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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  • takkischitt - Thursday, July 22, 2010 - link

    Hi,

    I've been looking everywhere to try and source the correct Kapton tape but I'm struggling.

    Can anyone give me an extact model number and description so I can hopefully find the right one to order on the net?

    Thanks!

    Great article btw
    Reply
  • crashnburn - Saturday, July 24, 2010 - link

    Question is.. Am I better off with getting an iPhone 4 or a 3GS Reply
  • v12v12 - Sunday, July 25, 2010 - link

    Wow this is hilarious... people are using tape and all kinds of MacGyver tricks and tinkering to get it to work "properly" or "better." WHO DOES THIS? Would any of you accept a washing machine that required a rubber stopper from home depot to properly seal in water? Or would you use some aluminum foil to increase your brand new flat screen's picture (hypothetically speaking of course)???

    RETURN the fscking thing, roll the dice and get another one, OR get a Droid or some other phone thats known to work?

    The stupid masses have gotten HAD again... "OMG I've GOT to get the newest, (BETA) latest, greatest phone so I can be cool and waste hours of daily productivity playing around with it like a toy like everyone else..." Baaaaah baaaaaaah, said the sheeple. WAKE UP noobs, YOU DO NOT BUY NEW PHONES, until they've been thoroughly tested and de-bugged. Christ, anyone that knows anything about technology knows TODAYS devices are public-BETAs that the corps use YOU to test, while they profit from your fully-paid dollars up front, which they immediately invest in hedge funds and the stock market to recoop massive short-sell interest from, while YOU all are not only footing the bill for, BUT doing all the leg-work by testing the phones and reporting (complaining) back to them... I digress.

    YOU all are the problem, there's NEVER been a "beta-test" type market like this crap we have today, bc YOU all have ALLOWED this POS beta-warez business model to flourish and take advantage OF US ALL... This is just a waste of txt, since you'll just keep buying and shopping to be "cool," and "hip."

    Thanks a lot sheep... See you at the slaughter house! Please... AFTER YOU... I insist ;-)
    Reply
  • kalleboo - Tuesday, July 27, 2010 - link

    The android bars vary by manufacturer, they're not all exactly the same. My Sony Ericsson X10 shows 3 bars for -85 dBm/14 asu, contrary to this graph Reply
  • bubblesmoney - Wednesday, July 28, 2010 - link

    On GSM or UMTS frequencies (between 870 MHz to 2170MHz around the world) used the noise signal floor is between -111 to -114dbm. The signal noise floor is that strength at which the signal cannot be distinguished from background radiation.

    The signal noise floor depends on the frequency of broadcast and not the device as far as i am aware. So the iphone cannot have a different noise floor compared to other mobile phones, unless Jobs got the governments all over the world to beam a special signal on a different frequency purely for the iphones!!!!

    Anandtech has made a basic error in this analysis i think and consequently this whole article is wrong and meant to favour the iphone either inadvertantly or deliberately.

    see post number 57 on the thread in this link for my explanation why this anandtech article is wrong http://forums.moneysavingexpert.com/showthread.php...

    please note that -113dbm (some resources say -111 instead of -113) is the noise floor where signal is indistinguishible from background radiation. look it up in science webpages if you doubt what i say. so if the iphone shows signal to be -120dbm then that is an error. just because it shows a number does not mean that the -113 noise floor value does not exist in physics for gsm broadcast frequencies. http://en.wikipedia.org/wiki/Johnson%E2%80%93Nyqui...

    see gsm freq bands for the world here http://en.wikipedia.org/wiki/GSM_frequency_bands the freq bands (GSM AND UMTS) used in uk are between 870 to 2170MHz and for that the noise floor is between -111dbm and -113dbm and for the noise floor to be 120dbm the freq would have to be in 180kilohertz which is !!!! and bull as the freq bands used are in MHz so the anandtech numbers dont add up in the 120dbm small print, as there is no 180kilohtz band for gsm in usa or uk as far as i can tell!!!! and 180khz is the freq used for AM band radio and looks like the iphone4 is getting interference from 180khz AM band radio signals too as far i can tell from what anandtech says about 120dbm etc !!! yikes!

    I would be happy to be proved wrong and to learn, but from what i have shown i am right! Noise floor depends on the frequency band of transmission rather than circuitry. In the UK Cellular mobile services operate within the frequency ranges 872-960 MHz, 1710-1875 MHz and 1920 - 2170 MHz so the noise floor would be between -114 to -111dbm.
    It would be similar in the USA too.

    see the rest of my detailed response elsewhere on other forums (post 57 of the thread on the link) http://forums.moneysavingexpert.com/showthread.php...

    I hope there is a response to this comment of mine, from this articles writers or some other RF engineers, as due to the reasons quoted in my ananlysis i think this article by anandtech is grossly wrong. I would be happy to stand corrected if my analysis is wrong.

    yes the signals can be sensed at -120dbm but that wouldnt be signals from commercial GSM or UMTS signals, it would be some other signals sensed by the iphone4 sensors and giving a wrong reading of the signal strength. As i said earlier the signal noise floor depends on the broadcast frequency and not the handset, so this article is wrong and grossly so as it is making assumptions of the iphone4 being able to sense GSM / UMTS signals of the order of 120dbm which isnt possible for the frequencies broadcast as per the physics involved. http://en.wikipedia.org/wiki/Johnson%E2%80%93Nyqui... that or Jobs and anandtech found some way to defy physics!
    Reply
  • amariofrmdamoon - Sunday, November 4, 2012 - link

    I've never seen kapton tape used that way, but a good source for it seems to be www.kaptontape.com. There are some other insulating tapes at www.maskingproducts.net. May be worth a try to see if some other cheaper tapes work in a similar manner. Anyone know if it works similarly with the iphone 5. Thanks! Reply

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