A Simple Guide to Understanding Signal Strength Terms
When you’re working with radio frequency systems,whether you’re managing a cell signal booster, planning a DAS (Distributed Antenna System), or checking network performance, two terms will keep showing up: dB and dBm. They sound similar, but mixing them up can lead to some serious misunderstandings.
Let’s break down what each one means, why they matter, and how to use them correctly in real-world wireless and RF setups.
What Is dB?
dB stands for decibel, a relative unit of measurement. It tells you how much something has increased or decreased compared to something else. It doesn’t tell you the actual power level, just how that power level compares to a baseline.
You can think of dB as a way to describe gain or loss. For instance:
- If your signal is +10 dB, it’s ten times stronger than the reference.
- If it’s -3 dB, it’s roughly half as strong.
One of the reasons dB is used so widely is because RF systems often deal with very large or very small numbers, and working in decibels makes everything more manageable.
A few common examples:
- Amplifiers are rated in dB of gain.
- Cable loss is measured in dB.
- Antenna performance is often specified in dB.
The key thing to remember? dB is always comparing two things, and unless you know what the baseline is, the number alone won’t tell you much.
What Is dBm?
dBm, on the other hand, is an absolute measurement. It tells you exactly how much power you’re dealing with, and it does so relative to one milliwatt.
So:
- 0 dBm = 1 milliwatt of power
- +10 dBm = 10 milliwatts
- +20 dBm = 100 milliwatts
- -10 dBm = 0.1 milliwatt
Because it’s fixed to 1 milliwatt, you can always take a dBm reading and know exactly how strong your signal is, no comparison necessary.
Most RF equipment, like cell phone signal boosters or radio transmitters, lists output power in dBm, because it gives a clear, direct measurement.
In a typical mobile environment:
- A strong signal might be around -50 dBm
- A usable signal is usually -85 to -95 dBm
- Anything weaker than -110 dBm starts causing dropped calls and slow data
The Practical Difference
Here’s how it works in practice.
Let’s say your cell booster receives a signal of -95 dBm from the tower. Your amplifier then boosts it by +20 dB. The final signal power is:
-95 dBm + 20 dB = -75 dBm
That signal is now strong enough to be useful inside your building.
So while dBm tells you the actual signal strength, dB tells you how much that signal is being increased or decreased as it moves through the system.
Why This Distinction Matters
If you confuse these units, you might misread your system’s performance, or worse, damage your equipment.
For example:
- Thinking an amplifier outputs +70 dBm when it actually outputs +70 dB gain is a huge mistake. That would suggest you’re pushing out 10,000,000 milliwatts, which is impossible and unsafe.
- When installing coaxial cable, knowing it causes 3 dB of loss helps you calculate your final signal strength in dBm. That loss matters when every dB counts.
In short: dBm tells you what you have, and dB tells you what changed.
Real-World Scenarios
Let’s say you’re setting up a public safety BDA system in a large building. You test the rooftop antenna and get -90 dBm from the nearest tower. Not great, but workable.
Your system includes:
- A booster with +70 dB gain
- 200 feet of coaxial cable that adds 6 dB of signal loss
- An indoor antenna with 3 dB of gain
The final signal hitting your indoor antenna would be:
-90 dBm + 70 dB – 6 dB + 3 dB = -23 dBm
That’s a strong indoor signal.
You’ll also hear these units used in:
- Antenna specs (dBi and dBd, both based on dB)
- Wireless access point settings
- Power outputs from RF transmitters
- Link budget calculations
Understanding the difference between dB and dBm helps you interpret all these readings correctly and design better systems.
Quick Conversion Table
| dBm | Power in Milliwatts |
| 0 dBm | 1 mW |
| +10 dBm | 10 mW |
| +20 dBm | 100 mW |
| -10 dBm | 0.1 mW |
| -20 dBm | 0.01 mW |
| -30 dBm | 0.001 mW |
As you can see, every 10 dB represents a 10x change in power.
Common Mistakes to Avoid
- Don’t treat dB and dBm like interchangeable units. They serve completely different roles.
- Don’t forget cable and connector losses. Those dB losses reduce your signal power (in dBm).
- Don’t just look at signal strength without considering gain. A system might show weak dBm, but still function well if there’s enough dB gain downstream.
Final Thoughts
Understanding dB vs. dBm isn’t just for engineers, it’s a key part of anyone installing, maintaining, or evaluating wireless systems. Whether you’re in charge of boosting cellular signals in a warehouse, deploying a BDA system in a high-rise, or installing a new antenna for better Wi-Fi coverage, you’ll make better decisions when you know how to read the numbers.
In short:
- Use dBm to measure your signal.
- Use dB to describe what’s happening to that signal.
Get those two straight, and you’ll have a much clearer view of how your RF system performs, and what to adjust when something goes wrong.
