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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  • screensurfer - Saturday, July 17, 2010 - link

    No. Care to explain? It's not my words, it's his (something like this): "we haven't found a solution to defy the laws of physics. YET!", i.e., after September 30th we'll have a different antenna system that doesn't drop calls so easily
  • barelyinaudible - Saturday, July 17, 2010 - link

    I keep seeing this article quoted as "proof" to apple's signal problem. However, I am finding that you are leaving out one rather important piece of information -- Your reference.

    Are you measuring in dBc, dBJ, dBm, dBi, dBd, dBq? There are literally hundreds of different reference points to choose from?

    Also, dB is a logarithmic function - What is starting point for the attenuation? 100 dB? 30 dB?

    Going from 120 - 100 dB is a MUCH MUCH bigger drop than from 30 - 10 dB -- since dB increases exponentially.

    I think this article is being intellectually dishonest by not providing proper information to your readers (read: 85% of statistics are made up on the spot.)
  • ameridian - Saturday, July 17, 2010 - link

    There are 22 occurrences in this article referrings to dBm's as the unit of measure. I'd advise reading the article before posting comments.
  • leeseng - Saturday, July 17, 2010 - link

    Hi anand,
    Full tight grip is about tight grip with another palm cover up the phone as well. That's very similar to how a tight-grip-fellow talking over the phone call.

    I just did a test. Full-tight-grip cost -24dBm in my htc desire. Where as tight grip cost -12dBm. What about iPhone4?
  • rushbc - Saturday, July 17, 2010 - link

    my favorite quote from the article is this one:
    "Ultimately this is why I consider Apple’s design here to be unnecessarily risky. Introducing a change with stylistic and technical benefits where the downside is limited but potentially very noticeable is just ballsy."

    i agree and would like to add the following:

    so is creating and mass-producing a $700 smartphone out of glass!!
    that is pure Steve Jobs "balls to the wall, damn the torpedos, full speed ahead, F 'em if they don't like it" ballsy. love ya, Steve (and J.Ive), but you guys may be a little too aggressive sometimes in your design decisions.

    i love my iphone 4, it's great, it's fast, it's beautiful, it works super for ALL of my needs--yes even phone calling/signal strength is great--but i am in constant fear of dropping it. dropping a $700 piece of glass would suck, because, guess what? glass breaks when you drop it!

    ...but try not dropping something that you carry around with you every single day, wherever you go, and are constantly using it throughout each day, picking it up, putting it down, taking it out of pocket, putting it back in pocket, etc. not an easy requires constant care and vigilance to not drop the darn thing. and yes, glass can be slippery, so theres that, too.

    love the phone, hate the stress!
  • Griswold - Saturday, July 17, 2010 - link

    Get one of the free bumpers. It would be the first I'd do with a glass sandwich like that.
  • ajb - Saturday, July 17, 2010 - link

    I notice that in the photos of the iPhone 4 compared to the 3GS you were holding the iPhone 4 in your left hand (presumably having much more contact with the sensitive area of the antenna aray) while the photo of the 3GS depicts you holding the phone in your right hand. Could this have resulted in whole or in part for the larger drop of signal strength with the iPhone 4? Trust me, I'm not trying to defend the iPhone 4 - I've had enough problems of my own with maintaining a signal. I only ask to make sure that the numbers quoted are accurate.

    Also, I have noticed a very discernable difference in wi-fi performance at the weak end of the wi-fi bar scale (2 out of 3 wi-fi bars). My 3GS did much better in performance than does my 4. I sometimes get the impression that the phone's alogorithm at that wif-fi signal strength can't make up its mind whether it wants to be on 3G or wi-fi, which even with 2 bars should be and with the 3GS always was much faster than 3G. I do believe I read somewhere that there might be a problem with not releasing expired IP addresses, but I don't think that accounts for all of the problems I have been having because even after turning off wi-fi, moving closer to my wireless router, turning wi-fi back and, and walking back to where I originally was, I continue to experience the problem. Have you done any wi-fi testing? Do you have any insights into this problem? Any idea if this is a wide-spread problem?
  • codeachrome - Saturday, July 17, 2010 - link

  • anandreader - Sunday, July 18, 2010 - link

    Could you clarify what you did there?

    I'd like to enable dBm instead of bars on my iPhone.

  • Fri13 - Sunday, July 18, 2010 - link

    "While the software update obviously does not and cannot address the design of the antenna itself - or make the drop from holding the phone any less - it does change the way the issue is perceived among users. "

    Actually the algorithm should effect the reception. The firmware (where the algorithm is placed) is responsible to use more or less power (W) to boost the signal what phone antenna is receiving. I have heard that max output is 2W. With 1W boost you gain -30dB.

    While the bars tells the signal strength from cellphone to tover, the algorithm needs to compensate the drop/rise of the signal strength so it would not drop off. So if the max drop of is -24dB and before the iOS 4.0.1 the 1-4 bars were merely calculating -22dB range before drop out, the boost was never enough to keep the cellphone in the network (like you had -22dB signal, you touched and you got -24dB drop and algorithm boosted only -15dB, you were -7dB over the needed signal strength).

    As anandtech has noted, software can not affect the hardware itself. The software can not eliminate the -24dB drop what is coming from hand (and touch). But the software can make the drop smaller by boosting the signal to needed level.

    What is the needed level? Stronger signal = bigger power consuming -> less talk time / stand-by time. Needed level is simply that what gives good enough voice quality and keeps data transfer at maximum. It is not wise to overrun the power use so the battery is empty more faster.

    And do people remember when Apple did improve the battery life in the 3Gs time with software patch? That would be very easy to do just by making the signal boost algorithm such that it does not use so much power = weaker boosting.

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