When the iPhone 4 antenna was announced I commented “This is a very difficult thing to do” in an article on the Wall Street Journal website. (I gave lots of other comments too, though these mostly weren’t used.)
Now the internet is abuzz with discussion of reception problems that have been found with the iPhone 4. Many are blaming the new antenna design. I don’t know for certain what the problem is, but I can make some informed speculation.
Customers who have bought the iPhone have noticed that when they touch a certain place then the reception gets much worse. That place is the small band of plastic in the out metal sidewall on the bottom left.
This video isn’t me, but it’s one of the better videos demonstrating the problem on the net.
A lots of folks on the internet have been asking if this problem is common to other phones. In a press release Apple said:
“Gripping any phone will result in some attenuation of its antenna performance with certain places being worse than others depending on the placement of the antennas. This is a fact of life for every wireless phone. If you ever experience this on your Phone 4, avoid gripping it in the lower left corner in a way that covers both sides of the black strip in the metal band, or simply use one of many available cases.”
It’s quite true that other phones suffer from similar problems. There are now several videos on the internet showing the signal strength report on various phones deteriorating when they are held. This has been shown using old Nokia phones, Blackberries and earlier iPhones. This is a well known problem. The important question though is: does the iPhone 4 suffer from it *more* than other handsets? In my opinion from the evidence available online the answer to that question is “Yes”.
The human hand contains a lot of water and other materials which absorb EM waves. It is also strongly dielectric, muscle has a dielectric constant of ~56. So, any capacitive effects are strengthened by the presence of the hand. Both of these have an effect on antenna performance. Suppose you cover over the antenna on your handset with your hand. Because of the absorbtion less radiation reaches the basestation. Also, the antenna contains distributed capacitance, this capacitance is increased by the dielectric properties of the hand, the antenna become more dielectrically loaded. The antenna has been tuned to work at the design frequencies with the normal capacitances. This disturbance causes “detuning” and the performance of the antenna reduces at frequencies bands it was designed for. Often there is a frequency shift and the antenna’s bands of good performance move up in frequency or down. People talk a lot about the absorption effect, but in practical cases the dielectric effect is often more important.
The magnitude of this problem was measured by G.F.Pedersen in several papers that comprise his PhD thesis. Pedersen got many people to hold a mobile phone like they would in a phone call and measured it’s performance as they held it, to find out the loss caused by the proximity of the head and hand. He collected the data for several types of antenna, including an internal PIFA antenna mounted at the top of the handset. Pedersen’s measurements show a great range of performance depending on how the specific characteristics of the user, such as how they hold the phone, the size of their head, etc. He found that for the PIFA antenna the average degradation was ~3dB. In the past I’ve found that modern handsets with internal PIFA antennas close to the top are a little worse than the one Pedersen used, but not much worse, I’d estimate the loss at 3 – 6dB. Those with PIFA antennas at the bottom are worse because because the user tends to cover over the antenna with their hand. I don’t have an estimate for the loss in that case.
The iPhone 4 doesn’t report the signal strength in dBs. Like most phones, it indicates it to the user by showing “strength bars”. At present nobody outside Apple knows how those strength bars work. The may include data other than strength too, such as signal to noise ratio. However, the various videos in the internet do show the signal strength dropping from 5-bars down to no-signal. Normally when a phone gives “full-bars” that means it’s receiving at least 10dB more than the minimum it can cope with. Often full-bars means that the phone is receiving 20dB more than the minimum. This indicates that the performance degradation for the iPhone 4 is especially bad. I can’t be sure of that, but that’s my view from the available evidence.
There are several ways that this problem could occur. Firstly, the loss introduced by placing a hand around the phone may be especially high. I don’t think this is likely because there have been demonstrations on the internet that show the problem is associated with a particular area on the phone. If the plastic band at the bottom left is touched then the performance deteriorates. The most likely explanation for this is associated with dielectric properties. If there is a lot of fringing capacitance between the two pieces of metal at either side of the gap then placing a dielectric object – such as a finger – across the slot will radically change the capacitance. That it turn will change detune the antenna and reduce the performance in some bands.
Some folks have suggested that the problem could be due to conductivity. They’ve suggested that the moisture on people’s fingers may cause a current path between the two pieces of metal. This is possible, if it did happen it would certainly cause big problems. Some folks on the internet have put a coin across the gap and showed that this causes performance degradation. However, there are some videos now showing the problem occurring with iPhones in rubber and plastic cases. These cases are unlikely to conduct well at GHz frequencies, so I think the dielectric explanation is more likely.
Some people aren’t reporting any problems with their iPhone. That’s not really so surprising since lots of people live in areas of high signal-strength (see my post on networks). Also, if dielectric detuning is the problem then it will not affect all bands and channels equally. It may be that some bands are entirely unaffected. So, we should expect only a subset of users to be affected.
Starting with the iPhone 4 Apple have started offering a cover, called a “bumper”, for the iPhone. It’s interesting that the experiments with covers and outer cases have been inconsistent. Sometimes the cover has solved the problem, sometimes it hasn’t, sometimes it’s helped a bit. This could be because the various covers are different, some dielectrically load the critical parts of the antenna, and some don’t. Perhaps some allows the users hand close enough that it causes dielectric loading but others keep the hand further away.
Lastly, an answer to something that has puzzled a few people – why does the signal strength display change so slowly? That’s because these sort of displays use averaging. They average the signal strength measurements over the last few seconds. So, even if the signal strength changes quickly the display doesn’t. This is a feature to make the display more user friends, all handsets do it.