BosonQuest

Learn Physics or Get Bored Trying

iPhone 4 versus duct tape

There has been an awful lot of talk lately about the iPhone 4’s antenna troubles. And indeed, from having used one for a few weeks, it does indeed behave badly if held in a particular manner in a weak signal area. But one rumour has been following this story around like a bad smell—the legendary Duct Tape “Solution”.

It seems obvious enough—two antennae next to each other, separated by an insulated gap; fine until you “bridge” the gap by having your hand in contact with both sides, causing a short-circuit. Hey presto: no signal. So a small square of insulating tape over the gap, and problem solved! Alas, no.

Most people have heard of a capacitor, though perhaps don’t know what one is or does. Quite simply, it is two conductive “plates” separated by a thin dielectric, or insulating, layer. So an antenna with a layer of tape being held by a person such that an area of skin is in contact with the tape is basically a large capacitor. When a voltage is applied across a capacitor, charge builds up on the two conducting plates over time. As the charge builds up, the capacitor starts to resist more and more current from flowing, eventually stopping the flow altogether. The amount of charge that can accumulate is called the capacitance, and this varies directly with the area of the conducting plates (area of hand covering antenna), and inversely with the thickness of the dielectric layer (tape).

In an alternating, or oscillating, current, both the voltage across a component and the current flowing through it resemble a sine-wave. If you divide the amplitude (height) of the voltage wave by the amplitude of the current wave across a component, you get a number that is called the impedance of the component. Note the similarity with the resistance of a component in a DC circuit, which you might recall from school is equal to the (constant) voltage divided by the (also constant) current. The impedance of a component in an alternating current is essentially how much it… well… impedes the flow of current.

What this is leading up to is the fact that, across a capacitor in an alternating current, the impedance varies inversely with both the capacitance and the alternating frequency of the signal, which for mobile phone radio waves is pretty high—around 400-1800 Mhz. So the impedance of a piece of tape separating a radio antenna and your hand is negligible. You may as well be touching the antenna directly.

Roughly speaking, in an alternating current, rather than charge flowing round a circuit, it oscillates back and forth. With a capacitor in the way, charge builds up on the plates just as in a DC circuit, but at high frequencies, it turns round and goes the other way too fast for enough charge to build up on the plates to impact the current.

The good news for people whose phones work with a bit of duct tape on the antenna is that your phone works without it too. Trust me. The bad news is that if you have gone around saying your phone is unusable without tape, then you are fooling yourself. And the really bad news for people who have published damning reviews of the iPhone 4 while suggesting that duct tape will fix it, is that you are lying to your readers. Plain as. This is not to say that the phone doesn’t have an antenna problem—it does—but if you say your tests showed improvement with duct tape, you cast serious doubt on the validity of your tests.

As for solutions, well, with a bumper, or other case, you are increasing the thickness of the dielectric layer in the “capacitor” between your hand and the antenna, dropping the capacitance way down, and to the point that its impedance is too great to cause the antenna detuning issue, even at radio frequencies. You can also minimise the capacitance by altering your grasp of the phone—lower the contact area and you lower the capacitance. So Apple’s well-published suggestions “not to hold it that way” or “use a case” are scientifically the best ways to reduce the problem. But as many have pointed out, a phone for which the natural grip is the wrong one (all phones have bad areas, most aren’t right where you want to put your hand), or for which you need to spend additional cash to get a decent performance is a poorly designed phone. Apple should really address this somehow; right now it seems that the best course would be the free case, and iPhone users putting up with the fact that our phones are awesome at pretty much everything… except being phones. Or return it, of couse.

If you read the rest of my blog here you will see that I am but an amateur physicist, so it’s fair to say I could have this wrong. But the level of physics in this material is not particularly high, and since there’s no real way that I can be setting up experiments in the areas I’m currently studying, I jumped at the chance to test this theory out for myself. And while the results were more qualitative than quantitative (since I lack the means), I have tested duct tape, electrical tape, Sellotape and the plastic overlays that came off the phone when I unboxed it. No difference was noticeable with any of these materials entirely covering the parts of the antenna that were in contact with my hand. If you want a more credible source for these findings, there are a number of posts worth reading at www.antennasys.com/antennasys-blog.

Ain’t science a bitch?

Update: thanks to the nice folk at Anandtech, it seems I’m right and wrong. High impedance tape does exist, and can be applied to the phone. Even with such tape, the impedance is not enough to completely isolate you from the antenna, though perhaps it could do so enough to bring the phone into line with others. I would guess the tape must be made from a material with an impressively low dielectric constant; I still believe that the DIY fixes involving bog-standard tape are spurious, and if you want the best performance in a weak signal, a case would be a good idea. But it’s nice to think that there is a potential solution out there—also nice to know I was on the right tracks.

July 14, 2010 - Posted by | Science in the media, Tech | , , ,

8 Comments »

  1. Hi crosswordbob.

    I can’t help but notice the article you’ve linked to disagrees with you.

    They did disagree with the testing methods of Consumer Reports, but the overall opinion was certainly that any ‘bumper’ will help alleviate the problem, duct tape included.

    What about the Consumer Reports “Duct Tape” fix. Yep, it will help. Any insulator over the “gap” area of the antenna is going to help in direct proportion to its thickness. I think Consumer Reports was going for style points in the selection of Duct Tape. Nice move – they sure dominated the news cycle.

    But, hey, Consumer Reports guys: you don’t do radiated tests in a shield room. That’s like measuring the light output of a desk lamp in a house of mirrors. It’s amateur hour. Either you didn’t really explain your experimental technique fully in your video and text on your website, or perhaps you did and it really stinks. In either case, we end up agreeing with each other, so let’s not dwell on that too much.

    Comment by Julian Wearne | July 15, 2010 | Reply

    • Hi,

      Fair comment. The article containing that arrived after mine, and is certainly very interesting reading. But it doesn’t entirely disagree with what I’ve written here—to say the impedance varies inversely with the thickness of the dielectric layer is equivalent to saying it’ll “help in direct proportion to its thickness”, though approaches it from a more pessimistic direction. I’d say “going for style points” rather implies that they’re not entirely convinced by the effectiveness of Consumer Report’s “solution”.

      Comment by crosswordbob | July 15, 2010 | Reply

  2. Its cool to see you applying your physics knowledge to the real world (something many physics majors suck at).

    But I’d be careful of outright saying something WILL NOT work before you test it properly.

    It hurts your credibility

    Comment by AFreshMint | July 16, 2010 | Reply

    • Meh—lost the last of that ages ago ;-)

      But seriously, that’s a fair comment also, and on a new blog I appreciate getting any reply. As I re-read, I think I must’ve been a bit tetchy when writing this, and probably felt like laying down the law (s of physics).

      So yeah, there should definitely be a clear disclaimer that I might be talking out my hat, and I would genuinely appreciate being shown so; it’s all part of the learning process.

      Comment by crosswordbob | July 16, 2010 | Reply

  3. If there’s one quote that has stuck with me for years and driven me nuts it’s “Perception is reality.” Your perception may be your reality but that does not make what you said truly factual.

    The issue could be capacitance, true. It could also be a voltage offset between the antennas with the mere conduction of DC voltage between the antennas from perspiration causing an issue for the “failing” circuit. It could even be the direct conductivity messing with the effective antenna length. If it’s either of those than even minimal insulation would provide serious improvement.

    Beyond signal strength, this conductivity could explain why some people experience it and others do not. People naturally have different amounts of perspiration with differing salt content.

    Consumer Reports may have limited knowledge/ability, but at least they tested their hypothesis. If they found it helped, and others stated it helped, then I can feel just fine in saying it helped.

    Comment by Greg | July 16, 2010 | Reply

    • I can’t disagree—if the issue has nothing to do with capacitance, a simple insulator would help. But the fact is that I did test with several forms of isulator; duct tape, electrical tape, Sellotape and the (pretty thick) plastic sheeting that came off the phone in unboxing. In the last of these I had two such sheets, so was able to cover both sides of the phone with no part of me touching metal. No discernable difference in any case.

      And to be honest, I’m not convinced that CR actually did test their theory. They don’t make direct reference to it in the main paragraphs about their testing procedure—which was sketchy at best—they mention it almost as an afterthought, and frankly it looks to me like they were playing for laughs more than offering consumer advice. I could be wrong, but I have asked them to clarify whether the tape was part of their controlled test and had no response.

      I stand by the physics and tests (allbethey amateur; I recommend people do their own tests rather rely on mine). And I would add that Apple have now had two great opportunities to go with a protective layer—right now I’d expect them to jump at any opportunity to be seen to be doing something—but clearly don’t think it’s a viable solution. Time may prove me wrong; and if someone knows more about the science, I’d love them to tell me so, since the whole point of the blog is to learn about this stuff.

      Comment by crosswordbob | July 16, 2010 | Reply

  4. Wrong model? I think your model of this problem as: “antenna + dielectric(tape) + a hand” might be flawed. I see it as “antenna#1 + dielectric(a hand) + antenna#2″ because there are two antennae involved, as shown in a picture here:
    http://www.anandtech.com/show/3794/the-iphone-4-review/2
    So it’s maybe more about the difference between an oily/sweaty hand bridging the two antennae versus a tape covering a hand?

    Also, in your own tests there may not have been enough difference between your control (your bare hand) and the different tapes you tried. Are your hands oily/sweaty or very dry? Do you see the signal degrade when you put a finger over the black gap between the two antennae?

    Comment by George | July 17, 2010 | Reply

  5. Whether the model is antenna + hand or antenna1 + hand + antenna2 I’m not going to pretend to know for sure. Rudimentary testing does seem to support the conjecture that it is sharpest when the gap is bridged. But my contention is that adding a thin dielectric bettween any hand/antenna interface doesn’t qualitativly affect that interface’s dynamics; so in the model in which your hand bridges two antennae, that is not greatly improved either.

    As for local properties of my hand, I really don’t know, which is why I would urge people to do their own testing. I was able to observe degradation with one finger, with and without insulating layer. But the message of the post is more to make people aware that the physics is not so simple as “stick something non-conductive in and you’re done”.

    In terms of something more practical, I’m taking a leaf out of Anandtech’s book and looking into what sort of tapes are available. Kapton tape that they used is said to have high resistance, and dielectric strength, but their data sheet doesn’t specify dielectric constant, which is what I believe will be the key to a useful tape-solution. I have left a query with their tech support, and will post my findings.

    Update: The wikipedia article on Relative permittivity (essentially the same as dielectric constant but measured differently) gives Teflon an impressively low permittivity of 2.1, so that’s certainly where I’ll be looking now

    Comment by crosswordbob | July 17, 2010 | Reply


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