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.

In the WSJ article on the Apple iPhone antenna Ian Sherr mentions problems with dropped calls. I often here about dropped calls when I’m talking to Americans about cellular technology, but I hardly ever here this from Europeans. There are good reasons for this. Before discussing the iPhone antenna I think it’s useful to explain the differences between cellular networks across the world.

Suppose you work for a fast-food chain selling hamburgers or pizzas, you are a manager and you have to decide where to locate your next restaurant. You could put it in a small rural town. In that case there would only be a few other restaurants competing with it. The land to put the restaurant on would be cheap. But, on the other hand, there wouldn’t be many customers. Alternatively, you could put it in a big city. In that city there would be lots of competing restaurants and the rent would be high. But there would be lots of potential customers, if you could out-compete the other fast food restaurants then the potential for profit is very high. In both cases the restaurant itself would be quite similar, though you many need a few more seats, staff, friers and parking spaces if the restaurant were in the big city.

Cellphone network companies are in a similar situation. A cellular network in a sparsely populated area may have little or no competition. It’s relatively cheap to buy the land to put the cell towers on, but the number of potential customers is low. In a big city there is lots of competition and the land is expensive. However, the number of potential customers is very high. This is the logic that drives much of the cellular network business.

When cellphone networks first became practical they were applied first in the most densely populated cities in the richest countries. This was purely for the economic reasons I mention. The big cities allowed early cell networks to cover a large customer-base with few masts. Then, in time the cellular networks spread into more sparsely populated places and poorer places. These forces led to two distinct technical issue “coverage” and “capacity”. What concerned the network in big cities was capacity. They had lots of customers and lots of data to move. They had to make sure that the networks could absorb more customers without bandwidth limitations being reached. The people dealing with more sparsely populated areas had other concerns, they wanted to build a network to cover as wide an area as possible as cheaply as possible. They needed longer range base-stations and cheaper ones.

So, we could draw a capacity/coverage map of the world, and it would approximately follow population density. For example, in places like central Tokyo capacity is the concern, in the American countryside coverage is the concern. Lots of places fall somewhere in-between.

These problem have driven many specific technical solutions. For example, there are base-station antennas that are specially designed to deal with the situation where the network must cover a long road, but need not cover the adjoining countryside. A special type of small cell, the microcell, has been created to deal with areas where capacity is important. Many features of WCDMA are targeted at providing high capacity.

Whole countries can be looked at in a similar way, and this is where history comes in. In some countries the population is centred on large densely populated cities. In Japan, Korea, China and Spain it’s quite normal for people to live in apartments buildings in large cities. In some of these places the local culture and the law encourages this. In Britain there are strict planning regulations and taxes on transport fuel to prevent the growth of suburbs. In the US and Canada, for example, the culture is different people generally prefer to live in suburbs and the planning laws are set up to encourage use of the car rather than discourage it. This leads to different problems for network operators. In the US and Canada there are large areas of moderately populated surburbia, with some areas of high population and large areas of low population. European and Asian countries have areas of low population too, but the cities and suburbs are more highly populated.

This has direct effects on the technological problems. In areas where coverage is important the performance of the handset is more important. Below I’ve drawn a rough diagram of the differences between cellular networks, the scale is the same here…

In order to tackle the capacity problem in the densely populated Spanish city the network planners have used a large number of small cells. In the American suburb the planners have used a few large cells to serve their more spread-out customer base. In the diagram of the US case the cell tower is in the centre. It has four antennas on and four sets of base-station electronics producing four cells which I’ve marked in red, yellow, green and blue.

This has a strong effect on the cellphone market. In the US case a handset at the far side of the red cell is a long way from the base-station. So, the signal from the base-station is weak, and the signal received by the base-station from the handset is weak. If the receiver, transmitter or antenna on the handset is poor then it will drop calls on the edges of the cells. This isn’t the case in the Spanish city situation. There the cells are very close together and a cellphone would always be close to one of them. So, the quality of the RF electronics on the handset is less critical. Indirectly, coverage problems make the performance of the RF electronics (the antenna, low-noise amplifier, power amplifier, etc) more important. This is why Americans so often complain about dropped calls and bad coverage.

Cellular network operators worry about this because customers often blame the network for dropped calls. So, one way or another they encourage customers to use handsets with good RF electronics and encourage manufacturers to make them. A common method used by US carriers is to offer “good” handsets in their stores and to subsidise buying those phones with a contract. European cellular networks look at things differently, they don’t have to worry so much about coverage and dropped calls. The cellular networks in more densely populated places also offer special subsidies to the handset manufacturers but they’re often for different purposes. In that case the cellphone maker is encouraged to make a phone where the user has easy access to features that earn the network operator high profits. All network operators are “open to negotiation” on this point. If they think a new feature or a new phone will generate high revenue per user then they may be willing to sacrifice RF performance and put up with a few more people complaining about their network.

I was quoted very briefly on the Wall Street Journal website today. The subject was the new antenna in the Apple iPhone 4. Tomorrow I’ll write some more about this interesting antenna.

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My first job in Antenna design was designing internal antennas for cellphones. The cellphone market is enormous and every handset needs at least one antenna, and often two. It’s important for many of the people involved to understand the problems, so this is a very basic overview.

Due to the differences between handsets nearly every new model requires a new bespoke antenna design. The volume allotted for the antenna is different in each new handset, and it’s a different shape. The circuit board and the metal parts of the handset provide a groundplane for the antenna. This groundplane generally changes in size and shape with every new model. The groundplane characteristics affect the antenna significantly. An antenna built for a particular groundplane will not work well on a groundplane with a different size and shape. The plastics surrounding the antenna, and other nearby components also affect the antenna’s parameters.

Because of these issues it is virtually impossible to reuse designs directly. It can sometimes be done with external antennas such as whip antennas and stubby antennas, but not with the internal antennas commonly used today. It can sometimes be done with Bluetooth and Wi-Fi antennas, but not often with the main antenna for the cellular service.

So, Antenna Designers use a different sort of reuse. They gather knowledge about certain types of antennas, such as PIFAs and sub-types of them, they then learn how those sub-types behave and what factors affect their performance. An Antenna Designer learns how to “tune up” a few types of antenna. This allows him or her to quickly respond when given a particular product to work on.

To make this process as efficient as possible the major antenna companies have invested in rapid prototyping equipment, rapid antenna measurement equipment and other infrastructure.