Another Antenna Engineer on the iPhone

Spencer Webb at AntennaSys has written a good post on the iPhone antenna problems. What’s he’s said about detuning and attenuation is very similar to what I’ve said in other posts. I doubt this is because he reads my blog, it’s more likely because we’ve both had similar experience with handsets before and know many of the same things.

I have been thinking about this more though, and there is a possible explanation for why damp hands may cause the problem. As I said earlier at GHz frequencies damp hands don’t conduct well. But, what if the problem is at much lower frequencies? It could be that the WLAN/BT/GPS antenna is polluted with low frequency interference in the KHz or MHz region. Then, when the user puts a damp finger over the gap that interference is conducted to the cellular antenna. This would only cause a problem if the cellular antenna radio were sensitive to the low frequency interference, that shouldn’t happen but it could happen if mistakes in the design were made.

That would be a very odd cause though, I still think detuning is more likely.

Apple Claim Signal Strength Display Causes iPhone 4 Issue

Apple are claiming that the problem with the iPhone 4 is only down to the signal strength display. But, many users have demonstrated that the drop in signal strength affects both data throughput and can cause dropped calls. As I mentioned yesterday Anandtech have measured the signal strength drop and found that it’s very large.

Apple have said:

Upon investigation, we were stunned to find that the formula we use to calculate how many bars of signal strength to display is totally wrong.

Our formula, in many instances, mistakenly displays two more bars than it should for a given signal strength. For example, we sometimes display four bars when we should be displaying as few as two bars.

This does make some sense. The users affected may be in poor signal areas that are being reported as good signal areas. So, the degradation is exaggerated. However, that doesn’t show that Anandtech or the other testers are wrong. Anandtech’s test didn’t depend on the bar display, they used a piece of software that extracts the signal strength in dB from the radio.

Also, the problem is reported to affect both WCDMA(3G) and GSM EDGE. The signal levels involved in these two protocols are quite different. The algorithm or formula that takes signal strength information (and sometimes other info) and produces the display of signal strength bars is generally different. That is, WCDMA has an algorithm to produce the bar display and EDGE has a different algorithm.

iPhone 4 antenna problem – measurements of the signal strength drop

The folks at Anandtech have hacked an iPhone 4 to produce a signal strength indication in dB. They’ve then experimented to find the signal strength drop when the phone is held in various orientations. See the article here. Since they’ve measured this with a normal network in a real-world environment I don’t expect that the results will be very accurate. The propagation path between the test site and the nearest base-station will sometimes change. However, the measurements give a ball-park idea of what the drop in signal is like. If the handset is “cupped tightly” the drop in signal-strength is 24.6dB, it it’s “held naturally” the drop is 19.8dB.

One of the handsets they compare it with is the Nexus One. Interestingly the Nexus One doesn’t do too well either.

What Could Apple do to Fix the iPhone 4 Performance Issues?

A few folks have asked me what Apple could to to fix the problems. As I wrote in earlier posts I think this is an antenna issue, that makes software fixes unlikely. It may be that the problem has a software component. For example, the signal strength bar display could be exaggerating the problem.

There may be a way to redesign the antenna to remove the problem without changing it much. In my opinion the dielectric properties of the hand are changing the capacitance between the metal bands on either side of the slot. That capacitance might not be a necessary part of the design. In that case it may be possible to solve the problem by making the plastic gap wider, that would reduce the capacitance in any scenario. If the capacitance is necessary though then things become more complex. It may be possible to move the sensitive area a little into the inside of the handset.

The iPhone 4 Reception Problems

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.

The iPhone 4 Antenna

New types of antenna are being invented all the time. Existing types of antenna are constantly being adapted to new roles. But, in most cases these efforts aren’t successful, most new antennas are outperformed by existing designs. The existing well-known types of antenna have their own niches. When designers attempt to apply them outside their normal niches in other niches where other antenna types predominate this usually fails. Even if the antenna works in its new application its normally found that the type normally used in that niche can outperform it. These stories are the tails of the failure of many research programmes and start-up companies.

The new antenna in the iPhone 4 is surprising and impressive because it’s designers have overcome these barriers. Most new handsets use one of the common types of antennas, the monopole, the vertical helix (or stubby) antenna or the PIFA. The antenna design is almost always new for every new handset, but it’s based on an existing archetype. When a new type of antenna being developed it will normally be released first in an obscure low-volume product.

In the iPhone 4 Apple have imported a new type of antenna into the handset niche. They’ve done this on their headline product, not just some minor experimental product that could fail without financial consequences. Steve Jobs mentioned the antenna in his keynote speech, he said that the stainless steel rim around the edge of the phone forms the antenna. He put up a slide showing it:

This slide doesn’t make it clear what type of antenna we’re dealing with here because it doesn’t show the feeds or the ground-plane. I think I’ve figured it out though by looking through some of Apple’s patent submissions. Apple have patented a lot of antenna designs, but two patents in particular looks very like the antenna in the iPhone 4 slides and teardowns on the internet. The first is US patent application #20100123632 and the second a granted US patent #7551142 and these describe types of slot antenna.

Like most patents it doesn’t describe the relevant ideas very clearly. So, I’ll explain it in my own way…. A normal slot antenna is a narrow rectangular hole cut in a large conductive surface.

The slot antenna is closely related to the dipole, it is the “complement” through the Babinet-Booker principle of the dipole. There’s an analysis of slot antennas from this perspective in Kraus & Marhefka. These simple slot antennas are used in antenna arrays for radars and various other types of electrically large antennas. But, they aren’t generally used for electrically small antennas. (Some folks may know of “slotted-PIFAs” these are PIFA antennas with slots in them but they aren’t slot antennas their design is very different). Slot antennas can be dielectrically loaded by filling the slot with a dielectric material such as plastic or ceramic.

The patent application suggests that Apple have designed a slot antenna that runs around the edge of the handset. The introduction says:

The antenna may be a slot antenna having a dielectric slot opening. The slot opening may have a shape such as a U shape or an L shape in which elongated regions of the slot run parallel to the edges of the portable electronic device. The portable electronic device may have a housing with conductive sidewalls. The conductive sidewalls may help define the shape of the slot. Antenna feed arrangements may be used to feed the slot antenna in a way that excites harmonic frequencies and that supports multiband operation while being shielded from proximity effects.

In the middle of phone there is a circuit board, battery, and display. All of these can be connected together, so that at RF frequencies they behave quite like a large block of conductive metal. This is common practice in handsets, as far as I can tell this is what’s being done in the iPhone 4. Around that there is a metal sidewall. The patent suggest that using a configuration like this slot antennas can formed from the gap between the metal sidewall and the internal conductive block.

The diagram above shows too slots both feed slightly off centre. One runs around the top and left side of the handset and the other around the rest. There are several ways of feeding such an antenna and the patent describes some of them. The simplest is a feed between the sidewall and main chassis driving a voltage between them. In the diagram above I’ve put on feeds and places where the slots end, these are just an illustration of the principle. I don’t know for sure if Apple have used this patent, if they have I don’t know where the feeds are. The patent explains that non-conducting front and back covers can be used to conceal and protect the antennas and internal electronics. Since these are covers of a dielectric material, that means there will be some dielectric load, and the antennas will be dielectrically loaded slots.

There’s lots more to talk about here. I haven’t covered much of the ideas in patent #7551142, I’ll do that in a future post. If I have time I’ll compare this to more conventional types of antennas like PIFAs. Also, I’ll comment on the problems recently uncovered with iPhone reception very soon.