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

  • canontk - Thursday, July 15, 2010 - link

    I'm not trying to defend Anand and his love for Apple, but you must understand this site started from his love for computers and things technical. I've been coming to this site for over 12 years now and I dislike seeing the constant Apple articles just as much as the next guy, but I respect Anand and most of his staff.

    You'll just have to learn to deal with it or find another site. The problem is that most sites are big on Apple news and reviews just to get the traffic. I don't think this is why Anandtech posts Apple news and reviews, I think it's because they really like their devices.

    We get good information from their interests, we just have to hope some of those are the same as ours.
  • jonup - Thursday, July 15, 2010 - link

    I think what you said is worrisome(?). Fact is that Apple has a marginal presence in the cell phne market (though growing at very fast pace) but they products receive dispropotional presence on review site, even on sites like AT where cell phones are not the core content.
    Apple is getting PR (intential or not) that sometimes even I want to get an iPhone. I almost installed a Hachintosh a few months ago. I meen I will never (!?) get an iPhone cause they do not make them in the form factor I prefer (small, light, and durable).
  • SunSamurai - Thursday, July 15, 2010 - link

    Marginal presence? Apple has a 15-20% share in the smart phone industry. How is that marginal?

    They dont make ANY smartphone in a small light form factor. What in the h3ll are you talking about and how does that have anything to do with the iphone vs any other smart phone?
  • jonup - Friday, July 16, 2010 - link

    15-20% of the US smartphone market and that is namely because pin-point marketing like the one in question. (At one point even I was convinced to by an iPhone, silly me!)
    A year old Nokia E52 - 3.4Oz. 54cc. You didn't hear about this one, did you? Sorry they were to busy reviewing iPhone 3GS.
  • SunSamurai - Friday, July 16, 2010 - link

    15-20% is not negligible. Your excuses are pathetic.

    Nokia E52:
    Dimensions: 116 x 49 x 9.9 mm
    Weight: 98 g

    Dimentions: 115 mm x 58.6 mm x 9.3 mm
    Weight: 137g


    Have fun with your small-ass screen. I have no connection issues on my iphone. No one cares about the E52.
  • jonup - Saturday, July 17, 2010 - link

    That is 40% more weight and 12% more volume. The E52 is a year old phone. Compared to the year old 3Gs that is 40% and 55% more weight and volume.
    And then comes the building quality. Check out a review of the phone to see what I mean. As a testimony, my old 6301 had only a small dent on the forged aluminum battery cover after 2.5+ years of abuse of falling on concrete.
    3.5" is definately better than 2.4" but it does not provide for significantly better usability. if 2.4" is a limitation for performing a particular task, chances are 3.5" would still not be enough. Besides you give up the single handed imput capability.
  • Tegeril - Monday, July 19, 2010 - link

    Sorry, SunSamurai is right. No one cares about the E52. Reply
  • canontk - Friday, July 16, 2010 - link

    How is what I said worrisome? Worrisome because the people that write for this site like Apple products? A lot of people do. I don't. I have an ipod nano (first gen) and I'm sorry I wasted the money on it.

    I won't get an iphone and I don't like seeing constant Apple news. But like I said, Anand likes his Macs and Apple devices, good for him. He's going to do articles on what he likes.

    He also seems to be very fond of SSD's and their market share is extremely low. I don't see people complaining about his 15 page super detailed articles and constant updates on firmware.
  • SunSamurai - Friday, July 16, 2010 - link

    Yeah exactly. Anand is doing articals on what is changing the industry. SSDs or Apple or the Droid review.

    Love them or hate them, Apple is an industry lead, and things they produce are copied and often improved upon. We owe them a dept of gratitude. Do not think for a moment we'd have something like Driod if not for them. We'd have the MS Kin phones and Google would not have had to make anything close to the current droids.

    This is business.
  • jonup - Saturday, July 17, 2010 - link

    Except that Anand is reviewing all kinds of SSDs not just one brand. If Anand was reviewing Intel of OCZ SSds 75%+ of the time then your analogy would have been correct. How many Symbian phones has this or most computer-centric site have reviewed since July 2007? The platform is used by Nokia and SonyEricsson. The two combined maintain more than 50% of the cell phone market. You would think they need a little more representation.
    @ SunSamurai bellow:
    I've grown up to like Apple products lately (not the brand) but Apple is the one that copies and improves upon others. Neither iPod was the first portable player, nor the iPhone the first touchscreen phone, nor iPad the first tablet, nor iPhone 4 the first videophone. I do not question Apples influence on the IT market, but simply asking how much of the hype is result of dispropotionate review coverage.

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