What does WiFi signal -65dBm mean? -60 is ten times better than -70, not 10; dB, dBm, and dBi finally distinguish

dB is a gap , without benchmarks. Saying '90dB attenuated' means the power has become one billionth of its original power; Saying 'difference of 3dB' means a difference of double. It's not a reading, but the distance between two readings. Remember this difference in one sentence: dBm measure height, dB measure height difference .

commonly used conversions are listed here; I will repeat them later:

doubling for every 3dB difference: the gap between -60 and -70 is tenfold, not 10

this is the most counterintuitive one. On a logarithmic scale, a difference of 3dB is a difference of one double, a difference of 10dB is a tenfold difference. So -60 and -70 are not just '10 differences', but a tenfold difference; My living room is -44 to bathroom -79 ( the ledger from the blind spot self-check), a difference of 35dB, which adds up to over 3,000 times.

this conversion rule explains two things I've always been vague about. One is the misleading signal bars: a phone drops from four bars to three, possibly only 3dB, with power cut in half but the feel almost unchanged; Dropping from three bars to one often means a difference of more than ten dB, and the rate has long since dropped from hundreds of megabytes to tens of megabytes. Another issue is the router and phone secretly shifting gears (QAM article covered the entire gear chart): each step of modulation consumption consumes about 6dB of signal-to-noise ratio, so the signal drops sharply, speeds drop rapidly, and the network speed is cut in half, losing only one bar. Watching the frame count is always a beat slow; watching dBm gives you a chance to get a headshot.

by the way, here's a pitfall I made myself: a couple of years ago, when choosing antennas, one was marked 5dBi, the other was 8dBi. I thought, 'Isn't it just 3 inches short? I'll buy the bigger one with my eyes closed.' Later, I realized that 3dB is the account of doubling power—but that's not the calculation at all; the dBi section is a separate topic.

three scenarios for a chain of Suantong: 20dBm when you go out, 90dB on the road, and the final 70dBm

link the gap with the reading, and the whole chain connects. The national standard for 2.4G router transmission power is 20dBm, which is 100 milliwatts ( the transmission power article I read the national limit in the table). After the signal goes out, every loss is a dB account:

5 meters from the same room, just the "radio spreading in all directions" means the 5G band consumes about 62dB. There is a ready-made formula: free space loss equals 32.44 plus 20 times the logarithm of distance kilometers, plus 20 times the logarithm of frequency megahertz. Substituting 5 meters at 5800MHz gives 62dB. 20 minus 62 equals -42. In my living room, it was -44, with a margin of error of 2dB. The formula is surprisingly reliable. Eat another 10 to 15 after a brick wall, restaurant test -55; Adding distance to the load-bearing wall in the master bedroom, actual measurement -71. All five points of reading can be reconciled, and this unit is instantly revitalized.

the same formula, calculate 2.4G: the same 5 meters only lose 54dB, which is 8dB less than 5G, and the wall penetration also absorbs less than 5G. This is the physical ledger of "2.4G wall penetration is good" (the from the frequency band test article), not mystical but dB-recorded.

looking at the fiber optic chain, the style is completely different. The optical modem transmits power around +3dBm (visible in the background, usually between 0.5 and 5), and at my home it receives -22.5dBm, with only a 26dB drop throughout the process. Why does wireless lose 90 while fiber only drops 26? The laser is locked inside a fiber core as thick as a hair, running by total reflection and preventing leakage; Radio waves really spread throughout the entire house. Moreover, of the 26dB fiber optic fiber, the main end isn't even a wire—the 1:64 splitter in the hallway naturally consumes about 20dB (dozens of households share one beam of light per block at a building, theoretically 18dB, but the engineering leaves a 20dB margin). The remaining dB is the fiber itself (0.35dB per kilometer) separated from the fusion joint.

So I have an answer to my cousin's question: media are different, and family finances are different Fiber -22.5 is only 26dB away from the transmitter, so the link budget is generous and healthy; the -71 fiber is 90dB from the 20dBm transmitter, so the phone's reception sensitivity is already struggling. Light decay -27 and WiFi -71 are both called "still usable," but you can't directly compare the values to size.

Four Tiered Tables: For the same unit, check its own table

dBm are everywhere, but each health zone is independently calibrated and cannot be seen in a single context. I have compiled a table of four frequently checked items at home:

table hides a detail that needs to be singled out: SNR is measured in dB, not dBm. Because it's calculated by subtracting two dBm — signal -70dBm minus noise at -97dBm, subtracting two absolute readings and canceling out the baseline, leaving a pure difference of 27dB. dBm subtracting dBm gives dB; adding or subtracting dB from dBm is still dBm. This logarithmic arithmetic follows just these two rules: think of the last m of the unit as 'meters', subtract the absolute value from the difference, and it works smoothly in one minute.

dBi is a different matter: the gain doesn't deliver power, only the spread method is changed

router antenna labeled dBi, the 'i' at the end means 'relatively ideal omnidirectional antenna.' 0dBi is essentially a theoretical perfect spherical ejection, spreading evenly in all directions; 5dBi isn't about increasing power; it's about flattening the sphere into a pie shape, thicker horizontally, and thinner vertically. about antenna gain in the article I tested switching to three antennas. After switching to 9dBi, the bedroom signal shifted from -72 to -70, because all the extra gain was filled horizontally, making vertical upstairs and downstairs even thinner.

also has dBd, which is based on half-wave dipole antennas; 0dBd equals 2.15dBi. Sellers love to label dBi because the same antenna is labeled one size larger than dBi, which looks intimidating. When choosing an antenna, first identify the last letter of the wire, then compare the numbers.

the real power ceiling is the national standard EIRP—Equivalent Omnidirectional Radiation Power, which equals the dBm number of transmitted power plus the antenna's dBi number. 2.4G is limited to 20dBm, 5.8G to 33dBm, and both are limited to this general ledger. So when the antenna gain is increased, the machine has to lower the transmission power to make up the total, and the ceiling is welded shut from the factory. This is also the root of the 878 article's long test where "wall-penetration mode only yields two or three dB."

order of checking the numbers: set the release first, then get started

use this unit system, the order of household inspections can be half the hassle of fussing around.

WiFi slow, first take out your phone to check dBm, which is much more solid than signal bars. Don't blame the signal for below -50; check channel interference ( packet loss method); Between -60 and -70, change routes and locations, and follow the blind spot self-inspection method; After -70, just consider overlaying solutions, Mesh or relay, and stop competing with a single router. At home, whether to choose between 2.4G and 5G, it's all about using dBm.

the optical modem, if the foundation is unstable during network disconnection, it reads the light degradation and handles it according to the table above. -28 is a hard line; fluctuations inside the line should be smoothed out first, then repaired outside the line.

At the end of the

, I left three sentences, which is enough: dBm look at absolute, dB looks at the gap, 3dB doubles, 10dB multiplies tenfold . Next time you see a reading of negative tens, don't panic—it's not 'how many signals are owed,' it's the abbreviation for one millionth of a milliwatt—humans invented this awkward but truly useful scale to avoid writing a string of zeros.