The iPhone 4 Fiasco
by Mark W. Hibben
A Simple and Inexpensive Solution
Almost as soon as the iPhone 4 became available to the public, technical problems surfaced that are very much related to the innovative design of the phone: the use of glass for the phone body shell and the use of the external metal band for the phone antennas. Users have reported shattered glass from merely dropping the phone. Many more users have reported cellular signal loss just by touching certain parts of the metal frame. Both problems are real, and Apple’s official response has left much to be desired. Apple management appear once again to be exhibiting an institutionalized inability to acknowledge any deficiency in their products, thereby compounding what already has the makings of a public relations disaster. The irony of this is that the solution to both problems is so easy and cheap: a simple rubberized sheath that fits over the back of the phone that protects the back glass plate and insulates the metal frame. This solution only slightly diminishes the cosmetic appeal of the phone but greatly enhances its functional utility. Apple sells a rubber “bumper” for the phone which is not as good since it doesn’t protect the back of the phone, but does at least solve the antenna signal problems. Apple should be giving these away with each phone, rather than forcing customers to shell out an additional US$29 for them.
The Problem with Glass
The iP4 promotional video shows the glass back plate bending under pressure without breaking. Yes, we realize that glass can be very strong, but it’s also brittle and easily shattered. Perhaps customers will just accept this limitation and learn to live with it, buying a protective sheath or case if need be. This seems to be a case of Apple going overboard in their Quest for Thin. I can only hope that Apple opts for a more durable alternative, such as graphite composite sheet. Composite sheet is a relatively cheap commodity these days, and is available in the right thickness for this application. It’s lighter than glass, just as rigid, and it won’t break even if you hit it with a hammer. Plus, it just looks cool.
The problems with the iP4’s GSM cellular network antenna appear to be more vexing for the consumer and more difficult to solve within the existing design. The stainless steel metal frame is composed of two separate machined pieces, with little gaps separating them, which are the black strips visible from the outside.
There’s been some confusion about the purpose of the black strips, with some people thinking that they are the antennas for the phone. The black strips are a way to join the pieces of the frame together while maintaining electrical isolation between them, as I show in this close-up of a typical junction.
As shown in the exploded view of the frame adjacent, the larger J-shaped piece serves as the GSM/UMTS antenna, and the smaller inverted L-shaped piece serves as the antenna for the Bluetooth, Wi-Fi, and GPS radios.
When a user experiences difficulty with signal strength changing depending on how the phone is held, there are two effects that can come into play: 1) the spatial orientation of the phone, and 2) the way the phone is gripped in the hand.
All radio antennas have a variability in reception/transmission signal strength depending on the orientation of the antenna with respect to the location of the other transmit/receive antenna making up the two-way radio link. The simplest antenna is just a straight piece of wire (technically referred to as a monopole) that has a length equal to ¼ the wavelength of the radio frequency being used for the link. If the wire is pointing straight up (perpendicular to the ground), then the antenna reception has no preference in the azimuthal (think of a compass bearing) direction. For any given straight line distance between transmitter and receiver, the monopole antenna has no preference for direction north, south, east or west. The antenna gain is not spatially uniform however, but merely rotationally symmetric about an axis parallel to the direction of the wire, as shown above where the z axis is in the wire direction.
If the monopole antenna gets flipped on its side, the donut pattern above becomes more like a tire shape as mounted on a car. Now the antenna has a strong azimuth signal dependence, where the optimal orientation of the antenna is perpendicular to the line of sight between transmitter and receiver.
The spatial antenna gain pattern for the iP4 GSM antenna is more complex than the simple monopole pattern shown above, and is also influenced by objects nearby, especially the body of the phone user. Nevertheless, we can get some idea of the iP4 radiation pattern by regarding the GSM antenna as a monopole antenna with the wire oriented along the length of the phone.
To reduce azimuth angle sensitivity, the GSM antenna piece should be oriented vertically. When holding the phone to your ear, that’s more or less what you would have. I point this out because many demonstrations of connection problems that have been shown on YouTube have the phone lying flat on a table top or in the palm of the user’s hand. If you're holding the phone away from the body while making a call, it’s probably best to hold it vertically in “portrait” rather than “landscape” orientation to reduce azimuth angle dependence.
Finger Contact Effects
When a finger or palm or metal object is placed over the insulating gap on the lower left side of the iP4 frame, the insulating gap is bridged, and the GSM antenna is effectively changed. The antenna effectively gets larger and changes shape by virtue of the electrical addition of the other side piece of the frame. Both of these effects (effective size and shape change) may reduce the radiated power in the direction of the cellular radio tower. Likewise radio waves from the cellular tower may not be as efficiently collected by the “altered” antenna, causing deterioration in the signal strength measured at the phone. Furthermore, depending on the circuitry attached to the other frame piece, some of the radiated power from the GSM antenna may be dissipated within the phone itself, rather than being efficiently radiated into space. Likewise, received signal may also be dissipated in unintended locations of the iPhone circuitry rather than being funneled into the GSM radio receiver circuits. As of now, I have no way of knowing which of these effects predominates in causing signal deterioration.
Readers may wonder how it is that skin contact can bridge the metal pieces of the frame seemingly as efficiently as a metal object. Here, the problem is the high frequency of the radio waves used for GSM cellular networks, 850 MHz to 2100 MHz. At these frequencies, human skin can conduct the waves over short distances with little loss, even though skin is not particularly conductive for direct electrical current. Because of the construction of the frame, with relatively small gaps (~ 1 mm), the radio waves can jump across the gap, using the skin as the conductor, with relatively little loss, about 3-5% by my calculations. I’m sure this was never considered when the frame design concept was being developed.
What to Do
I’ve read suggestions that the finger contact problem is due to the lack of an insulating coating being applied to the stainless frame. I doubt that this is correct. At the GSM frequencies being used, the radio waves can very effectively couple capacitively into the skin through non-conductive membranes. A thin insulating film will not suffice. A coating would also be difficult to apply to stainless steel and would peel or chip easily. A much better solution is the thick (2-3 mm) insulating bumper or sleeve. This should be sufficient to minimize capacitive coupling into the hand, as well as help protect the phone. It’s a rare opportunity when the same approach can simultaneously solve two completely unrelated problems. Of course, that still leaves unresolved the third and biggest problem: Apple’s presumption of infallibility.