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.

Update

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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  • relentlessfocus - Thursday, July 15, 2010 - link

    Thanks for your professional approach and your in-depth analysis. It's a shame that Apple didn't handle this better, maybe they've learned something for the future and will be a better company as a result. I have to say that I love my iPhone 4.

    Keep up the good work!
    Eric

    Reply
  • AlexE - Thursday, July 15, 2010 - link

    "...we identified a worst case signal drop of around 24 dBm..."

    should say "a drop of around 24 dB". The difference between two referenced dB values (-10 dBm and -34 dBm for example) is actually expressed in dB, not dBm. As an electrical engineer, I couldn't leave that uncommented :p
    Reply
  • Brian Klug - Thursday, July 15, 2010 - link

    Dang, I thought I fixed all those! I totally agree - drops are dB, while the values themselves are dBm. Fixed now! ;)

    -Brian
    Reply
  • Cat - Thursday, July 15, 2010 - link

    Please don't use a marketing term. Many people hold the phone in a way that is bad for the iPhone 4, and have held it that way for more than a decade. Calling this grip "wrong" is wrong. Reply
  • cmdrdredd - Thursday, July 15, 2010 - link

    Yep...There is no wrong. A Phone should always work if the signal is reading good no matter how I hold it.

    As an aside, I came to this site originally because I built a PC and wanted info on the latest PC hardware and needed reviews on what was the best overclocking components and best bang for the buck video cards etc. There is a reason I didn't buy a Mac and am not reading a Mac centric website instead. I don't give a crap about Apple products or their newest laptop that is $2000 too expensive (I don't really care what type of battery life you can claim, I am always near an outlet and if I'm not, I'm only hours away from one and my laptop will work fine for those short couple hours). If I wanted that info I'd go to an Apple/Mac site and read it all there.

    So in truth, I agree with those who say that the Apple articles and apologetic tone is out of hand.
    Reply
  • Snaafu - Thursday, July 15, 2010 - link

    For someone that doesn't give a crap about Apple products you know a lot about price, battery life, and how to hold the iphone :p Reply
  • cmdrdredd - Saturday, July 17, 2010 - link

    Yeah...your argument is what? Exactly...as worthless as Apple articles defending flawed hardware. Reply
  • SharksFan - Monday, July 19, 2010 - link

    What a painful argument.

    So -24db drops means "broken", then what is the Nexus One's -17 drop?

    3/4 broken?

    What about -17db drops against a -113 cutoff? Isn't that identical to -24db drops against a -121db cutoff?

    So I guess the Nexus One is broken too?
    Reply
  • mesiah - Thursday, July 15, 2010 - link

    Did anyone else chuckle at the thought of Anand's cheek wanting to get some "Facetime" with a contact? Reply
  • eliotw - Thursday, July 15, 2010 - link

    If the iPhone 4 is able to transmit voice and data at lower signal levels then in most scenarios doesn't that compensate for the possible increased attenuation? So doesn't it generally perform on par with the 3GS and sometimes it's better? Why would this deserve some rushed urgent "fix" ? Reply

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