33 dBm signal, and a -84 dBm signal is 4 times as powerful as a -90 dBm signal. Going back to the -95 dBm original signal you have. If you boost this signal to -92 dBm, you have just doubled your signal strength. If you get to -89 dBm, you have just boosted your signal strength by a factor of 4. It doesn’t take much boost to make a big difference in signal strength.
Cell phone signal boosters cover a particular square footage. A booster rated at -20 dBm can cover about 3000 square feet of your home. We did this calculation earlier related to dB and dBm. Signal Boosters claims signal strength of up to 32x your current signal. This takes a poor signal and make it an excellent one.
What do I do with all this information?
Now that you know how to check signal strength, you also have enough information to explore options to boost that signal if it’s not enough for your use. You have the knowledge to scout your home for the best place to install your router or signal booster to best optimize the available cell phone signal.