Make your own signal map of where coverage is weak

A personal signal map is a set of real signal readings you take on your own routes, pinned to the places where you took them. It shows exactly where coverage is weak for your phone and your carrier, which a carrier's coverage map cannot. You build one by recording readings as you go about your normal week, then looking for the patterns.

Why make your own signal map?

Carrier coverage maps are predictions. They are made from computer models of masts, terrain and buildings, and they usually show outdoor coverage at street level. They are useful for a quick look, but they cannot know about the dip on your platform, the dead stretch on your commute or the corner of the office where calls always fail.

Your own record is different because it is:

• Measured, not modelled. Each point is a reading your phone actually took.
• Specific to your phone and SIM. Different phones and carriers behave differently in the same place.
• About the places you go. Home, work, the school run, the gym, the route in between.
• Time-stamped. You can see if a place is fine at noon but poor in the evening.

What each reading should record

Every point on your map should capture a few details. Without them, two readings in the same spot cannot be compared fairly.

• Signal strength in dBm. The number behind the bars. What dBm means explains the scale.
• Network type. 4G, 5G or an older network.
• Carrier. Useful if you have two SIMs or test a second network.
• Band and cell ID, if available. These show which mast and which frequency served you.
• Date and time. Busy hours can change the picture.
• Indoors or outdoors. Note this, because walls make a big difference.

How to build your signal map, step by step

1. Choose your routes. List the five to ten places and journeys that matter most to you, such as home, work and your commute.
2. Decide how you will hold the phone. Keep it the same way each time: in your hand at chest height, or in the same pocket, for example. Mixing methods muddies the results.
3. Record readings at each stop. Pause for about 30 seconds and note the typical value, not the best one.
4. Add readings along the way. On a walk or bus ride, take a reading every few minutes, or let an app log them as you go.
5. Repeat on different days and times. Aim for at least a weekday morning, a weekday evening and a weekend.
6. Mark the weak spots clearly. Anything below about −105 dBm on 4G is worth flagging, along with places where the phone dropped to an older network or lost service.
7. Review the map after a week or two. Look for patterns rather than one-off bad readings.

Reading the patterns

Once you have enough points, the map starts to tell a story.

• A weak patch that is always weak points to a gap in coverage, often caused by terrain, a tunnel or a building in the way.
• A spot that is weak only at busy times suggests congestion rather than a gap.
• A sudden change across many points after a certain date may mean a mast was changed, moved or switched off.
• Good outdoor readings but poor indoor readings at the same address point to the building itself. See why signal is weak inside buildings and basements.

What to do with the map once you have it

A map is only useful if it leads to a decision. Here are the most common ones:

• Plan around weak spots. Download music, maps and podcasts before you reach the dead stretch.
• Pick better places for calls. If a café on your route always has strong signal, make important calls there.
• Compare carriers. Borrow a friend's SIM or use a pay-as-you-go SIM on the same routes, and compare the two maps. How to compare carriers' coverage where you live covers how to make it a fair test.
• Report problems with evidence. A map with dates, times and readings is much more convincing to a carrier than "the signal is bad". Keeping a signal history explains why.

Privacy: keep your map to yourself

A map of your readings is also a record of where you go. Keep it on your phone, think twice before sharing a full map publicly, and when you send it to a carrier, share only the area and dates that relate to your complaint.

How CellRadar helps

CellRadar puts your own readings on a signal map, with offline maps and place search, so you can see weak spots on your routes at a glance. The live readout shows strength in dBm, network type, carrier, band and cell ID. The signal history can be exported as a PDF or CSV report, and your readings and saved places stay on your phone, with no account needed.

Frequently asked questions

How many readings do I need for a useful map?

A few readings per place, taken on different days and at different times, is enough to spot patterns. More is better on long routes.

Is my map more accurate than my carrier's coverage map?

For the exact places you measured, with your phone and SIM, yes. A carrier map is better for a quick look at areas you have never visited.

Do I need mobile data to build a signal map?

No. Signal readings come from the phone's radio, not from data. You only need data, or offline maps, to see a background map.

Can I compare two phones on the same map?

You can, but note which phone took each reading. Phones differ by a few dB in the same spot, so compare like with like.