There are several different schools of thought on the question “How should PIFAs be analysed?”, or “How do PIFAs work?”. I’ve found that some of them are useful and some less so, one day I’ll do a series of posts about them.

Right now though, what I’m interested in is the matching, specifically how the ground and feed pins affect it. According to some theories the ground pin plays a role in determining various antenna characteristics. What I’m concerned with here though is it’s role in affecting the input impedance. This part of the theory of PIFA antennas is really quite simple: if the ground pin is close to the feed pin then it forms a shunt inductance to ground. The size of that shunt inductance depends on the thickness and length of the ground pin and the metal between the two pins. So, the ground pin is effectively a matching element, it can be used along with the matching circuit in order to solve the overall matching problem. This is a very useful technique.

PIFAs with different ground and feed points

The diagram shows two prototypes for handset PIFA antennas. The groundplane is in green, though I haven’t shown it we’ll suppose that there is a full covering of groundplane below the PIFA area. The PIFA is in yellow, slots cut into the PIFA are in black, the feed pin is marked in blue and the ground pin is marked in pink. These pins could be “pogo” pins from the PCB below up to the PIFA.

The only difference between these two antennas is the position of the feed and ground pins, this is a big difference though. The smith chart or return loss chart for these two types antennas would be quite different. This is a dual-band design and both bands would be different. Even the antenna patterns would be different. Those are all subjects for another time. From the matching viewpoint the most interesting thing is that the path from the feed pin to ground is longer on the antenna on the right, and shorter on the one on the left. This is a shunt circuit so we should think in terms of admittance. From that view the design on the left has a shunt inductor to ground with higher admittance than the one on the right.

This fact gives the PIFA designer an extra degree of freedom in matching. Let’s suppose the matching circuit is a pi network of three components. Let’s suppose a Smith Chart analysis indicates that a shunt inductor is needed on the side of the pi network connected to the antenna. In that case the designer should think about changing the distance between the feed and ground pin. It is often much easier to change that inductance by altering that distance than by using the matching circuit. But, changing the pin positions also changes the antenna behaviour, making the problem more complicated. This added complexity though produces extra options which can be useful in design.

Often the pin positions are fixed early in a project before the PIFA design is anything like complete. Even in this case though the inductance can be varied by cutting a slot between the ground and feed points. This change will also affect many other characteristics of the PIFA, but it can often be made without difficult mechanical changes. This method can be used to reduce the shunt admittance (or if you prefer increase the shunt reactance), unfortunately there is no similar easy way to raise the shunt admittance.

Cutting a slot between the ground and feed points

Cellphones and laptops need multi-band antennas. Practically every cellular device must support at least two bands and most must support more. WiFi devices often need to support the 2.4Ghz and 5Ghz bands. So, there’s great demand for multi-band antennas today.

A multi-band antenna must have good radiation characteristics at all of the relevant bands, that’s the first challenge. I’ll talk about that another time. The second problem is to impedance match the antenna to the front-end. Often a lot of performance is left on the table because designers don’t understand multi-band matching. I was taught the right approach by Devis Iellici.

One approach is to design a matching circuit appropriate for one particular band. For example, suppose the unmatched antenna has worse performance in one band. The designer could make a circuit for that band and simply see what happens to the other bands. This is a bad plan since the effect on the other bands is most often detrimental.

A better method is to design the matching circuit with all bands in mind. For any narrow band there are several simple circuits that can improve the match, its best to find them all. I measure the Smith chart of the unmatched antenna and find the worst performing band using a radiation measurement such as efficiency. I then use Smith chart software to find all reasonably good matching circuits that can be made with three elements. I then pick the circuit that has the best performance in the other bands, that can be estimated using the Smith chart software too.

For example, suppose we have a quad-band PIFA antenna for cellular. It must cover 824Mhz-960Mhz and 1710Mhz-1990Mhz. Suppose the lower band has the worst performance. I would then find every three element LC circuit that matches the low band reasonably using Smith chart software. I’d then try each one out in the high band and see which performs best there. Then I’d implement it, test it and tune further using trial and error.

For this to work best the designer must take into account the parasitics of the components. Sometimes that can be done using the Smith chart software or by using a full microwave circuit analysis package.

The iPhone 4 has shown that hand effect problems can be very serious. This is generally true of antenna problems, I know of several handsets that were developed at huge cost and never shipped because satisfactory antenna performance couldn’t be achieved.

There are five directions this problem can be approached from:

• The Industrial Design direction. Some IDs are easier for antenna designers to work with than others.
• The Antenna Type – an antenna type can be picked that is more resistant to problems.
• Antenna placement – where in the handset the antenna is.
• Details of Antenna Design – just as some antenna types are better some features within antennas can be better or worse.
• Radio influences such as the radio and the band support can help.

Industrial Design
If the industrial designers and the antenna designers are prepared to be cooperative then that can help a lot. The industrial designers should to be flexible about the type and layout of the antenna. The antenna designers for their part should not try to build the easiest type of antenna possible, they should aide the industrial designers in achieving a good design. Generally the more cubic centimeters of space that are apportioned to the antenna the better the performance that the antenna designers can achieve on every spec. But, there are often more elegant ways to solve the problems than throwing CCs of space at them.

In some situations the industrial designer can design a case that encourages the user to hold the handset in a particular way. The case can encourage the user not to hold the antenna. For example, if there’s a bump around the rear-facing camera then the user is less likely to wrap their hand around it and the area above it.

Antenna Type.
The main types of antenna used in handsets are shown below:

• External
  • Stubby or Helical
  • Whip or Retractable
• Internal
  • Antennas with a groundplane below them – PIFAs
  • Antennas without groundplanes below them – Monopoles & Inverted-F antennas

Externals are rarely an issue these days, but they teach important lessons for internal antenna design. A retractable whip antenna is an external monopole that is parallel to groundplane and above it. The other main type of external is the stubby, these are a small round cylinder of plastic containing a type of shortened monopole antenna. Often stubby antennas are called “helicals” in books because it’s assumed that the antenna inside is a normal mode helix. Often though designers used other shapes, such as meanders. Anyway, both meanders and normal mode helicals are shortened monopole antennas. So, the two main types of external antenna, the retractable and the stubby are respectively a monopole and a shortened monopole. Investigations into these antennas years ago showed that the stubby antenna suffers more from the effects of the nearby head than the retractable. The reason for this is quite clear if you imagine a person holding a phone with both types of antenna. Part of the retractable antenna is close to the head, but all of the stubby antenna is close to the head. By compressing the antenna into a small space the stubby antenna creates a small space where the nearby dielectric materials have a large effect. So, stubbys suffer seriously from detuning problems, this problem carries over into internal antennas.

Internal antennas are often all called “PIFAs”, but the word is used for types of antenna that are really quite different to each other. Since PIFA means “planar inverted-F antenna” some people use it to refer to any antenna that has a feed and short pin that is made out of flat conductors. There are really at least two types though, firstly there are the sort built on top of a groundplane, those are somewhat like size-reduced patch antennas. Then there are the sort where the groundplane is parallel to the antenna but not underneath it, those are more like size-reduced monopole antennas. The diagram below is a side view of the internals of two handsets. The PCB material is in green and the groundplane is in orange.

The type with the groundplane underneath is what’s normally called a PIFA. The type with no groundplane underneath is closer to a monopole, even if it has a ground pin that only really affects the matching. Some types of normal mode helix have ground pins too. These two types of antenna have their advantages and disadvantages, it’s worth mentioning them all not only those connected with the hand & head effect problems.

• PIFAs with groundplanes underneath
• Pros
  • Components can be put in the side of the opposite side of the groundplane to the antenna, and sometimes on the antenna side too.
  • With many types of design radiation is directed away from the head especially in the high frequency bands. It’s not that difficult to meet SAR specifications.
  • Dielectric objects, such as the head, don’t detune the antenna much if they’re behind the groundplane. Since that side of the groundplane doesn’t transmit much radiation blocking it with an absorbing object doesn’t cause that much signal to be wasted.
• Cons
  • Generally require a lot of volume.
  • The distance from the antenna to the groundplane is critical. That gap must be large for a multi-band antenna. But, the size of that gap directly affects the thickness of the handset.
  • Difficult to design.
• Monopole-like antennas without groundplanes underneath
• Pros
  • Generally require less volume than with-groundplane antennas.
  • A bit easier to design.
  • A multi-bands antenna doesn’t require a thick handset.
• Cons
  • The PCB area around the antenna can’t be used. Other types of metal components often can’t be placed nearby because they cause detuning.
  • The antenna detunes when a dielectric object is placed nearby at any side. The head causes more detuning compared to an antenna with a groundplane, and the hand generally does too.
  • If the antenna is close to the head in the normal call position then the handset will probably fail the SAR tests.

The main lesson here is if you want the antenna to perform well in the presence of the head and hand then use an antenna with a groundplane.

Antenna Placement.

The conventional type of handset antennas I’ve discussed here only work well if they’re at the top of the handset or the bottom. They don’t work well in the centre of the PCB. So, the choice is between the top and bottom.

Generally users hold their phones by the bottom part, so hand detuning is worse if the antenna is at a the bottom. Sometimes the ID designers can help here by creating a case that encourages the user to grip the phone in a different way, but that’s difficult. Similarly, head detuning is generally worse if the antenna is at the top of the handset.

PIFAs with groundplanes below can be put at the top or bottom of a handset. If they’re put at the bottom then the user is likely to cover the antenna with their hand and cause serious attenuation and detuning. If the antenna is put at the top then the head is on the groundplane side not the antenna side, so it causes less detuning. So, this type of antenna should be put at the top.

The situation is different for antennas with no groundplane below them. These will generally fail the SAR test if they’re put at the top, so they have to be put at the bottom for that reason. This means that these type of antennas have a two-fold problem. They are sensitive to detuning by dielectric objects and since they generally have to be put at the bottom of the handset they are more likely to be close to the hand or even covered by it.

The main lesson here is to use a PIFA with a groundplane and put it at the top if you can. The situation for slide handsets and clamshell handsets is quite complicated, I might discuss that in the future in another post.

Details of the Antenna Design.

The closer the antenna is to the outside of the case the more it will be affected by proximity detuning. But, if the designer leaves a gap between the case and the antenna then that’s forsaking space and it will probably decrease the antenna’s performance in empty space. This trade-off has to be made on a handset-by-handset basis. It’s difficult and can require a lot of prototyping and simulation.

Another problem is areas of high E-field. Generally the sensitive parts of an antenna follow the E-field. If the E-field is high in a particular region then a dielectric object close to that place will cause a large frequency shift. It’s best to avoid creating such regions as much as possible, that can be done by widening out conductors on the antenna close to the high E-field region. Another possibility is to move the region further inside the handset.

There have generally been two theories about the iPhone 4’s problem. Firstly, lots of people online have said that dampness on the users fingers causes a conductive bridge over the gap in the case. As I said in previous posts I don’t think that’s very likely. Secondly, when the user touchs the gap that causes detuning. I think the detuning explanation is more likely, but the conduction explanation could be right, or it could be a bit of both. In either case this demonstrates a problem with putting the antenna on the outside of the case.

If GPS, Bluetooth or WLAN antenna is needed then it’s best to use a separate antenna for those bands. That helps the detuning problem because it makes the main antenna design easier.

Other Influences such as the Radio.

The output amplifiers in the transmitter part of the radio suffer from the problem of “load-pull”. The amplifier is built to work when applied to a particular output impedance which is generally 50 ohms. If the antenna’s input impedance is different than 50 ohms then there is mismatch loss. But, there’s also an additional loss caused because the amplifier’s performance degrades, this is load-pull.

Suppose due to detuning the VSWR on a particular channel gets worse, it goes from 3:1 to 2:1. The amplifer may not be able to supply full power when mismatched 2:1, so it’s power output may drop by a dBm or two. That would cause a big drop in performance. So, an amplifier with better load pull characteristics gives better performance when there’s detuning. This was a great problem in the early days of digital cellular in the 90s and early 00s, amplifiers then had much worse load-pull characteristics than those we have today. But, the problem has never completely gone away. (Exactly why it’s never gone away is a story for another time).

I’ve seen a few folks try to redesign the matching circuit to cope with detuning better. I’ve never seen this enterprise succeed though. I don’t think there’s much that matching circuits can do to overcome this problem.

ESD protection circuits are important if metal parts of the antenna are exposed to the user. The radio must be protected from static shocks that can occur if the user accumulates a charge by walking across a carpet for example.

Cellphones today cover many bands. A cheap European handset will cover the European GSM/EDGE bands at 900MHz and 1800MHz, the US PCS band at 1900MHz and the European WCDMA/UMTS band at 2100MHz. A cheap US handset will cover the US 800MHz and 1900MHz bands, GPS at 1575MHz and often the European 1800MHz band too. But, these handsets don’t compare to high-end Smartphones, those often support all of the bands I’ve mentioned above, that’s six bands. Such phones are called “world phones” because they support so many bands that they can be used almost anywhere there is a cellular network. Supporting all those bands requires a large antenna especially if a PIFA with a groundplane below it is used. This often results in detuning taking a back-seat in antenna design because band support becomes the big issue.

This problem can be solved by regional variants. The makers of cheaper handsets tend to make a US version and a European version. Most countries use the US or European band layout, so doing that provides a large market. These phones will still work if taken abroad because there is provision for that. For example, a typical European handset will work on the 1900MHz PCS band in the US, assuming the carriers involved have agreements. If Smartphone designers were to take this approach then they would have to build more handset variants, but it would make the antenna less difficult. It would make with-groundplane PIFAs more practical and they were used that would help prevent detuning.

A Concluding Thought…
If handset designers paid a little bit more attention to the issues above then the user experience would be better. From the user’s point of view “network coverage” would magically increase. But, users mostly blame networks for poor coverage rather than handsets. That means the handset companies often have little incentive to improve. But, after the well-publicised iPhone 4 problems users may be more critical of handsets in the future.

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.

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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.