7 - Wi-Fi dBm Calculations

The primary purpose of learning RF math is to enhance our capabilities as wireless engineers and gain a deeper understanding of wireless deployments. In this blog, we will examine things from a Wi-Fi perspective, ensure our comprehension of mathematical operations, practice additional calculations, and explore how this knowledge will impact our performance during wireless site surveys and signal readings.

Let's consider a scenario where a Wi-Fi access point is transmitting at 200 milliwatts. What dBm value would accurately represent this power level?

Rule of thumb

0 dBm = 1 mW

+10 = x10 power

-10 = 1/10 power

+3 = x2 power

-3 = /2 power

So, remember that 0 dBm represents 1 milliwatt. From here, we need to reach 100 milliwatts, and we have two options.

We can either use a factor of 10, which increases power by tenfold, or we can use a 3 dBm change, which corresponds to a 2x factor. To reach 10 milliwatts and then 100 milliwatts, we apply a 10x factor and another 10x factor. Finally, we need a 2x multiplier to get us to 200 milliwatts.

So, we add 10 dBm, then another 10 dBm, and finally, a 3 dBm increase. This totals 23 dBm. That's how we solve these math problems.

Answer: 200 mW = 23dBm (10 ‘10mW’ + 10 ‘100mW’ + 3 ‘200mW’)

There are plenty of online calculators that can provide us with these numbers for practice, ensuring that we are getting the correct answers. However, practicing with random numbers helps us understand basic calculations, which in turn improves our understanding of these values.

Let's consider another example: How many dBm is equivalent to 250 mW?

• We know that 0 dBm is equal to 1 mW.
• Adding 10 dBm gives us 10 mW.
• Adding another 10 dBm gives us 100 mW.
• Adding another 10 dBm gives us 1000 mW.

Now we have exceeded our requirement of 250 mW, so we need to apply the other rule (negative 3 dB reduces power by half).

• A negative -3 dBm corresponds to 500 mW.
• Another negative -3 dBm corresponds to 250 mW.
• Hence, the answer is 24 dBm (0 + 10 + 10 + 10 - 3 - 3), which can be calculated as follows:

Answer: 0 dBm (1 mW) + 10 dBm (10 mW) + 10 dBm (100 mW) + 10 dBm (1000 mW) - 3 dBm (500 mW) - 3 dBm (250 mW)

Now, on the other side, we must consider the concept of negative dBms, as mentioned in my previous blog. Many of our wireless readings will indeed be negative.

For instance, let's assume that we're broadcasting at 200 milliwatts. However, if the signal reaches our wireless receiver, we may find that we're receiving a signal at -53 dBm. This raises a question: How many milliwatts does -53 dBm represent?

At the end-user level, signal levels are expressed in negative dBm values. Therefore, it's equally important that we know how to perform calculations to convert them into mW values.

So, for the signal at -53 dBm, we can start by examining the tens place because we know that every 10 dB decrease corresponds to a division by 10. So, we can begin with -50 dB and account for the remaining -3 dB later.

Again our “frame of reference” is going to be 0dBm = 1mW

If we start with 1.0 milliwatt, we move the decimal point to the left five times, which is equivalent to dividing by 10 five times (1st time -10dBm: 0.1, 2nd time -10dBm: 0.01, 3rd time -10dBm: 0.001, 4th time -10dBm: 0.0001, 5th time -10dBm: 0.00001). Now, our decimal is in the correct position, and we remove the trailing zeros. Thus, the answer is 0.00001 milliwatts, representing -50 dBm.

Now, considering the remaining -3 dB, we know that this involves dividing by 2. Starting with the result from -50 dB (0.00001 milliwatts), we divide it by 2. This results in 0.000005 milliwatts, which corresponds to -53 dBm.

It's important to note that while we use these calculations to understand the concept, we typically do not use them directly for reading wireless signals. It's more practical to interpret -53 dBm simply as a negative dBm value.

What constitutes a good wireless signal? And what represents a bad wireless signal? Even though we're trying to move away from using dBm numbers like -53, it's crucial to understand these values.

Signal Strength Analysis:

Going from 23 dBm to -53 dBm may seem like a significant drop. But, surprisingly, -53 dBm is an excellent signal. It's rare to encounter a signal that strong in most wireless environments. Generally, a signal of -72 dBm or stronger is considered good.

Special Considerations:

However, certain scenarios demand even better signal quality. For real-time applications like VoIP calls over Wi-Fi, we may require -67 dBm or better. For improved data throughput, we might aim for -62 dBm or similar values. Achieving signals in the negative 50s is often challenging.

Why Use dBm?

Using dBm instead of milliwatts may seem odd, but it provides a more human-readable and linear way to measure signal strength. As we move away from an access point, dBm values decrease linearly, allowing us to make informed estimates about signal behavior.

Predicting Signal Propagation:

Understanding dBm lets us make predictions about signal propagation. For example, if we know that drywall reduces the signal strength by 3 dBm, we can anticipate signal changes in a wireless design when such barriers are introduced.

Math Operations:

To become proficient in working with dBm values, practice converting between dBm and milliwatts. Websites that provide these conversions can be helpful. Remember that increasing dBm is multiplication while decreasing dBm involves division.

Closing Thoughts:

In summary, dBm values offer a human-readable and linear method for measuring signal strength, helping us make predictions about signal propagation. Becoming comfortable with dBm math is crucial for effective wireless engineering.

Below is the table that provides dBm to mW calculations and vice versa.

Linear Growth	Exponential Growth
decibel-milliwatts (dBm)	milliwatts (mW)
-90	0.000000001	Unlikely Connection
-89	0.0000000012
-88	0.0000000015
-87	0.0000000019
-86	0.0000000025
-85	0.0000000031
-84	0.0000000039
-83	0.000000005
-82	0.0000000063	unstable connection
-81	0.0000000079
-80	0.00000001
-79	0.000000012
-78	0.000000015
-77	0.000000019
-76	0.000000025
-75	0.000000031
-74	0.000000039
-73	0.00000005
-72	0.000000063
-71	0.000000079
-70	0.0000001	Light Browsing and Email
-69	0.00000012
-68	0.00000015
-67	0.00000019	Min for Voice and non-HD Video
-66	0.00000025
-65	0.00000031
-64	0.00000039
-63	0.0000005
-62	0.00000063
-61	0.00000079
-60	0.000001	Good reliable Signal
-59	0.0000012
-58	0.0000015
-57	0.0000019
-56	0.0000025
-55	0.0000031
-54	0.0000039
-53	0.000005
-52	0.0000063
-51	0.0000079
-50	0.00001	Excellent Signal
-49	0.000012
-48	0.000015
-47	0.000019
-46	0.000025
-45	0.000031
-44	0.000039
-43	0.00005
-42	0.000063
-41	0.000079
-40	0.0001
-39	0.00012
-38	0.00015
-37	0.00019
-36	0.00025
-35	0.00031
-34	0.00039
-33	0.0005
-32	0.00063
-31	0.00079
-30	0.001	Perfect Signal
-29	0.0012
-28	0.0015
-27	0.0019
-26	0.0025
-25	0.0031
-24	0.0039
-23	0.005
-22	0.0063
-21	0.0079
-20	0.01
-19	0.012
-18	0.015
-17	0.019
-16	0.025
-15	0.031
-14	0.039
-13	0.05
-12	0.063
-11	0.079
-10	0.1	Wireless routers emits about 0.1 W of power or Lower
-9	0.12
-8	0.15
-7	0.19
-6	0.25
-5	0.31
-4	0.39
-3	0.5
-2	0.63
-1	0.79
0	1
1	1.25
2	1.58
3	1.99
4	2.51
5	3.16
6	3.98
7	5.01
8	6.3
9	7.94
10	10
11	12.58
12	15.84
13	19.95
14	25.11
15	31.62
16	39.81
17	50.11
18	63.09
19	79.43
20	100	Bluetooth Transmit Power Min
21	125.89
22	158.48
23	199.52
24	251.18
25	316.22
26	398.1
27	501.18
28	630.95
29	794.32
30	1000	1 Watt - cell phone emits about 1 W & outdoor APs
31	1258.92
32	1584.89
33	1995.26
34	2511.88
35	3162.27
36	3981.07
37	5011.87
38	6309.57
39	7943.28
40	10000
41	12589.25
42	15848.93
43	19952.62
44	25118.86
45	31622.77
46	39810.71
47	50118.72
48	63095.73
49	79432.82
50	100000
51	125892.54
52	158489.31
53	199526.23
54	251188.64
55	316227.76
56	398107.17
57	501187.23
58	630957.34
59	794328.23
60	1000000

Happy Learning!!