Analyzing iPhone 4 reception vs. signal attenuation
[Via Edible Apple]
Brian Klug & Anand Lal Shimpi of Anandtech have posted a thorough analysis of the the iPhone 4 antenna and how the device’s signal is affected when held in various positions. Setting up the testing environment, however, was a bit tricky since Apple removed the Field Test from the applications directory in the iPhone 4 filesystem.
Still, Klug and Shimi were able, after a bit of iPhone tomfoolery, to get a “numeric readout of signal strength on a non-jailbroken iPhone 4.” Here’s what they found.
To give you perspective, for a UMTS “3G” plant, -51 dBm is the best reported signal you can get – it’s quite literally standing next to, or under a block away from a tower. At the other extreme, -113 dBm is the worst possible signal you can have before disconnecting entirely. With a few exceptions, signal power as low as -107 dBm is actually perfectly fine for calls and data, and below that is where trouble usually starts. However, you can see just how little dynamic range iOS 4 has for reporting signal; over 40% of the range of possible signal levels (from -99 dBm to -51 dBm) is reported as 5 bars.
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One of the things they found was that, yes, there is greater attenuation of the signal holding the iPhone but the iPhone had higher sensitivity to begin with, so you could still make calls.
The writers at Anandtech had this to say:
From my day of testing, I’ve determined that the iPhone 4 performs much better than the 3GS in situations where signal is very low, at -113 dBm (1 bar). Previously, dropping this low all but guaranteed that calls would drop, fail to be placed, and data would no longer be transacted at all. I can honestly say that I’ve never held onto so many calls and data simultaneously on 1 bar at -113 dBm as I have with the iPhone 4, so it’s readily apparent that the new baseband hardware is much more sensitive compared to what was in the 3GS. The difference is that reception is massively better on the iPhone 4 in actual use.
Putting the antenna outside the phone provides greater reception of signal but also allows greater attenuation with the hand. So you gain a lot in signal strength and reception, some of which may be lost by the hand. On balance, people are reporting being able to receive and make calls in areas they were unable to before.
It seems that the main reason this was noticed had to do more with the way Apple decided to create its meter. It is actually not very informative since most of the signal strength will always be reported as 5 bars, even when you have had attenuation that corresponds to 4 orders of magnitude – dB is a log scale.
To go from 5 bars to 4, there is a loss of 10,000 in signal strength. But to go from 4 bars to 1 is a loss of only 5! In a weak area, you could go from 5 bars to 1 with only a loss 17 dB or about about a 50-fold reduction in signal strength.
In other words, at 5 bars, there can be a change in signal strength of up to 10,000 – about 40 dB – without seeing the bar change at all. But to go from 5 bars to 1, it could take a change in strength of 50 – about 17 dB.
Apple has set the meters such that people think they are in much better reception areas than they are used to. That is, the reception of signal can vary over a range of 10,000 without seeming to change the meter. But a simple loss of signal by 50-fold could take the meter from 5 bars to 1.
So, a lot of people in strong areas are reporting seeing no change in the meter, because a 20 dB attenuation by holding the phone will not appear to change the meter much when it takes up to 40 dB to change it from 5 bars to 4.
Simply adjusting the meter, so that the signal strengths for each bar are set up differently would make the perception quite different.
Any phone manufacturer could make the meter read as many bars as they want. Here, it appears that Apple may have decided that instead of giving each bar equal weight of about 15 dB, it overweighted the meter to make 5 bars cover 40 dB.
If it had weighted things evenly, then people would see a 3 bar dropping to 2 or 1 bars, not 5 bars dropping to 1. Not as much of a surprise.
Someone said the the meter readings are bogus on any phone because the manufacturer can make them read anything.
What is key, though, is that a phone can still make great calls even when highly attenuated if it starts with a higher signal reception.
A Man With A Ph.D. 