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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  • MichaelEmpire - Sunday, July 18, 2010 - link

    If anybody can suggest the way or utility to have indicator in DB ? Other than resore from the previuose jailbroken phone.

    Thank you.
  • 386DX - Sunday, July 18, 2010 - link

    Great job once again. While your look at the iPhone4 and its antenna issue is fairly complete there is one more thing I and I'm sure others would be interest in. We know that the bumper appears to fix the antenna issue with the iphone 4 but it would be interested in seeing if using a bumper has any effect on the battery life. Looking at the original iPhone4 review comparing battery life I can't help notice that the 4 has a 16% higher cappacity battery but only gave 12% running time when using wifi over the 3GS. Yet when it came to 3G and talk time it gave 38-58% more running time. This leads me to believe the gain in battery life is mainly due to the external antenna design (allowing the phone to use less W to maintain signal). I wonder if using a bumper would affect battery life possibily because the phone may need to run the transmitter at a higer power to maintain the same signal.
  • tzikis - Sunday, July 18, 2010 - link

    Hi Brian, sorry to bring this up again, but the link you provided does not mention the width of the tape. Could you please measure it and let us know what is the proper width for a tape that will perfectly fit the iPhone?
  • dsj123 - Monday, July 19, 2010 - link

    Thanks very much for all the effort to do the iPhone4 review(s); very much appreciated!
  • The0ne - Monday, July 19, 2010 - link

    I'm seriously laughing my ass off just reading the comments for the review. It is clear we have tons of consumers that can and will "live" with just about any crap that they can't find themselves to peel away from. I'm always amazed at this phenomenon. It's got to be the greatest mystery ever.

    And it is astoundingly clear most members have little to absolutely NO CLUE how RF testing is carried out, nor apparently does Anandtech. This is why writing something that is not consistent and bias persuades the masses of readers to believe what they don't even know in the first place. All they know and want to believe is that Anandtech, or in this case, Anand and Brian said it's AT&Ts fault, REGARDLESS that others AT&T phones do not have the issue nor require any special tape/cases.

    Look how many loyal members you have supporting you when they really don't have much experience or expertise in the engineering, including RF, field. As one of the original poster stated, this is so bias it's not even funny. An article detailing how to scotch tape the iphone to normal working conditions while at the same time claiming it's the providers fault? You have got to be kidding me. Unbelievable but not surprising I guess.
  • v12v12 - Tuesday, July 20, 2010 - link

    1) I'm lmao bc of all the boasting and bellowing about numbers and credibility of "engineering" regarding this article that YOU, yes YOU haven't even shown you can grasp nor expand our knowledge base on the subject lol.

    2) STFU and prove you know more than what the article's OP(s) have stated, or GTFO?

    3) Again you have ZERO proof of your claim, even when you try and disparage other's; you are the clown here until so forth.

    4) Everything you've stated thus far is 100% conjecture, speculation and BASELESS w/o PROOF.

    5) WAITING FOR PROOF of your claims, kid... Lets SEE just how much you actually know... Or as said—STFU + GTFO.

    So... just WHO really doesn't have an understanding of the RF/Engineering field; apparently you haven't shown any either... laughing my ass off.

  • Janet55 - Monday, July 19, 2010 - link

    hmm, I read the news "Live from Apple's iPhone 4 press conference".
    Dude. I'm 99% sure there will be no recalls or h/w fixes. Like some dude called it, they'll say
    1) 99% phones are okay
    2) 1% suffer from that issue, which still makes us the best and greatest
    3) we're giving you free/discounted bumpers so worship us forever
    While I love their products, of course I do hope Apple steps up on the iPhone 4 improving as soon.
    The iPhone 4 tempting features, HD Video Recording and lovely pictures you're surprised in iFunia iPhone column
  • tlindaas - Tuesday, July 20, 2010 - link

    It seems the iPhone 4 gets better overall reception compared to other phones because of the large, exposed, external antennas, but at the same time this alternative design means is is more sensitive to the disturbances from human hands.

    What I would like to know is this: When holding the iPhone 4 as badly as possible, is it better or worse reception-wise compared to holding a "normal" phone as badly as possible, under poor conditions?

    If a normal phone start to drop calls in the 107-113 dBm range, and the iPhone 4 hold the calls all the way down to -121 dBm, shouldn't this imply that deathgripped iPhone 4s gets roughly the same reception as deathgripped normal phones? Point in case: HTC Nexus One, which drops 10 dBm held naturally, compared to 20 dBm for the iPhone 4:
  • viewfly - Tuesday, July 20, 2010 - link

    Brian & Anand:

    Great fair review guys. I too can display RSSI in dBm in my iPhone 4: it was passed down from my 3G ->3GS and now IP4 with iTunes. I also examined data speeds with

    My results. Have you seen the same?

    1. I'm right handed and don't normally do a tight grip. I have a bumper, but I can place my fingers anywhere on the metal SS band (except lower left 'sweet' spot) and see no change in data speeds or RSSI.

    2. I can take my thumb and index finger and bridge the gap above and below, with no adverse effects...only when I place my finger directly on the gap is there a problem ( 20+ dB loss). Also, placing a finger directly on the top gap (by earplug) produces no effect. The top gap also separates the two antenna, just like the lower gap. Interesting that it has no effect.

    3. When in a lower RF signal range ( -110 to -95 dBm) I can see data stop and loss of 22 dB when placing my finger directly on the lower fingers elsewhere, or even a few mm's near the gap have no great effect.

    4. When in a hight RF signal range ( -41 to -78 dBM) I see NO data or RSSI dB adverse effect at all. I had expected to see it drop by 22 dB too...but that would be ok...but instead I mostly see no change. Only once out of 10 tries did I see a dB change. I think I understand why some see to antenna issue at all.

    5. I tried kapton tape too. Just over the gap. It helped (data speeds where 850 kbps instead of 2200 kbps) and dB dropped less...but it was not perfect. I'm guessing that a coating would not help either. Only the bumper helped completely. I'm afraid many people will be ripped off buying tape from vendors...

    In general, being right handed and the phone made for right handers ( volume on left side of phone), and the way that I hold the iP4...I have really no problem going without a case. I was surprised how easy it is to avoid. Even in horizontal browsing fingers can touch the metal but avoid the lower gap.

    Maybe Apple should give people the RSSI display would help give good feedback to the user...we humans are easily trained!
  • viewfly - Tuesday, July 20, 2010 - link


    On item 4, I meant to say that i see no effect with my finger even directly on the lower left gap when in a high RF signal area ( -41 to -78 dB). I haven't mapped out carefully the range of -78 to takes time!

    Given that the 1mm gap is only about 0.01% of the total SS metal surface area...I am finding it pretty easy to avoid. So I'm having no problem operating without a case.

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