iPhone 4 Attenuation Only Half the Story

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Something about Apple’s press conference yesterday just didn’t sit right with me. Apple has put up a page with videos of various other smart phones displaying the same type of behavior when griped in a certain way. It also has put up a page where its explains itsĀ $100 million dollar testing facilities it uses for testing reception and signal in various conditions, just to let us know how much the company cares. Steve Jobs said that they love us. They seem to be doing the right thing by giving out the free bumper cases, but how they explained why the cases are needed in some instances didn’t quite cover everything. Attenuation is only half the story.

Way back when, a couple of lifetimes ago, I was a Radioman in the Navy, and as part of my education and advancement requirements I had to study antenna and wave propagation theory. For the sake of brevity, I’m going to over-simplify this to the point where real engineers might get a headache if they continue on, but here goes anyway.

Radio signals traveling through the air look like waves if drawn on paper. These waves travel at a constant speed, the speed of light, so to send more waves through on a signal, the waves must be smaller. The number of waves traveling along a signal is referred to as hertz, and the size of the wave is its wavelength. The antennas used to generate and receive these waves need to be the right size, and the right shape. Back when I was in the Navy, we were transmitting waves that required a 35 foot whip antenna because we were transmitting in the High Frequency (HF) range. As the frequency of the wave gets higher, the size of the waves and the antenna used to transmit and receive them become smaller, and more precise.

Cell phones operate in the Ultra High Frequency (UHF) frequency range, so the antenna that they use is much smaller than lower frequency transmissions. The iPhone 4 has two antenna that wrap around its case, one for cellular use and the other for Wi-Fi and bluetooth. The two antennas are separated by the small gap in the lower left hand side of the case, which Apple has identified as the antenna’s most sensitive part. This is true, in part because of the attenuation (or “blocking of the signal”) when you put your hand over the gap.

The other part of the equation that Apple is not talking about is that while your hand doesn’t make a particularly good medium for radio waves to travel through, it does make a fairly good electrical conductor. When you place your hand over that gap, you are actually bridging the two antennas together and making a larger antenna. A larger antenna that is not the right size for the frequency of AT&T’s cellular network, and the bars drop right off.

I don’t have the equipment on hand anymore to test this, but the video below, linked to by an expert on antennas (via Daring Fireball), seems to show the behavior I would expect. At around the 1:30 mark, a key placed over the gap drops the iPhone 4’s reception down from five bars to one, and the narrator says that eventually there will be no signal at all. When the key is removed, the bars return.

The free bumpers solve both problems for the iPhone 4. They prevent the antenna gap from being bridged by anything conductive, and they give a little more room between the antenna and your hand, to help with attenuation. Unfortunately, the bumpers do not address the actual design of the antenna. It may be possible for Apple to move the antenna gap to a different spot on the phone in future revisions of the iPhone. For example, why not put the antenna at the top of the phone where people are less likely to hold it? As answered by AntennaSys in the link above, physical placement of the antenna is mandated by the FCC. If bridging the antenna gap is the problem, it may be possible to move the gap to the bottom of the phone, but since that would change the shape of the antenna, I’m not sure if that’s possible or not. Anandtech has a beautiful solution using Kapton insulating tape, which makes the iPhone look like it’s been plated in gold. If Apple were to add a layer of insulation to the iPhone, that might alleviate the symptoms.

I’m not the only one who thought that Apple’s explanation seemed lacking. TidBITS writer Rich Mogull has a very detailed article where he draws a similar conclusion. If you’d like to know more about the issue, I’d suggest dropping his article into Instapaper. If you’ve got any ideas about the iPhone 4’s antenna “situation,” I’d love to hear from you in the comments. Especially if anyone can reproduce the key trick from the video.

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