Introduction

The problem has bedevilled designers for decades: How do you squeeze an increasing number of antennas into a decreasing size of mobile devices? It is tempting to think that the answer is to use smaller antennas or to pack them closer together, but there are fundamental constraints on antenna size and placement. Antennas emit energy in the form of electromagnetic waves, and they do so in many directions. With one antenna, all is well. But put another antenna next to it, and that second one can be swamped by the strong signal coming from the first, making it deaf to the weaker signal it should be receiving.

Two antennas are bad enough. But these days, a typical smart device can have many more. Their ranks usually include one or two antennas for Wi-Fi, one for Bluetooth, one for GPS, and two or four for 4G LTE cellular communications. This multiplicity of LTE antennas is the norm because it enables the phones to avoid dropouts—including those that occur when your hand obscures one antenna, which can be particularly frustrating during a conversation. Multiple
antennas for the same communications link also allow cellphone carriers to combine multiple streams for improved data transfer rates. And soon, 5G communications will add more complexity to the mix by extending the frequency bands used for cellular below six gigahertz, making existing antennas work harder, and requiring antennas that operate at the 28- and 37-
GHz bands.

Device designers must assume that all the antennas in this gaggle will operate simultaneously. After all, it is not unusual for a device to run apps that rely on GPS and Bluetooth while streaming a video over Wi-Fi or the cellular network. Chipset manufacturers have been working to combine functions, such as using one antenna for both Wi-Fi and Bluetooth. Designing two systems to cooperate like this is helpful but only chips away at the problem.

So, you have a small physical space and many antennas that need to operate simultaneously without interfering with one another. Designers have tried to solve this problem by making antennas smaller and more directional. They have also tried to isolate the antennas more effectively, which is tricky given the small amount of real estate to work with.

The Placement Puzzle

Antennas can create mutual interference. A simple solution could mean either spacing the antennas wide apart or physically blocking the signal path. The first requires more space, and the second would cause the antennas to miss signals from the blocked direction. A better solution is compact isolation, incorporated into the antenna design to stop the interference but also let the desired signals through.
The only sensible solution is an innovative approach to antenna design. Instead of creating antennas as individual elements and leaving it to the designers to choose where to fit them into a device, designing a suite of antennas that work well together as a system and then installing that system into the mobile device as a single cohesive unit.

Before we get into the details of this approach, we should go over some antenna basics. An antenna is simply a transducer, typically made of copper, that (when receiving) picks up energy being sent to it and passes that signal onto a radio chipset. That chipset takes this analogue signal and turns it into a digital one, which the main processor in the device can then use. When transmitting, this process is reversed.

Like tuning forks, Antennas pick up energy well only at the frequencies they naturally resonate. The antenna’s resonant frequency depends on its physical size, although the frequency can also be modified without changing the size byte by adding electronic components to “tune” the antenna. Designers twist antennas into various shapes to fit antennas into a small space. There are, however, some physical limitations involving just how close and in what shapes you can make these antennas.
Originally, when mobile phones just had one antenna for cellular reception and transmission, it extended from the handset as a recognizable aerial. Sometimes there was a retractable piece; other times, it was just a bump. That was mostly an aesthetic decision, but it gave antenna designers plenty of room to optimize the antenna for the best reception.

Today’s sleeker devices have taken away that choice, but leaving it there would not help much: The growing variety of phone functions has multiplied the number of antennas, so they would never work all crammed into a single antenna bump—mutual interference would destroy their capabilities.
These days, designers are instead distributing the different antennas at various spots around the phone, typically at the edge and rear of the casing. That makes antennas less likely to interfere with one another, but it also makes it difficult to prevent signals from being blocked by the user’s hand.

Remember the iPhone 4 debacle, when users were furious that merely changing the position of their hands while holding their phones caused the cellular signal to drop? That episode in part, triggered the use of multiple antennas at the same frequency, typically paired at the bottom and top of the phone and the top left and bottom right corners.

AS TIME HAS GONE ON, mobile devices have become slimmer and packed with more functions, thanks in large part to the influence of Moore’s Law on most of the electronics they hold. But Moore’s Law has not helped antennas. Their design is one of the last bastions of analogue electronics. With clever engineering finding new shapes in which to arrange the conductive elements or pairing those elements in various combinations, you can keep the antenna resonating at the desired frequency and make it smaller. Still, usually, you are not making it better or cheaper. You reduce its performance, sometimes to the extent that it barely functions as an antenna. One of us personally recalls buying one of the early smartphones from Asia and finding that turning on the Wi-Fi prevented the GPS from functioning.
Laptop manufacturers have more room to work with but increasingly encase their devices in metal and carbon fibre. Strong and attractive as they may be, these materials are conductive or absorptive, so they block radio signals. As a result, the antennas end up in the hinge or in plastic screen bezels, which are increasingly small strips of real estate.

A company based in Barcelona is trying to address this problem. The company has developed a tiny antenna measuring just a few millimetres on a side called “invisible” that can perform acceptably in a narrow bandwidth for, say, a single Wi-Fi channel.

Another strategy is to use multiple directional antennas and dynamically select whichever antenna gets the strongest reception at the desired frequency. This technique works well and is extensively used in Wi-Fi routers. But it is less effective in smaller spaces simply because there are fewer antenna shapes and position options, so it is generally not used in compact mobile devices.

A more traditional way of fitting antennas into a small space without risking interference involves polarization. The concept is like what goes on in polarized sunglasses, in which molecules contained in a film coating on the glasses are aligned so that only light waves oriented in a certain way get through, reducing glare.

In the simplest scenario, antennas that are placed vertically send out radio waves with polarization at right angles to those placed horizontally, so when antennas in both directions are transmitting, they interfere with one another significantly less than if they were in the same orientation. Similarly, each antenna will have a better chance of receiving a weak, distant signal. Unfortunately, designers find this technique challenging to implement within the physical constraints of today’s products, especially when more than a couple of signals are involved.

Our Approach to Solving the Paradox

At Novocomms Limited, We are taking a somewhat different approach. We accept that the antennas will be crowded together and emit radio-frequency signals that can interfere with one another, so we believe that focusing on the design of one perfect antenna is pointless.

We can and do use standard polarisation techniques to prevent antennas from being victims of interference. But we also make the antennas directional and point them away from one another. We do this by modifying the physical shape of the antenna in three dimensions.

We also try to make it easier for antennas to ignore extraneous signals, using filters in a nontraditional way. Normally, the filters used in antennas are bandpass filters. That is, they allow only signals in the desired frequency band through. Today, this kind of filtering is typically done digitally. The problem with digital bandpass filters is that they don’t get applied until after the analogue signal reaches the receiver and is converted to a digital stream. That’s too late to avoid what radio engineers call the “desense” problem: If the out-of-band signals that need to be blocked have enough energy, the receiver can’t detect quieter signals while those high-powered signals are present, and for a brief period afterwards, the difference in energy can be vast—say, 1 watt for the unwanted signals compared with three milli watts for the target signals.

Our approach is essentially the opposite. We know the frequencies at which the surrounding antennas transmit, so our analogue filters look for those specific frequencies and, in effect, short those signals to the ground, dissipating their energy while passing the desired signals undisturbed to the receiver. We also use standard analogue design techniques. For example, we use signal matching to identify transmissions at the desired frequency. But the key to our approach is doing the filtering at the front end, in the analogue realm, and dynamically changing the signal before it passes to a standard receiver and an analogue-to-digital converter.

With our innovation, we can place antennas physically close together, within just a few centimetres. But positioning them properly in three dimensions so they all work harmoniously is difficult. We can’t leave that step up to the device designers or manufacturers, so we aren’t producing individual components. Instead, we’re manufacturing the entire set of antennas needed for a mobile device on one flexible 0.44-millimetre-thick printed circuit board. Flexibility reduces the cost of manufacturing because it lets us start the process in two dimensions and then turn it into a 3D package—by moulding a plastic shape and then wrapping it with the flexible circuit board.

One of those shapes appears long and thin. In cross-section, though, it is triangular. Our circuit board, wrapped around that, folds at the triangle's points. Some of our antennas form L shapes, while others are rectangular. We also supply custom design services for antenna specifications that need to fit a specified space inside a particular device.

For very high-performance antennas, we have started using a more precise manufacturing method—Laser Direct Structuring—that allows us to create far more complex plastic shapes and build our antenna structure directly on top of them. That technique starts with an injection-moulded part made from plastic doped with a nonconductive metal. A laser then draws the pattern of circuits on the plastic. The metal additive causes the areas hit by the laser to roughen. When the material undergoes electrodeless plating, these rough areas fill with copper or other conductive metals.

WE THINK THIS KIND of approach to antenna design is the way of the future because the demands on mobile device antennas will only get tougher.
Data transfer speed will be pushed higher. Today’s 4G is typically capable of delivering up to 100 megabits per second, but 5G is expected to deliver between 1 and 10 gigabits per second. That will require increasing bandwidth; there’s some talk about bandwidths as large as 2 GHz. And frequencies are increasing, with 28-GHz and 70-GHz frequencies slated for 5G.

Because of the high frequencies at which 5G will operate, mobile devices will use arrays of similar antennas to get reasonable performance, aligning the signals by phase shifting and then combining them into one signal of the required strength. Given that phones will also have to carry the full set of legacy antennas, at least for the immediate future, the increased competition for space inside mobile devices will make an integrated antenna a necessity.

The first commercial antennas from our company using these techniques started to show up in tablets and laptops from major manufacturers in July. And by the end of this year, we expect to see this technology appear in its first mass-market Internet-of-Things product. That may well be the largest application of this technology in the future.

Conclusion

Sure, jamming antennas into phones is hard. But the challenge of fitting multiple antennas into the tiny space available in a typical IoT device is even more daunting. Nevertheless, commercial examples showing that this challenge can be overcome are just around the corner. When you see them, you’ll now have a good idea of how they can work as well as they do.