Editor’s Note: The following article is reprinted from Network World.
If I buy an iPhone 4, am I going to have big radio frequency reception problems, apart from being on AT&T’s network?
It depends on how you define “big” and on a whole bunch of other variables, such as what 3G frequency you’re on, maybe your body mass index and even whether your hands are sweaty. There have been a rash of complaints from new iPhone 4 users that when they wrap their hand around the phone to take or make a call the phone’s signal strength indicator — those bars on the screen — can show a big drop.
What do those bars actually mean?
Apparently, not very much, at least from an RF engineering viewpoint. There’s been an interesting discussion online about this issue. The consensus is that Apple’s bar display of signal strength is highly relative: it’s actually showing only a small part of the lowest end of possible signal strength, according to two bloggers, Steve Gibson, and Simon Byrnand. (Byrnand first identified potential antenna problems in a June 8 post on the Talk3G mobile phone forum.)
That seems weird, doesn’t it?
Keep in mind that the five-bar display—and this is true for all cell phones, not just iPhone 4, according to Byrnand—is in effect “only supposed to give a relative indication of likely call quality—any signal stronger than 5 bars whilst stronger, won’t lead to better call quality so [it] isn’t indicated to the user to keep things simple,” he writes.
(According to Byrnand, the iPhone operating system used to have a very detailed signal strength reader, a hidden app that was activated by dialing a specific numeric string and capable of showing the exact received signal strength in –dBm. But it was removed from an early beta version of iOS 4.)
To get a good, clear conversation, Gibson says, you might only need 5 percent of the signal strength from a 3G base station. So the “5-bars” covers any signal strength from 100 percent down to 5 percent. “It’s only when the received signal strength begins to drop below 5 percent that conversations suffer, calls get dropped, and Apple starts to take bars away from their 5-bar display,” Gibson writes.
So those Youtube videos that show the bars disappearing from the iPhone 4 when you hold it…?
Gibson speculates that in those videos, the phone’s signal strength is actually very, very low to begin with. So when you cover the antennas with your hand, or when your skin “bridges” the cellular and Wi-Fi antennas, it’s enough to cause the signal to drop still lower, and that’s when Apple’s “five bars” becomes “three bars” or two or none.
So what’s actually happening?
First, keep in mind that with iPhone 4, Apple integrated several antennas into the exterior stainless steel band around the phone’s edge, which is “cut” in three places by a dark slot of insulation. During the phone’s public unveiling, Apple claimed that moving the antennas to the outside of phone would actually improve reception.
It seems so, though other factors are involved, including the use of a glass back for iPhone 4 instead of the plastic in the 3GS model, and AT&T’s ongoing cellular upgrade to High Speed Packet Access (HSPA). One blogger, an antenna design specialist named Spencer Webb, says the design change “probably improves” the uniformity of the antenna’s radiation pattern “but only when the phone is suspended magically in air.”
PCWorld did a pretty extensive, but completely unscientific, field comparison in downtown San Francisco of the iPhone 4 and the previous iPhone 3GS model. The comparison found that the iPhone 4 generally connected at faster speeds to AT&T’s HSPA network: for example, close to 2Mbps download speed vs. about 1.5Mbps for the 3GS. The improvement on upload speeds was even more pronounced.
The reporter wrote that he “was surprised to find that in all the test calls I made with iPhone 4, not one call dropped, and on none of the calls did I hear any noticeable jitter, delay, or static.”
That’s great! But there was a catch, right?
Right. This unscientific trial also specifically looked at what happened to iPhone 4 reception when the reporter held the phone as many do: in your left hand, with your palm covering the bottom left edge of the phone. The reporter found “dramatic speed decreases” in four of five locations. The number of connection bars dropped from five out of five to only one or two.
So what’s actually causing the reception problem?
There are several issues, according to a couple of blog posts by Spencer Webb, the president and principal engineer for AntennaSys, a company that specializes in RF consulting and custom antenna design services.
First, iPhone 4, like almost every other modern cell phone, puts the cellular antennas at the bottom, where they are most likely to be covered by your hand. The reason is that the Federal Communications Commission (FCC) has strict limits on the amount of energy that can be absorbed by the human body from a handheld device, Webb says in his first post last week. The energy limit is called the Specific Absorption Rate or SAR. So the phone designers move the antennas as far away from the head as possible—to the bottom of the phone.
Where your hand messes them up. How come?
Webb says in his post this week that the hand contact has two effects on any antennas.
One is “detuning.” Webb describes the human hand as a bag of salty water. As a result, the hand is dielectric, which means it can concentrate an electric field. When your hand touches an antenna, it loads the antenna with this concentrated field, which in turn lowers the antenna’s resonating frequency. And that means it may be “harder to squirt energy into [the antenna] at the frequency we want” — the frequency to make a cell phone call, for example, Web writes. In some cases, this detuning can completely “kill” the antenna, according to Webb.
What’s the second issue?
Signal attenuation, often called reduction or loss. Your hand, Webb says, can also conduct electricity, though not very well. In addition, it’s a “lossy” conductor, because “RF energy impinging on your hand (or head) is partially going to be turned into heat.” That’s what the SAR limits are all about. “This leads to an attenuation (reduction) in the signal being radiated into space by the antenna…Once turned into heat, the RF energy is gone.”
He and others commenting on this issue agree that this kind of attenuation is common to all cell phones. The detuning seems to be problematic uniquely for iPhone because the antennas are exposed in the external steel band rather than being packed away under the covers.
How bad is this?
It depends. Byrnand argues that there is for iPhone 4 a “substantial additional reduction of signal” due to the direct electrical contact between the hand and the antenna operating at a range of mobile frequencies (he’s based in the United Kingdom, where mobile carriers use some frequencies different from those in the United States.) Such contact will “severely detune” the antenna, he says. Depending on conditions, the detuning could weaken the antenna to the point where the signal strength is too weak to support a 3G connection.
But Webb insists that it’s far too early and the rough experiments far too lacking in engineering rigor to form any conclusions about how iPhone 4 performance will be affected. The informal measurements and observations, such as those by PC World, don’t and can’t “see” into the cell sites that from the network, for example.
“While you are making observations, you have no idea whether your iPhone is staying on one cell site, or switching between several,” he writes. “This will completely obfuscate any measurements, even if you decided that the [indicator] bars are useful.”
What’s needed, he says, is simply more time and more testing in more places. Until then, he’s waiting for his own iPhone 4 to arrive.
Has Apple responded?
Late last week, the company put out a statement on the matter.
Essentially, the statement says that attenuation of antenna performance is a problem with any cell phone, and the scale of the problem varies widely depending on the antennas are placed.
What’s their recommendation?
“If you ever experience this on your iPhone 4, avoid gripping it in the lower left corner in a way that covers both sides of the black strip in the metal band, or simply use one of many available cases.”
Apple CEO Steve Jobs considers the whole thing a “non issue.” One MacRumors reader sent Jobs an e-mail, asking what is going to be done about the signal dropping issue. His e-mail reply: “Non issue. Just avoid holding it in that way.”
There’s also fact that to use one of the “many available cases,” you first have to buy it: Apple charges $29 for its iPhone 4 “bumper” case, which is, in effect, a glorified rubber band. Which, by the way, completely covers up that cool stainless steel sideband.
What does the bumper actually do, technically?
According to Webb, it essentially pushes your hand further away from the antenna. That’s a good thing, or more precisely, a better thing. “The so-called bumper case is a much thicker insulator (or dielectric) than a piece of tape [one of the fixes being touted on the Web]. It pushes the lossy dielectric (your hand) further away, significantly reducing the capacitance. I would expect this to reduce the detuning effect, but not the attenuating effect. Will it help? You betcha’.”
He says the bumper case will still “load” the antenna, causing some detuning, just not as much as your hand.
My head hurts.
It’s probably due to holding your iPhone against your ear.
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This story, "FAQ: The iPhone 4 antenna flap" was originally published by Network World.