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
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  • SharksFan - Monday, July 19, 2010 - link

    They probably just read the article and realized they were as well off with an iPhone 4 signal wise vs the compared Nexus One, so they stopped worrying.

    -121db cutoff and -24 signal attentuation means the phone should be good till -97db on average, before you grab it hard.

    -121 cutoff and -20 signal attenuation means the phone should be good till -101db on average.

    -97db vs -98db when compared to Nexus One when held "hard", and -101 v -102 when compared to Nexus One when held "normally".

    Those are pretty much identical cutoff's btw.

    So I guess you should be clamoring for a Nexus One recall too....

    Maybe we can sue Google?
  • leexgx - Friday, July 16, 2010 - link

    i did not even read that comment
  • gilesrulz - Friday, July 16, 2010 - link

    First, thank you for your two well reasoned and reported articles on this subject. Would you be willing to update the article to discuss Apple's indication that attenuation caused by conductivity is not the issue, and that it is caused by the normal physics of being meat bags?
  • steve.h - Monday, July 19, 2010 - link

    Yes! This is exactly what I am also wondering, and something I haven't seen addressed anywhere else. If the problem is mostly due to the conductivity of the skin interfering with the antenna, Apple hasn't been completely truthful in comparing their problems with problems faced by other manufacturers. It seems like a fairly simple test to perform, I am surprised none of the tech blogs I read have tried to address this yet. Of course anandtech is the only place any actual information gathering was done and reported in the first place, most of the rest was just fear mongering.

    Again, I am convinced that this is a non-issue either way for most users, since it only affects performance in a small number of cases. However, I'd be interested to see if Apple really was being intentionally misleading in their press conference by comparing their issues to other manufacturers.
  • atomez - Friday, July 16, 2010 - link

    I got an iPhone 3G with OS4 (iPhone 4 no yet available here) and the same "grip of deat" happens! It usually got 5 bars signal strenght. Using the left hand GOD it dropped to 2 bars. Now with OS4.1 it shows 3 bars and drops to the same 2 bars on GOD. And you know what? It never dropped a call! So it's working fine for me. I understand the iPhone 4 will be the same.

    It's not a problem, it's a feature. Get used to it.

    No one is fooling you. If you don't like it, just don't buy it. If you have it, go get the free bumper. Or just return it and get your money back.
  • Polakapalooza - Friday, July 16, 2010 - link

    Essentially, then, you are equating — and, it appears, correctly so — the iPhone 4 with Henry Wadsworth Longfellow's "little girl," to wit:

    There was a little girl,
    Who had a little curl,
    Right in the middle of her forehead.
    When she was good,
    She was very good indeed,
    But when she was bad she was horrid.
  • jms102285 - Friday, July 16, 2010 - link

    Might I recommend to the readers who are upset at the passive-apple/anti-MS dynamic here an alternative:

    For the past few years I have been coming here for the news on the latest/greatest reviews digging into the meat and potatoes of cutting edge hardware. Lately I've been very very disappointed at the disproportionate amount of content regarding Apple vs. the release of other new phones.

    I understand that website hits lead to more advertising revenue and that Apple news gets hits, but couple that with the dare I say passive nature towards Apples mistakes versus other manufacturers' mistakes and it really hurts the integrity of what I come here for.

    I also have been less than happy with the IT computing section as they really don't discuss much there. It seems like an afterthought compared to other sections of the site.

    So peace out Anand, good luck here.
  • SunSamurai - Friday, July 16, 2010 - link

    You wont be missed.

    Running off because of one too many apple related articles. Talk about childish.
  • screensurfer - Friday, July 16, 2010 - link

    Just a few of things I noticed:

    1) Apple found 3 phones that have similar problems. Fine. I can find a dozen that don't have such "grave" problem. The problem with iPhone 4 is that its attenuation is "a record" for a smartphone when the hand touches it.

    2) Apple took 22 days to find that the solution... is to patch it with a case. Bah!

    3) AppleCare data: it's flawed in 2 ways: First, they should only present it when the 30 days are over, to have a more apples-to-apples comparison vs. 3GS (who's data might not be the same time period). Second, the fact that iPhone 4 is a much better device than 3GS: people are holding to them. Regardless, this is not an excuse to keep a design flaw.

    4) ATT returns rate only. Where are Apple stores numbers? Could they show a different figure? I believe so, otherwise Steve would have shown their own numbers.

    5) (My favorite lie with numbers technique): Average cell phone drops 1 call per 100. If iPhone4 drops <1 pp more than 3GS, it means that it drops up to more 100% more calls than 3GS!!!!!!!

    6) They have figured out a better hardware that prevents this, they're just unwilling to recall the inicial production. Why? First, 30th September deadline; second, Steve implied it during the presentation that they were working on a hardware fix that "defies physics".

    Please don't lie with numbers. I hope this helps to clarify the PR BS!
  • Snotling - Friday, July 16, 2010 - link

    "Steve implied it during the presentation that they were working on a hardware fix that "defies physics"."

    yeah, I'm sure Steve Jobs will try hard to defeat the laws of physics...

    do you realize how silly you sound?

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