This article originally appeared in TidBITS on 2010-07-16 at 2:25 p.m.
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Why Using an iPhone 4 Case May Improve Signal Strength

by Rich Mogull

Consumer Reports [1] said on 12 July 2010 that it could duplicate iPhone 4 call and data quality drop-off problems in its lab when part of a hand covered the external antenna gap on the phone's left edge. Based on its testing, holding the phone in your left hand so that you create an electrical bridge over the small plastic gap between the two metal pieces has a dramatic effect on the phone's connection, large enough to disconnect active calls.

Consumer Reports considers the problem so serious that the organization cannot recommend purchasing an iPhone 4, even though it otherwise topped their smartphone ratings. At the end of the article, Consumer Reports recommends a quick, simple fix that's been widely reported in other publications. Merely placing a small piece of tape or clear nail polish over the gap on the lower left side of the iPhone 4 can fix the problem, as can putting your phone in a case or using Apple's own bumper case that wraps around the phone.

The testing organization's report was followed later in the week by Apple's press conference, when the company announced it will be providing free bumper cases to all iPhone 4 owners to address the issue (and Apple's PR woes). Apple will also refund the purchase price of bumpers purchased from its stores. (See "Apple Responds to iPhone 4 Antenna Issue [2]," 16 July 2010.)

Yet since many people think the main problem is that our hands absorb the radio signal as we hold the phone - an issue a piece of tape can't possibly fix - the recommended solution can seem a bit confusing.

The reason a case (or some tape) helps is that the iPhone 4 actually suffers from two different antenna issues. One is common to all phones, and the other results from the iPhone's unusual external antenna design.

Of Radio Waves and Absorption -- While I'm not a radio antenna engineer, deep in my past I spent a considerable amount of time in the rescue and emergency services community, including over a decade with Rocky Mountain Rescue. Aside from handling our own radio communications in austere, remote environments, we also spent a fair bit of time practicing how to find emergency rescue beacons used by backcountry skiers and located in aircraft (and boats - not that I went hunting for many of those in the Colorado Rockies).

Much to my surprise at the time, this work required some basic knowledge of radio frequency propagation and even antenna design. It's awfully hard to find that crashed airplane if you plug the wrong antenna into your receiver.

The first issue with the iPhone 4 is that when you hold it in your left hand, your hand covers the antenna used for cellular voice and data transmission and reception, and attenuates some of the signal. From an antenna's perspective, the human body is a big bag of water, and water absorbs radio waves superbly. (Attenuation is a fancy term for blocking a signal through absorption, which can be by a body part, a wall, or other materials. Even the air around us attenuates a signal.)

This is a problem for all mobile phones, not just the iPhone, but it has never manifested itself quite so clearly before. During its press conference, Apple showed competing smartphones dropping signals when Apple engineers held those phones in a typical manner that covered the "sour spot" where the worst results from attenuation occurred.

Like other mobile phone manufacturers, Apple positioned the iPhone 4 antenna's maximum power output as far away from the head as possible. An increasingly large number of studies involving long periods of time and lots of people have failed to demonstrate a relationship between cell phone use and health conditions, such as cancer in the head closest to where a phone is typically used. But there is no dispute that having the least amount of electromagnetic radiation focused on the head to avoid heating effects is the best course of action. It's also worth noting that the FCC's absorption rules on emissions are based on proximity to the human head, and ignore the hand.

In the old days, the problem of where to place the antenna was solved via a big antenna sticking out the top, but big pop-up antennas are no longer fashionable in civilized society, even though they worked quite well.

If you can position the antenna in such a way as to reduce how much radiation the head absorbs, you can increase the overall power output while keeping the FCC happy. Unfortunately, different locations for the antenna also increase the potential for signal obstruction.

Different Frequencies Have Different Behaviors -- Adding to the problem is that current phones use a variety of frequencies, most of which are at much higher ranges than our old analog phones. It used to be quite expensive to make chips that could operate at higher frequencies (in this case, typically between 1700 MHz and 2100 MHz). That's no longer a bar, and spectrum around the world has been continuously freed up to allow greater use of mobile phones and other devices.

As you might know from looking at a classic sine wave, the higher the frequency, the shorter the distance between two peaks or troughs; that distance is the wavelength. For example, an 850 MHz signal runs through 850 million cycles per second.

The downside to using higher frequencies is that signal strength drops off faster than at lower frequencies. The higher the frequency, the more "energetic" it is, allowing the signal to drop quickly as it bounces off and is absorbed everything from buildings and trees to the air itself.

That's why 2.4 GHz Wi-Fi, used with 802.11b and g originally, and supported with 802.11n, can cover an entire house, while the 5 GHz band used by 802.11a and supported in n, may cover just a couple of rooms. It's also one reason why VHF television signals were easier to receive than UHF back in the day - VHF (very high frequency) was lower frequency than UHF (ultra high frequency), and would thus travel further with less signal loss.

Practically speaking, this meant that in my rescue days we preferred lower frequency radio channels since they would travel longer for a given power output (a big factor for battery size and life) and pass through objects better. Higher frequencies required more energy to travel a given distance (outside a vacuum), and were more likely to "bounce" off objects. When you are hanging from a rope a thousand feet off the ground with someone else controlling the brakes, you tend to like a nice, clear signal.

This has direct implications for mobile phones. Higher frequencies effectively require more energy to cover a given distance, and signals struggle more to penetrate objects and buildings. Believe it or not, cell carriers can tell when trees lose their leaves based on the signal changes in the towers near the trees.

It also means phones are designed to minimize the interference with the antenna. That's why the original iPhone needed a plastic section on its otherwise-metal back. Metal would block the signal, which passes through plastic fairly well. The iPhone 3G and 3GS used an all-plastic back, solving the problem.

The iPhone 4 takes this a step further and sticks the antenna on the outside of the phone. This is one reason for overall improvements in signal strength, but if you block the strip on the left side of the phone with a big bag of water (your hand), some of the signal is lost. There isn't a phone on the planet that doesn't lose some signal if you block the antenna with your hand.

It's why Steve Jobs joked at the antenna press conference that Apple had provided a target for blocking - no other phone has as clear a location where, when covered, you will guarantee higher attenuation than other areas.

While placing the antenna on the outside reduces signal attenuation from the body of the phone, it does cause a second problem.

Bridging the iPhone 4 Antennas -- That metal band around the iPhone 4 isn't just a single antenna for the cell phone. It's actually two different antennas, which explains those little black bands. One antenna is for the cell phone itself, and the other is for Wi-Fi, GPS, and Bluetooth. The black band on the right is decorative, with the antenna continuing underneath the plastic, but the one on the left side is a physical split.

Crafting external antennas is actually a tough problem. The size and shape of an antenna is dictated by our old friend, the wavelength. Antennas aren't merely random sticks of metal, but carefully designed components sized to match the frequencies they work with. Lower frequencies have longer wavelengths, and thus require longer antennas.

The antennas used to communicate with submarines in the Very Low Frequency band are a heck of a lot bigger than the little ones in our phones. There are even Ultra Low Frequency communications systems used by the military and in mining that can travel through the Earth itself. For instance, a 60 KHz signal broadcast from Fort Collins, Colorado, sets the time [3] for the entire United States on those radio-set clocks that adjust themselves.

Antennas have sweet spots which are different factors (multiples) of their wavelength. Thus the antenna on a cell tower is bigger than the one in your phone, but they both operate on the same frequency. There are many more factors involved, but this is the important bit for the iPhone 4 problems. Bigger antennas do a much better job for any given frequency, but only if they are the right size and shape for the wavelength they are handling.

Technically, our phone antennas are fractions of the optimum size for the given wavelengths. As one commenter noted, at 850 MHz (one of the GSM bands supported by the iPhone) the wavelength is 35.27 cm (13.89 inches), and antenna of that size wouldn't fit even in an iPad, much less in an iPhone. Thus we use fractions of the optimum length antennas, and higher frequencies, with their shorter wavelengths, allow us to use more-effective, yet smaller, antennas.

The second problem with the iPhone 4 isn't your hand absorbing the signal, but instead your hand, as an electrical conductor, acting as a bridge between the antenna on the bottom of the phone and the one on the left side. This not only creates potential interference by allowing the signals to interfere with each other (probably not a huge issue since they are independent frequencies), but it also changes the "size" of the antenna and thus its capability to function properly.

Although each antenna handles multiple services on different frequencies, they are all within the design constraints of the antenna. Consumer GPS receivers listen in at 1500 MHz and Bluetooth and Wi-Fi (all on the right-side antenna) employ 2400 MHz, but because 2400 is not far from 1500, it works out well enough. (The iPhone doesn't support 5 GHz 802.11a/n.) However, bridging the GPS/Bluetooth/Wi-Fi antenna with the cellular antenna (mostly on the left side), which handles 850, 900, 1800, 1900, and 2100 MHz, doesn't fall within the engineers' plans.

That's why placing tape over that spot on the left side of the iPhone 4 can resolve the problem. It insulates the antenna from your hand, preventing it from bridging the two antennas and messing up the signal propagation/reception. When you insulate the antenna with tape or a case, your bag-of-water hand is still absorbing some of the signal, but it doesn't interfere with the antenna's operation by changing its physical characteristics. You probably need to cover only the break point since your hand isn't that good of a conductor, and bridging from the far ends of the antennas doesn't have as pronounced an effect. (AnandTech [4] has some good coverage of this as well.)

Testing the Theory -- Again, I'm no radio engineer, but I performed a simple test at home to see if this theory holds water. Placing my phone on my desk, I first bridged the two antennas with my finger and noted a signal drop of one bar in less than 10 seconds. When I licked my finger (to increase conductivity) and bridged the gap, I saw a drop of two bars instead of one. The signal bars show an average of the last 10 seconds, which is why you won't notice the change immediately.

I then tried bridging the gap with a small piece of electrical wire. I still lost about a bar on average, but it seemed to take longer than when using my finger, and I could never get it to drop two bars.

Finally, if I placed my finger right next to the gap, on either side, I didn't see any signal loss at all.

Using my finger, which both absorbs radiation and bridges the gap, resulted in the greatest loss. But since I also lost some signal using a piece of wire, that seems to support my theory that bridging the antennas with a conductor is also a factor. Especially since placing my finger right next to the gap has no effect unless I bridge it.

Thus a bumper or case helps with both parts of the problem. It prevents your hand from bridging the antennas, and creates a small gap that lets a little more signal hit the antenna instead of being absorbed by your hand. This contradicts some of Apple's explanation, which focused on your hand absorbing the signal.

In one sense, the iPhone 4's design is an advantage over competing approaches, since the external antenna enables Apple to use a larger antenna that's blocked less by the phone's case and innards. But the drawback is that the iPhone 4 has a single, small spot where any signal loss effects are magnified.

I don't have the equipment or software for a scientific test, but my informal results are very consistent and are something you can try yourself (though if you're in a good coverage area it's unlikely any of this will matter; for once my terrible AT&T coverage is an advantage).

That may be more than you wanted to know about radio waves and antenna design, but hopefully it gives you a little insight into the seemingly strange recommendations coming from Apple and others.

And if I'm wrong? Well, you still got to learn some interesting trivia for dinner party conversation.