The Antennagate Code Patch ⇥ hachyderm.io

Yes, it is 2025, but Sam Henri Gold decided to pick apart the changes between iOS 4.0 and iOS 4.0.1, which changed the number of bars shown in moderate-to-low reception areas:

The actual calculation is dead simple. When converting signal strength to bars, CommCenter loads each threshold from memory and compares until it finds the right range.

[…]

For example, here you need -107 [dBm] or better signal to see 3 bars.

These values are not the same as those found by AnandTech in 2010, but they are close. And, as Richard Gaywood pointed out at the time, the effect of the attenuation could bring a five-bar signal down to a single bar simply because iOS used to display five bars even in areas with mediocre coverage.