A word about receivers.

Apple’s iPhone 4 has a antenna problem. I’m not a radio expert but when I read about the external antenna design of the iPhone 4, my first thought was that they must have a somehow adaptive antenna. Touching and even moving something close to an antenna changes several factors important for both, sending and reception:

Placing anything near the antenna shields the signal or produces noise. This is the “fact of life” Steve Jobs was talking about. It affects all phones in the same way.

Touching the antenna changes its size. The longer, the better. But: Multiples of the wave length are better than values in-between. This factor has an impact on the iPhone 4. It seems however that this is not the main problem. If it was, placing a sheet of paper between finger and antenna would fix the problem. In some cases this worked but in many other it did not.

Electrical conductive objects near a antenna change its capacity. This is in my opinion the main problem of the iPhone 4. Other phones have their antenna inside the phone and the plastics around it prevents you from changing the near environment.

How do receivers work? They have to find the difference between noise and signal. The bigger the difference (The signal-to-noise-ratio), the easier to read the data. Due to production tolerances, every phone is slightly different. To compensate this, the receivers adapt to the environment they find themselves in. Usually this environment does not change too fast and the receivers can adapt their settings over time.

With the iPhone 4, this is different. You can touch the antenna and the receiver has to adapt within a fraction of a second to a completely different setup. I think, Apple thought about that. And they tried to solve that problem by adding an algorithm that detects sudden setup changes and triggers a fast reconfiguration of the receiver. However, in some situations, their algorithm seems to fail.

Changing signal

Take a look at the image above. Let’s simplify and assume, there is only one parameter you can change. That parameter is shown by the horizontal axis. Vertically, the graph shows the signal-noise-ratio. Higher values are better. The blue line represents the minimum you need to get a usable signal. The green line shows the situation before you touch the antenna. The algorithm was able to find the best setting and this results in a pretty good signal.

After touching the antenna, the signal changes as shown by the red line. It is still possible to find a setting that allows good reception but since the algorithm must maintain the connection, it can not simply test all possible settings. It does a neighborhood search. A little to the left: bad. A little to the right: Better. But better, in this case, is not good. The iPhone looses the connection.

Slow changes to the environment allows the receiver to “surf” the best signal as it does not jump outside the neighborhood.

If something like this is the root of iPhone 4’s antenna problem, I’m quite sure it can be fixed in software. Maybe we will see a chart similar to mine on the press conference today. No, we won’t.