Mitigating the Problem with Tape/Gloves

I originally tested the iPhone 4 in a number of different positions in the hand, and in a bumper case, and generated the signal strength drops reported in our previous article.

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

After getting those numbers, my first thoughts were that two dominant effects were responsible for the iPhone 4 signal drop being measured. The first was detuning due to capacitance added by the hand making galvanic contact with the stainless steel, and possibly even coupling the two discrete antennas together. The second was simply attenuation due to our meatbag extremities (read: hands) being not perfectly transparent to RF at 850 MHz and 1.8 GHz. I made some Star Trek references that some of you caught about us being bags of mostly water - it’s true, and it’s something Apple has emphasized heavily in its letter - that all phones drop signal when your hand is in between the path to the base station antenna. The real question was how much of that 24 dBm drop was due to galvanic contact with your capacitive hands (detuning), and how much was due to your hands being mostly water, and so close to the radiative surface.

Almost immediately after the issue was identified, many took it upon themselves to apply adhesive insulative tape to the troublesome area. Others suggested testing with rubber gloves on to see how much the issue changed. I set out to test both.

Allow me to introduce you to my friend Kapton tape.

No, it isn’t a gold iPhone 4, nor have I dipped the stainless steel band in gold (you have no idea how many people have asked) - it’s the native amber color of the world’s most awesome tape. I managed to find a roll of 1-mil thick Kapton tape that is exactly the right width of the iPhone 4’s stainless steel band. It’s miraculous really how exact the match is, without any cutting or tweaking, it just fits.

What makes Kapton the most conclusive choice of tape here ever (and not your grandpa’s electrical tape or duct tape) is that it’s the industry standard for flexible printed circuits. In fact, it’s what’s used to insulate just about every flex PCB antenna around. The tape obviously has a huge impedance, so when I hold it, I’m insulated completely from the stainless steel band.

I wrapped the tape all the way around the phone - not just the lower left trouble corner - to guarantee complete insulation. Of course, it’s impossible to use the phone like a phone this way since you cover the dock connector, speakers, and microphones, but just for testing. The other thing the tape simulates is how the iPhone 4’s antenna would behave with a thick 1-mil (25.4 µm) coating. To test, I cupped the phone just like I did to cause the 24 dBm drop before.

I also took an ordinary natural latex glove (yes, really) and held the iPhone, this time without any Kapton tape wrapped around the phone. Nothing special here, just a laboratory glove and cupping the phone.

The results speak for themselves.

Signal Attenuation Comparison from Cupping Tightly in dB - Lower is Better
  Bare Phone 1 mil Kapton Tape Coating Applied Natual Latex Rubber Glove on Hand
iPhone 4 24.6 16.6 14.7
iPhone 3GS 14.3 N/A N/A
HTC Nexus One 17.7 N/A N/A

Instead of a 24.6 dB drop from cupping the phone tightly without a case, with bare skin, we see a 16.6 dB drop with tape all the way around, and a 14.7 dB drop wearing a rubber glove. Insulating the stainless steel completely from the hand completely results in 9 dB less of signal drop. The remaining 16 dB is then due to the hand being so close to the phone.

The takeaway is that the best coatings Apple could possibly apply would bring the drop down to 15 or 16 dB - in league with the Nexus One’s worst case drop, and almost in league with the iPhone 3GS worst case drop. It’s hard to argue that bringing the signal drop down to levels other phones have been selling with for a year now isn’t a problem solved type solution.

However, adding tape won’t completely eliminate the drop in received signal, nor does it mitigate the problem nearly as much as getting a case. In fact, if you’re really concerned about dropping signal on any phone, you should get a case anyways. It demonstrably reduces the signal attenuation added by having your hand so close to the radiative surface of the antenna.

Oxide on Stainless

I talked with a number of materials science wizards, and picked their brains about the possibility of applying a nonconductive coating to the iPhone 4’s stainless steel antenna bands.

The response I got back was that stainless steel is difficult to coat by very nature of it being “stainless.” The metal unsurprisingly develops a dull oxide which itself is a poor conductor, but forms a protective layer that resists tarnishing and corrosion. This same layer that makes the metal stainless makes it difficult to coat. Some grades of stainless are apparently much easier to coat than others, but nearly all grades would require abrasion or chemical etching, followed by vapor deposition of the coating.

It’s not impossible to coat though, and if rumors that new iPhones built in recent weeks are rolling out with coatings turn out to be true, it’s obviously being done. But coating the stainless steel bands is obviously something Apple had to have considered.


I originally thought my Kapton tape was 5-mils thick, turns out it's 1-mil thick Kapton Polyimide, with adhesive for a total of about 2-mils of thickness. It's P-221 Permacel branded tape billed as the "ultimate" in electrical insulation.

Total Silent Recall?

Gizmodo reported (and iFixit followed up) on some users claiming that newer iPhone 4s had a different coating on their stainless steel band that mitigated the signal attenuation issue caused by tightly holding the phone. In theory, with the right coating, Apple could deliver the same sort of results we just showed using the Kapton tape. To date we haven’t been able to get our hands on one of these iPhone 4s with improved coating.

We found an iPhone 4 produced in week 28 of 2010 (digits 4 & 5 from the left of the iPhone 4’s serial number indicate production week) and took a multimeter to it. There was no measurable difference in resistance between it and our older iPhone 4s. In other words, the band was just as conductive. While this doesn’t rule out the possibility of Apple changing the manufacturing process on the phone, I wouldn’t waste time trying to hunt down a phone manufactured on a specific date just yet.

Proximity Sensor

Until two days ago neither one of us had experienced the proximity sensor issue with the iPhone 4. The proximity sensor on the iPhone detects if your face is close to the screen, like it would be during a phone call. If it does so, the iPhone turns off its screen to avoid any accidental input and save power. The proximity sensor issue manifests itself by the phone incorrectly assuming that you aren’t holding the phone up to your head and turning the screen back on. This happens in the middle of a call and often results in your cheek doing things on your phone without your knowledge.

Two days ago I was on a phone call when the proximity sensor all of the sudden decided that my face was no longer near the phone. My cheek then navigated into my contact list and tried to FaceTime with another contact while I was on the phone. I didn’t find out until the iPhone complained that a FaceTime connection couldn’t be established (due to the contact my cheek was trying to FaceTime with not having an iPhone).

I’m on the phone quite a bit and so far this was the first and only time the proximity sensor bug cropped up. We’re still looking into it but so far we can’t tell what the root cause is or if it’s helped by iOS 4.1.

Better at the Low End, Mixed Feelings Everywhere Else Final Words


View All Comments

  • Brian Klug - Thursday, July 15, 2010 - link


    So I don't think I elaborated enough on how Android's implementation of the signal bar visualization cutoffs mirror standard industry practice - practice which like you say shows just how tolerant the digital system is to noise.

    You're entirely right, there's a large range of bars that give you essentially full service. In fact, with the iPhone 4 it seems fine down to maybe -100 dBm.

    So there are really two schools of thought here:

    Option 1: The range could be compressed to show when signal is essentially good (we're between -100 dBm and -51 dBm), or bad (below -100 dBm) - this is basically what Apple used to be doing.

    Option 2: Distribute the cutoffs evenly (in dBm, not mW of course) so that signal drops are reported in an even, perhaps linear fashion as suggested by the bar heights themselves. This is the route that Apple has taken with iOS 4.1/4.0.1.

    Obviously the reason they've implemented Option 2 is so that the drop doesn't make all the bars disappear. Previously, if you were right above the 4 bar cutoff, and you gripped it tightly in a bare hand, all the bars went away. It made the problem inconsistent and led people to think some devices were affected while others weren't. That's the perception problem, which the bar changes do mitigate.

    The other problem is the drop itself, which nothing short of hardware changes/modifications will change.

  • rs1 - Thursday, July 15, 2010 - link

    Holding Apple accountable for fixing their bugs should be the only thing required to mitigate the problem. There should be no need to resort to things like tape and gloves. Reply
  • Snotling - Thursday, July 15, 2010 - link

    If I understand your statement, you apparently believe that blame and lawsuits are superior to experimentation and knowledge.

    The original article was cited in many other media and probably played an important role in getting the truth out there and help put an end to this issue faster.

    Douglas Adams, through one of his characters said: "I would rather be happy than right anyday"

    This is the king of wisdom of which your comment was deprived.
  • rs1 - Thursday, July 15, 2010 - link

    No, you assume too much. Experimentation and knowledge are great, they just don't substitute for holding the responsible-party responsible. My point is just that if Apple does the right thing, owns up to their mistake, and fixes it, there's no need for users to invest their own time and money into fixing someone else's problem. And if Apple does that, then there is no need for lawsuits of any kind.

    If people want to discover what kind of hacks and workarounds are available to fix the issue manually, then good for them. Most people, however, would likely prefer to have the manufacturer fix the defective product properly, so that they don't need to spend their own resources doing so. There is, after all, more valuable knowledge that people could be seeking than how to fix a phone that Apple broke. Especially people as clever as Anand and Brian.
  • dypeterc - Thursday, July 15, 2010 - link

    as the article states, "Whether or not the antenna design manifests itself as an issue really depends on AT&T’s coverage where you’re using the phone."

    just because iPhone 4 has a higher attenuation, doesn't mean there's a problem.

    at&t needs to get its act together and roll out better coverage.

    in addition, if you don't like the fact that it has a high attenuation, you can always return it. everyone is still within the 30-day grace period.
  • Tanker10a - Friday, July 16, 2010 - link

    I could not agree more with your comments. I am currently using the 32GS iPhone and I am absolutely frustrated with AT&T with this coverage issue. From my backyard, I am flanked by two 2G (EDGE) antennas that are approximately 10 miles away according to AT&T Consumer Service department and after screaming at them last Sunday; they still could not solve my problem. It took three to four phone calls plus waiting time in order for them to identify that those towers cannot deliver 3G signals. Contacted AppleCare and they told me to reset my iPhone (never mind the fact that I have done that since iOS4 was deployed)... I think that regardless what kind of phone that you throw at these towers, you will not get the coverage that you are paying for. Truthfully, the iPhone operates great when it is in the company of a Wi-Fi device or 3G. I have received AWESOME coverage in the Carolina Great Smokey mountains than I do from my backyard.
    Bottom line, AT&T needs to own up to this problem.
    As far as the Antenna is concerned, I think Apple is stuck with the positioning of its location due to FCC regulations... And, this is no different than grabbing your typical radio antenna and to experience a signal drop...
  • leexgx - Thursday, July 22, 2010 - link

    i have not seen any other mobile company follow the FCC rule like apple has

    again simple fix buy the bumper case or case like it (none conductive)
  • Snotling - Thursday, July 15, 2010 - link

    Actually, it is Anand and Brian's job to do those kind of experiment so that we do not have to, and so that we know precisely what kind of product is put on the market, what are their capabilities and if there is a huge amount of hype over something: to translate it to rational and factual information, so that we are not prey to basic marketing evil and limited to the information available on the shelf sticker at best buy.

    this is exactly what tomshardware stopped doing so I'm doubly happy that Anandtech sticks to it.

    so if wearing a glove to hold a phone can prove a point please, do it before I spend my money on the phone because I want to know this.
  • jonup - Thursday, July 15, 2010 - link

    If I understand your statement, you apparently believe that pulling a latex glove out in public and freaking everyone around you every time you need to make a phone call is unacceptable? Reply
  • aj28 - Thursday, July 15, 2010 - link

    This whole thing is ridiculous. You know what else doesn't work well when tightly cupped in your left hand? Asymmetrical mice. Reply

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