Free Shipping on Orders Over $99 - Learn More

Field Test Mode: What it is and How To Enable it on Your Phone

One of the first steps to selecting a cell phone signal booster for your home or office, vehicle, or large building is to determine how strong the outside signal is. When most people talk about cell phone signal strength, they talk about “bars” in reference to the signal strength bar indicator on the phone. While bars are an easy way to talk about signal strength, it turns out that it’s not a very accurate way to signal test. While many phones show the signal strength on a 5-bar scale, some phones only have 4 bars while others have 8. Even among phones with the same number of bars, there’s no standardization, so the strength of a 4 bar signal on one phone can be very different than a 4 bar signal on another. Finally, bars aren’t very granular so for example, saying 3 bars of signal is not being very specific.

When experts discuss cell phone signal strength, they measure the signal in decibels. Decibels are a logarithmic unit of measuring signal strength and are very precise making them ideal for performing a signal test of just how strong of a signal is that you’re currently receiving.

Most phones have a setting we call Field Test Mode that can show you useful information about your phone, including the signal strength in decibels. Here are a few things to keep in mind about taking signal readings in decibels:

  • Any signal readings you take are only valid for the phone’s individual carrier.
    To get readings from other carriers, you’ll need phones from each carrier.
  • When taking a signal reading, you must also keep track of the network (2G, 3G, 4G, LTE, etc.) your phone is connected to.
    LTE readings are sometimes measured differently from previous generation networks so without knowing if a signal is from an LTE network or a previous network, it’s impossible to interpret the results.

iPhone Field Test Mode

The iPhone has a hidden built-in Field Test Mode app. To access it, follow these steps:

  1. Go into Settings > WiFi and turn WiFi Off.
    This will enable you to see what network you're connected to (2G, 3G, 4G, LTE, etc).
  2. For iOS 9.3 and above: Go into Settings > Cellular > Cellular Data Options > Enable LTE and turn LTE to Off.
    For iOS 9.2 and below: Go into Settings > Cellular > Enable LTE and turn LTE to Off.
    LTE readings can be very different from previous networks so to best interpret the results, we ideally want signal readings from a previous generation network. You can also repeat the site survey with LTE enabled, just be sure to keep track of which readings are which.
  3. Go into the Phone app and switch to the Keypad, as you would do to dial a phone number. Dial *3001#12345#* and press the Call button.
    This will launch the Field Test Mode app and where the bars/dots were in the top left corner of the screen, you'll now see a negative number. The negative number is the decibel signal strength reading and should be followed by the carrier name and then the network type.

If you see "‹ Back to Phone" in the top left corner

In iOS 9+, Apple updated the Field Test Mode app so that instead of showing the decibel signal strength in the top left corner, it now says "Back to Phone". To take decibel signal readings on an iOS9 or newer iPhone, follow the steps below under the heading Always show Decibels instead of Dots

When taking signal readings, move to the location where you want to take the reading, wait for 30 to 60 seconds for the signal readings to catch up, and then record the signal strength, carrier, and network type (2G, 3G, 4G, LTE, etc.).

To exit and return your iPhone to normal status, all you need to do is hit the Home button. Also remember to go back into Settings and enable WiFi and Cellular LTE. This mode is available on any iPhone running iOS 4.1 and all later versions.

Always show Decibels instead of Dots

If you want your iPhone to always display numerical signal strength instead of signal bars, you can perform the following process. Once in Field-test mode (accessed by entering and dialing the code above), hold down the power button until you see “Slide to Power Off”, then release it. Then hold the Home button until you’re returned to your main app screen. You’ll now see your numerical signal strength while you use your phone, and you’ll be able to tap the signal numbers to switch to signal bars, and vice versa. To exit this permanent field-test mode, simply reboot the phone or re-load Field Test Mode and exit it via the Home button.

Android Field Test Mode

Accessing Field test mode on Android phones is also straightforward. You simply need to find your way to “Settings” > “About Phone”, and your numerical signal strength will be available under either Network or Status, depending on the model of the phone you own. There’s usually another option near the signal strength that says the Network Type.

There are also apps you can download that will show you the signal strength in decibels along with other information. Go to the App Store and search for “cell signal” to find available apps for your device.

When taking signal readings, move to the location where you want to take the reading, wait for 30 to 60 seconds for the signal readings to catch up, and then record the signal strength, carrier, and network type (2G, 3G, 4G, LTE, etc.).

Field Test Mode on Other Phones

Most other phones also support a Field Test Mode. Some more popular phones are listed in the following document: Field Test Modes.

What Is It Used For?

The most common use of Field Test Mode is to conduct a special kind of field signal test known as a site survey. If you find yourself living or working in an area with weak signal strength on your device, and you want to boost your signal, performing a site survey will be useful before you choose a signal boosting system. The site survey involves taking several accurate readings of signal strength in and around your building, in order to calculate whether a cellular boosting system will help, and if so, what sort of equipment will be necessary.

Field Test mode is an immensely useful tool in such a situation - switching from unreliable signal bars to a precise numerical value is essential if you want to quantitatively assess your smartphone’s signal strength in different places.

Although a site survey field test is a multi-step process, it isn’t overly complex, and it’s possible to follow the steps and conduct your own test in and around you home or office, using the field test mode on your phone.

The Numbers Explained

When connected to a 2G, 3G, or 4G H+ network, this numerical value is known as RSSI (Received Signal Strength Indicator) and is a method of measuring wireless signals. LTE networks on the other hand, are typically measured in RSRP (Reference Signal Receive Power), which is why LTE readings are sometimes very different from previous generation signal readings. To learn more about the difference between RSSI and RSRP, check out this blog post.

Decibel signal strengths will generally be double or triple digits, and it will be negative. Some phones don’t show a negative sign so if your phone shows just a number such as 90, the signal strength is really -90. The closer the number is to zero, the better the reception, so -80 is a stronger signal than -90. The unit of measurement is the decibel (dB), which is a measure of the power of a signal. Similar to the volume of a stereo, the numerical value is really telling you exactly how “loudly” your phone is receiving the signal from your provider’s cellular network.

The decibel scale is not linear – an increase of 3 dB corresponds a signal that is twice as strong while a 10 dB increase corresponds to a 10 times increase in signal strength. So an RSSI value of -60 is actually ten times stronger than an RSSI of -70, for example.

Having an accurate reading of your Received Signal Strength is crucial when you’re assessing an area with the purpose of installing a possible signal boosting system. Being able to see exactly how strong your signal is, outside and inside your building, is essential in finding out how a cellular signal boosting system can improve your reception.