A friend asked me why touching the iPhone antenna hurts the signal while holding a television antenna (rabbit ears) generally makes the signal better (so much so that it is sometimes hard to actually set them to get a good signal otherwise).
I know why TV antennas work that way, but I could not for the life of me figure out (or find via multiple on-line searches) why it’s the opposite for the iPhone. Is it just that the antennas in a phone are so much more tweaked to perfection, that there is nowhere to go but down? Or is it that you’re blocking the signal? Or is there some difference between digital and analog that I don’t know about? Or is it something completely different?
I haven’t looked at the antenna design in the iPhone, but I would suspect that it is very precisely tuned for the frequencies it is designed to receive. Adding a load (your hand) to the antenna de-tunes it, resulting in a signal strength reduction. Note that touching TV antennas doesn’t always improve the signal, either.
So in layman’s terms, would it be safe to say that touching an antenna does 2 things: changes the frequency it listens to, and increases its overall reception strength (perhaps not a lot)?
So touching a TV antenna causes the the strength to go up and modifies the ideal frequency it responds to. The former is (generally) a better “good” than the latter is a “bad”, so the net result is a better picture (in general).
And touching an iPhone antenna does both as well, though no increased overall strength will overcome the fact that you are changing the very precise frequency it needs to work?
Antenna design is very,very complicated.
The simplest antennas are a length of wire that is some fraction of the frequency you are interested in (usually 1/4 or 1/2). These antennas respond the the Electrical part of the signal, and are relatively insensitive to de-tuning by capacitive or resistive loading (like a hand). So, if you have a simple TV antenna, touching it may increase the strength of the received signal because your body changes the effective length of the antenna. If the length increase happens to be close to a multiple of the frequency you are trying to receive, then you get a stronger signal. With a simple antenna (these are called dipoles, BTW), the antenna length isn’t terribly critical - you can be off by 25% and still get a decent signal.
An optimized antenna like the ones in most cell phones is very carefully tuned, and is usually designed to work on multiple frequency bands. They also may also respond to both the electrical and the magnetic parts of the signal. This usually involves some mind-numbing simulations to determine how the antenna will perform at different orientations, and with the variable loading caused by the antenna being close to your hand and head. In the iPhone 4, the antenna is actually part of the external case. This means that for most situations, the antenna performs better than the one in the 3GS, but it also means that if you hold the phone in a “death grip” you load the antenna to the point where it is de-tuned enough to not work very well.
I had to do a bunch of research on antenna design for a 900MHz transceiver that I was working on. The bottom line is: there are people who are experts in antenna design, and if I ever need to have a custom antenna made for a project, I’m not going to try to do it myself - it’s just way too esoteric a field of engineering.
Part of the problem with the iPhone4 is that the death grip bridges the wifi antenna to the phone antenna via a short bridge of skin - this significantly degrades the performance. Just an insulating layer of sellotape (or the free silicone covers Apple are giving away) prevents this short and restores reception. The hand wrapping round the antenna is actually less significant than the bridging.
The other part of the problem was that OS4 was misrepresenting the signal strength in the first place. My iPhone 3Gs with OS4 was displaying 4 bars of signal strength in areas where it would’ve only shown 1 to 2 bars running on OS3. A small reduction in signal strength could then cause an apparent drop from 4 bars to 1 bar. It made the problem appear worse than it was.
In particular, si_blakely, thank you for your reply but if what you say is true (and I am not doubting you!) that just pushes my question back one step:
WHY is it bad to bridge the two antennas? Is it the same “bad” as has already been described, that the antenna length is so optimized that changing it can only make it worse, or is there something else going on that is particular to a wifi antenna?
I can think of a couple of reasons. Antenna length is one. However, an antenna is the open end of a tuned resonant circuit, and the wifi antenna circuit is tuned differently to the cellphone one. Coupling the two antenna creates a kind of closed loop (your skin is not a perfect conductor) with resonant elements - the performance (at either frequency) of such a setup is going to be appalling, as people discovered. It probably kills wifi as well, but the signals are likely to be stronger, so it may not be as apparent.
It was an obvious flaw and comes down to a terrible design decision by Apple, driven (I am guessing) by aesthetics. A clear insulating varnish or plastic buffer on the aerials would have made this whole thing go away. That this did not happen (as well as the entire product called the iPad) shows me that Apple have somewhat lost their way in the cool design vs actual usefulness battle.