The Real Story on iPhone 4's Antenna

"How are you holding it?" - Brian Klug

"Three fingers. Like a [redacted] ninja turtle." - Anand Shimpi

There's been a ton of discussion lately surrounding iPhone 4 cellular reception. Even before it was officially announced, the reason for the stainless steel band running along the outside of the phone seemed enigmatic; many called it un-apple and decidedly atypical of seamless apple design which eschews hard edges. The black strips were written off as aesthetic curiosities, possibly even markings which denoted a fake.

Then at the WWDC announcement, we learned the truth. The iPhone 4's antenna is the stainless steel band that runs around the edge of the phone. The antenna for WiFi, Bluetooth, and GPS is the smaller strip beginning in the bottom left and running to the top, and the cellular radio for voice and data is the much larger strip running around almost three quarters of the phone.

It's a design nod back to some of the earliest cellular phone designs which packed external whips that one could manually extend for improving reception. Since then, designs evolved, and until recently virtually all smartphones have packed internal antennas at the bottom of the phone. The iPhone 4's external antenna promises improved reception over the internal antenna in the iPhone 3GS.

Of course, the caveat is that as with all external antennas, the potential for both unintended attenuation and detuning is much, much greater. When I first saw the iPhone 4's design spelled out watching the keynote online, I immediately assumed that Apple was going to apply an insulative coating atop the stainless steel. Perhaps even use diamond vapor deposition (like they did with the glass screen atop the iPhone 3GS) to insulate the stainless steel from users. We now know rather definitively that this isn't the case. Of course, the result is that anything conductive which bridges the gap in the bottom left couples the antennas together, detuning the precisely engineered antennas. It's a problem of impedance matching with the body as an antenna, and the additional antenna that becomes part of the equation when you touch the bottom left.

The fact of the matter is that cupping the bottom left corner and making skin contact between the two antennas does result in a measurable difference in cellular reception. But as we'll show, RF is a strange beast.

Measuring Reception Without Bars

When I set out to characterize and understand the iPhone 4's antenna issue, I noticed that reports online varied wildly. Some claimed that they were always able to recreate a reception issue created by cupping the phone, yet others reported no change at all squeezing the phone tightly. After acquiring my iPhone 4, the first thing I did was try to fire up Field Test via the widely documented *3001#12345#* dialer code. Unfortunately, like iOS 4 running on the 3GS and 3G, Field Test is absent from the iPhone 4. It isn't a matter of the dialer code, it's that Field Test has been completely removed from the applications directory in the filesystem.

For those that don't know, Field Test variants exist on virtually every phone for purposes of debugging the air interface and baseband. Quality metrics like RSSI (raw signal strength) usually in dBm are reported alongside a wealth of other metrics like SNR and even what adjacent towers are visible to the phone for handing off. It's a tool usually buried deep in every phone because the amount of data would overwhelm normal mobile users, but is useful for engineers and curious but savvy users alike to find out what's going on with the cellular network. For whatever reason, Apple really doesn't want anyone running that tool anymore.

Just about everyone knows that although reporting signal strength in bars gets the job done, it's an absolutely worthless metric for comparison across devices and platforms due to lack of standardization. Further, iOS smoothes the quality metric with a moving average over as much as 10 seconds, masking how fast signal changes. There's also the matter of dynamic range, but more on that in a second. Without any numbers at all it would've been impossible to understand what's going on with iPhone 4. On my 3GS, I exclusively report signal numerically, and as a result have a very good feel for coverage in Tucson, AZ where I live.

But I found a way. Undeterred by the lack of field test on iOS 4, I was determined to enable numeric signal strength reporting in the top left where bars are normally displayed. If you've ever run a jailbroken iPhone and used SBSettings, or changed your carrier string, you've probably encountered the fact that iTunes will back up and restore the status bar configuration across OS restores. See where I'm going?

I took my iPhone 3GS, downgraded to 3.1.3, jailbroke, enabled numeric WiFi and GSM and backed up. I then took my iPhone 4 and restored with iOS 4, but pointed it to the backup of the jailbroken, numeric-GSM-reporting iPhone 3GS. You'll note that booting and activating the new phone required fitting the new iPhone 4 microSIM into a SIM carrier. I ordered one almost a month ago, but it still hasn't shown up.


My MicroSIM -> SIM adapter. The real one is still inexplicably in the mail 15 business days later.

Success ensued, and I had a numeric readout of signal strength on a non jailbroken iPhone 4. The results are interesting.

Before we dive in, let's talk about dynamic range for a second. For a while, I've talked about how iOS reports the quality metric with a compressed, optimistic dynamic range. On iOS, 4 bars begins at around -99 to -101 dBm. Three bars sits around -103 dBm, 2 bars extends down to -107 dBm, and 1 bar is -113 dBm. To give you perspective, for a UMTS "3G" plant, -51 dBm is the best reported signal you can get - it's quite literally standing next to, or under a block away from a tower. At the other extreme, -113 dBm is the worst possible signal you can have before disconnecting entirely. With a few exceptions, signal power as low as -107 dBm is actually perfectly fine for calls and data, and below that is where trouble usually starts. However, you can see just how little dynamic range iOS 4 has for reporting signal; over half of the range of possible signal levels in dBm (from -99 dBm to -51 dBm) is reported as 5 bars.

So, an entire day and more than a quarter tank of gas later, here are the results. Holding the iPhone 4 without a case, in your left hand, crossing the black strip can result in a worst case drop of 24 dB in signal. As we'll show in a second, how you hold the phone makes a huge difference across every smartphone - and we've tested thoroughly in 5 different positions.

Now, there are two vastly different possibilities for what happens to the bar visualization after you drop 24 dB. I happen to live less than one block from an AT&T UMTS tower (it's across the street, literally), and have exceptionally strong signal in all of my house - it's part of why I chose to live here, actually. Signal is above -65 dBm in every single room, in most cases it's at -51 dBm. When I incur that worst case drop of 24 dB from squeezing the phone, I fall down to -83 dBm, which is still visualized as 5 bars.

However, in locales that have less signal, but where iOS still displays 5 bars, the drop of 24 dB is visualized much differently. For example, at another test location, signal without holding the phone is -89 dBm, which is still displayed as 5 bars. Cup the phone, and you'll fall all the way to -113 dBm. All the bars dramatically disappear one after the other, people think they've dramatically lost all the signal, and you know the rest.

If you're at 4 bars already, (which puts you on the low end of possible signal strengths), cupping the phone even more delicately is enough to push you the remaining 10 or so dB to cutoff. It doesn't take much when you're at 4 bars, which is why the visualization is flawed. Complicating matters is that signal is completely fine until down around 2 bars at -107 dBm.

If you add a bumper case to the iPhone 4, the signal strength drop from holding the device is on par if not better than other phones. In the exact same location, in the exact same orientation, I carefully measured my iPhone 3GS and Nexus One with the same AT&T microSIM in my newly made SIM adapter. After lots of testing, I decided on 5 different positions for holding the phone, and tested signal repeatedly. 

1) Cupping tightly - This is the absolute worst case and involves squeezing the phone very tightly, like people are doing online in videos demonstrating all the bars going away. I squeeze the phone hard and make sure my palms are sweaty as well. You'd never hold the phone this way because it's physically painful.

2) Holding naturally, comfortably - This is just how one would hold the phone typically in a relaxed way. Not squeezing it to purposefully reduce signal, but making contact with the fingers and not an open palm.

3) Resting atop an open, flat palm.

4) Holding naturally, but inside a case - In this situation the Bumper for iPhone 4, an Otter Box for the 3GS, and a comparable generic case for the Nexus One.

5) Pinching the top and bottom - Our baseline, virtually no attenuation. Held only to keep the exact position constant. It's not reported since this is considered ideal.

Signal Attenuation Comparison in dB - Lower is Better
  Cupping Tightly Holding Naturally On an Open Palm Holding Naturally Inside Case
iPhone 4 24.6 19.8 9.2 7.2
iPhone 3GS 14.3 1.9 0.2 3.2
HTC Nexus One 17.7 10.7 6.7 7.7

It's difficult to be exact about the data, since signal is very sensitive to direction, ambient conditions, and cell breathing. To generate these numbers, I measured at least 6 times and took the average. The results are pretty self explanatory. Inside a case, the iPhone 4 performs slightly better than the Nexus One. However, attenuation gets measurably worse depending how you hold the phone. Squeezing it really tightly, you can drop as much as 24 dB. Holding it naturally, I measured an average drop of 20 dB. 

The drop in signal from cupping the device with a case on is purely a function of us being "ugly bags of mostly water." A material which happens to be pretty good at attenuating RF - thus increasing path loss between the handset and cellular base station. There's nothing Apple nor anyone else can do to get around physics, plain and simple. It's something which demonstrably affects every phone's cellular reception.

Add in an external antenna you're essentially forced to touch and bridge to another adjacent antenna while holding, and the signal attenuation is even worse. The fact of the matter is that either the most sensitive region of the antenna should have an insulative coating, or everyone should use a case. For a company that uses style heavily as a selling point, the latter isn't an option. And the former would require an unprecedented admission of fault on Apple's part.

That's not all there is to the story, however.

The Antenna is Improved

From my day of testing, I've determined that the iPhone 4 performs much better than the 3GS in situations where signal is very low, at -113 dBm (1 bar). Previously, dropping this low all but guaranteed that calls would drop, fail to be placed, and data would no longer be transacted at all. I can honestly say that I've never held onto so many calls and data simultaneously on 1 bar at -113 dBm as I have with the iPhone 4, so it's readily apparent that the new baseband hardware is much more sensitive compared to what was in the 3GS. The difference is that reception is massively better on the iPhone 4 in actual use.

With my bumper case on, I made it further into dead zones than ever before, and into marginal areas that would always drop calls without any problems at all. It's amazing really to experience the difference in sensitivity the iPhone 4 brings compared to the 3GS, and issues from holding the phone aside, reception is absolutely definitely improved. I felt like I was going places no iPhone had ever gone before. There's no doubt in my mind this iPhone gets the best cellular reception yet, even though measured signal is lower than the 3GS.


Conference call with three calls going at the same time, and transacting data, all at minimum signal. Impressive.

That brings me to the way that signal quality should really be reported - Signal to Noise Ratio (SNR). SNR is essentially a measure of how much of the signal is compromised by noise or interference. It's readily apparent that because the iPhone 4 works almost perfectly fine at -113 dBm, it has much better sensitivity. The deciding factor for reporting the signal quality metric is then SNR, something Apple and other handset manufacturers will have to move to eventually instead of just power. In reality, reporting based on SNR makes a lot more sense, since I couldn't make calls drop driving around an entire day cupping the phone, despite being at -113 dBm (1 bar) most of the time.

The drop in signal from holding the phone with your left hand arguably remains a problem. Changing the bars visualization may indeed help mask it, and to be fair the phone works fine all the way down to -113 dBm, but it will persist - software updates can change physics as much as they can change hardware design. At the end of the day, Apple should add an insulative coating to the stainless steel band, or subsidize bumper cases. It's that simple.

WiFi/Bluetooth Reception

But what about WiFi? Surely since the UMTS/GSM antenna interferes with it, the WiFi signal has changed as well. It has, though not how one would expect. Holding the phone with no case actually improves WiFi signal strength by a measurable 5 to 10 dB. In the following plot, the dips are me releasing the phone from a tight grip and going to the two finger pinch. I verified the same ballpark level of performance increase on the phone as well. RF is truly an odd beast indeed. It just depends whether you're adding or subtracting length from the antenna, and thus moving away from or closer to an optimal solution.


The blue line is the iPhone 4. Look at the rate as well - more on that in a second.

The last lingering question is how GPS fix accuracy and acquire time changes depending on how you hold the iPhone 4. I spent a few hours testing and came to the conclusion that there's an insignificant difference gripped or not gripped, or compared to the 3GS. It's also difficult to repeat the same measurement since location services seems to keep the GPS going even after you stop using it, so subsequent API calls to it within a few minutes are very speedy. 

Getting an accurate location is still nearly instantaneous using WiFi through skyhook, and then AGPS takes it the rest of the way. It's impressive that we're talking on the order of seconds for a location within tens of meters of accuracy - considering that a cold fix on a standalone GPS used to take minutes. If you don't have line of sight to the sky, GPS fixes will take longer no matter what smartphone you're using. I have to wonder whether improved WiFi reception has an effect or not on skyhook (WiFi MAC address and signal strengh based) trilateration accuracy. Again, I couldn't be certain.

Introduction Network Improvements
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  • JAS - Wednesday, June 30, 2010 - link

    Yes, "more open, more configurable" is a double-edged sword. A comparison of Microsoft Windows and Macintosh OS X is apt in this regard.

    P.S. -- Can we advance beyond the juvenile label of "fanboy" when criticizing a person's like for a product?
    Reply
  • bplewis24 - Wednesday, June 30, 2010 - link

    Juvenile comes with the intent. I have the utmost respect for Anand and his opinion/reviews. If you take "fanboy" as a disparaging remark, fine, but it essentially means you have a bias or preference that obscures some of your objectivity.

    Brandon
    Reply
  • The0ne - Wednesday, June 30, 2010 - link

    As I've often expressed here, I like the reviews to not have any 1st person views. I don't care if one waited for 6hours for a phone. I don't want to hear any "this phone is the all to be". I just want a detail review of the phone and it's features. If one can qualify and/or quantify the differences with other products great. If you can't then don't, rather then having the innate urge to add your own opinions.

    Yes, the apple UI is more smooth, the experience is more enjoyable. The druid UI is a tad slower but by no means going to destroy or ruin anyone's experience. If you can't justify it, don't!

    Lastly, the problem with 1st person perspectives being included in reviews and especially technical reports is that the reader will see it as favoritism. This is why absolutely NO credible technical and scientific review/report is written this way. I don't write my engineering tests and reports in 1st person. Just stick to the material and leave opinions out of it. This type of review ONLY happens online and sadly it's affecting technical and scientific materials as well.

    Do the job, state the facts and tests and let the reader decide how to deal with it. Don't offer the reader any types of suggestion or persuasive comments. If you do include it, like other websites, in the editorial section or something similar.
    Reply
  • kmmatney - Wednesday, June 30, 2010 - link

    I wouldn't want to read reviews that just state facts. I'm an engineer/scientist and have written many peer-reviewed scientific papers. For tech reviews, though, I really appreciate user experience. There are just too many intangibles that can't be expressed by facts and tests. That's why I have been reading Anadtech for over 11 years - I appreciate the blend of techiness and user experience. Reply
  • totenkopf - Wednesday, June 30, 2010 - link

    I have to agree with TheOne on this. Also, you can express "intangibles" without using the first person... If the iPhone's UI feels smoother than another phone then just say that and leave off the "I feel" bit.

    Remember that any statement comparing the iPhone to "Android" will have its flaws. You can only compare the iPhone 4 to other phones, not android. Any such statement will inherently be some form of generalization. Besides, Comparisons made with particular Android phones will be far more helpful as many android phones offer an experience distinct form any other Android phone.

    It's really not that uncommon for iPhone users to play with an Android phone and really like it. Many of them actually seem quite surprised that they actually like it; some merely think that iOS is the only show in town as it has been the best for a long time. Widgets, in particular, offer a lot of customization and, perhaps just as importantly, personalization, that many iPhone users seem to appreciate. If used correctly, widgets can multiply the functionality of your phone many times over, and in some cases preclude running many apps at all. That said, setting up an android with just the way you like it and hunting down the newest and best apps and widgets can be an ongoing struggle. However, many people will enjoy it immensely if for no other reason than to make their phone that much better in their own eyes.

    There! My experience with android without sounding too biased... I think ;) It certainly sounds better than "Android rulz 'cus widgets are soo good and apple doesn't even have them because apple is fail!"
    Reply
  • John Sawyer - Thursday, July 01, 2010 - link

    As an iPhone user for the past year, I can concur with your observation that many iPhone users would be impressed with the latest Android phones. I've tried the Evo for about ten minutes, and during that time, I did some web browsing, ran some apps, etc., and it was fast (even with a 3G connection), seemed polished, and I wouldn't complain too much if it was the only phone I had to use. If I had the chance to use it longer, I might start seeing its deficiencies, but a quick look, looked good.

    Though a week later, the iPhone 4 was released, and I was blown away by its display, which no other phone matches yet, though I'm somewhat biased about that since I like to be able to read tiny text.
    Reply
  • The0ne - Thursday, July 01, 2010 - link

    The problem with user experience is that it's just that and it affects how readers perceive the product. I don't mind really that reviews are done this way, but many are done extremely bad. Here are a few examples of comments people include in their reviews...

    "I let me wife play with it for a day or so and she loves it!" This is quite common for PMP player reviews.

    "This is the best thing ever to come out on a phone..." Best thing is more or less an exaggeration. Way too many improper adjectives are used in reviews. Anandtech is no exception to this.

    "I like it..." Okay, that means what, I should too?
    Reply
  • strikeback03 - Thursday, July 01, 2010 - link

    I disagree, in that I think the subjective "feel" of one phone (and OS) relative to another is largely subjective and can't be quantified in just plain numbers. Would you really feel better if he stated that "after observation with a high speed camera phone X illustrates a scroll with 5 frames and phone Y does so with 20" as opposed to "phone X is noticeably choppier"? Or say for example Sprint shipped a special edition EVO with 768MB RAM, we know that is 50% more, but would it actually make a difference in your interaction with the phone if you had less than 20 apps open?

    I have an HTC Touch Diamond, a WM 6.1 phone with TouchFlo 3d. None of the reviews I read before purchase adequately described how clunky the interaction between the TouchFlo plugin and the background OS is, or how poorly optimized WM6.1 is for a touchscreen, and certainly not how the speed of the device goes from marginal when new to completely unacceptable after a few months and requires a hard reset to restore.
    Reply
  • ipredroid - Saturday, July 03, 2010 - link

    What many fail to realize is with out opinion there would only be numbers and no reason to have site with user friendly technical data (a site like this). Everyone's opinion is biased or influenced by something, simply sticky by facts and zero emotion devoted towards every product is impossible. Many opinions itself is laced in bias preference for facts.

    I for one want as much information as possible not half the information.

    Facts and opinions have and always will be better than just one of them. Would you rather have technical data about someones trip to Mt. Everest or an opinion. I would want both, so would everyone else or else you are missing facts.

    Science and opinion go hand in hand. Cause and effect.

    Reply
  • noiserr - Friday, July 02, 2010 - link

    I am getting really tired of people who have no clue what OS X actually is.

    Android is much more similar to OS X than Windows. OS X is based on an Open Source kernel for starters and both Android and OS X are *nixes, OS X also uses open standards (OpenGL vs. DirectX for instance).

    OS X is in many ways much more configurable and open than Windows, compare how many file systems Windows supports vs. those OS X does.

    Apple is foremost a hardware company. They strive to create tightly integrated devices which come at the price of less configurability but they are renowned for their intuitive UI and ease of use. You may disagree with it but you can't argue against the fact that It's a business model that has served Apple well.

    Personally I am a geek. I also write code for a living, while I do indeed like Android a lot when it comes to my phone I just don't care nor do I have time to tinker with it.
    Reply

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