Antenna Design Grows Up

Apple’s iPhone 4 antenna issue represents a classic example of what can go wrong in modern antenna design. Put one in the wrong place, and a seemingly insignificant part can turn a cool new product into a public relations nightmare.

Ever since antennas dropped out of sight, most consumers don’t give them a second thought. In the 1960s, almost every home had a rooftop antenna. Fast forward to the present and most devices are connected either by cable or wirelessly. The antennas are still there, but they’re no longer visible. And they’re even more important than before, and significantly more complicated to design.

Fig. 1. Rooftop antennas once dominated the landscape.

“It’s a very complex issue becoming more complex. Automotive, unequivocally, is the most complex system for any wireless engagement from an antenna placement, selection, management perspective,” noted Richard Barrett, senior product marketing engineer, automotive wireless technology at Cypress Semiconductor. “I can’t think of any industry that would even approach the complexities of automotive, outside of something like aerospace or military. Automotive is more complex because of the many models that you have in a consumer industry.”

Engineers have to start off with understanding what their limitations are, and what they need to work with in order to find the optimal solution, he said. “Some of the basic fundamentals are there, and so many electronics are being packed into a vehicle, where the radio is going to exist in the vehicle will change dramatically. It can be in metal enclosures, behind firewalls, and coexist with other radios or emissive devices around it, which is a huge challenge unto itself. In addition, temperature considerations are driving whether things are on the roof, in the mirror, inside infotainment or inside of an engine compartment. This will also define what can be done with the radios, and so forth.”

Placement is key for antennas, and the best location generally is where there is the least influence from the environment on performance.

“Ideally, antennas placed in their intended operational environment should perform just as they usually do when they’re designed in isolation—without anything around them,” said Shawn Carpenter, high frequency product manager at Ansys. “However, the reality is that the desire for integrated product size reduction, aesthetics that are pleasing to consumers, and a simple lack of real estate for isolating integrated antennas lead to compromises of antenna performance.”

This performance impact usually comes from coupling to other parts of the product, platform or vehicle into which the antenna is installed. Platform features such as limited ground planes, batteries, curved surfaces, material coatings and paints, and supporting structures can couple unintended energy from the antenna and change the way it radiates and/or accepts power. As a result, the antenna design needs to factor the placement effects of the antenna into its basic design, compensating for the potential performance induced by the environment.

The best placement for an antenna also depends upon the intended use of the antenna, Carpenter said. “In some situations, the orientation of the product, platform or vehicle into which the antenna is installed can vary over time, so we require the antenna to radiate and receive in all directions as equally as possible.”

Fig. 2: Radiation gain in antennas frequently deviates from ideal isotropic behavior. Source: Cypress.

Consider an antenna integrated into a mobile phone. The phone’s orientation can vary continuously, and the phone needs to have continual contact with the nearest cell tower. If the mobile phone’s antenna has weak gain in the direction of the cell tower, then the weakness in that link has to be made up by using more transmit power, or by increasing the sensitivity of the receiver, which lead in turn to reduced battery life.

Then, for omnidirectional antennas, the antenna should be placed in a location where its radiation pattern will be as undisturbed as possible, which usually means placing it as far away from other conductors as possible. This runs counter to the frequent requirement that the overall product (containing the antenna) be as small as possible. Designers faced with these challenges will generally incorporate an antenna on an edge or side of such a product, and try to anticipate the effects of near-field coupling (the antenna coupling placement effects) to other parts of product or vehicle in the overall antenna design, he observed.

Fig. 3: Multiple antennas in a device. Source: ANSYS.

“Other antennas are intended to be directional in nature—they are intended to reliably concentrate power in a specific direction. Such antennas can be quite sensitive to coupling to nearby structures, which can in turn reduce the antenna’s ability to concentrate power in a specific direction. If this antenna’s directionality is reduced, then more transmit power would be needed to overcome the loss in link gain, and the receiver will have higher susceptibility to signals coming from unintended directions. Proper location of directional antennas has long been a concern of antenna systems engineers because of the environment’s impact on the overall performance of the RF system,” Carpenter said.

System-level perspective
A further mitigating complexity with automotive that isn’t seen elsewhere is that the core design team at the automotive OEM that’s developing a solution can design essentially the core electronics, the core box that has the radios (WiFi, Bluetooth, LTE), and can give some recommendations on how antennas should be placed, and implemented.

“But at the end of the day, they don’t necessarily control that because then it goes to a vehicle model,” Barrett said. “Taking the most extreme example, let’s look at a company like a GM. The same exact system that’s developed by a core engineering team within General Motors may be deployed in a Chevy Volt or a Cadillac Escalade ESV. It varies widely on what it’s physically going into. As a result, that engineering team can’t predict what the RF environment is going to look like in the end model, or what the placement of the box is going to be, and how they will have to route to antennas. No other system is like this. You buy a phone and the phone designers know exactly what was going into it and what it will be used for. But cars vary so dramatically, and everything that’s ugly for any electronics — wireless in particular, whether it’s environmental or RF emissions or just metal shielding — everything in the way is just compounded,” he said.

Fig. 4: Mixed signals. Source: ANSYS.

Further complicating matters is that fact there probably will be conflict between the antenna and other devices that are transmitting or receiving.

“You’re pretty much guaranteed to have conflict,” according to Michael Thompson, senior solutions architect at Cadence. “At least when I design an automobile, I can take that into account because I know where I’m going to place those antennas, and there are some unique things that happen inside of a car because of the Faraday cage. Depending on whether the antenna is on the inside or the outside of it, and whether or not there is coupling around the car, all of those things can be modeled in order to make provisions up front to make sure there isn’t coupling, or least minimizing or filtering it.”

The larger issue going forward will be when everyone starts to have multiple radars in their cars, Thompson said. “How do we know that the signal you’re getting back is the signal you transmitted for your radar, or some other signal, and not somebody else’s car? Interference is a different type of problem, but it can obviously happen. It’s a completely different market, but electronic warfare is all based on putting out signals to fool/feign some other response. That can be managed by some of the signal processing behind the antenna, but the antenna can’t sort out what’s a good signal and what’s a bad one. All it knows it that it receives this energy at a certain frequency, from a certain area. It absorbs that energy, and passes it onto the receiver.”

To address these issues, the best that chipmakers can do is provide recommendations. “If you’re in a metal box, you have to cut a slot in the box so there’s someplace for the RF to get into,” said Barrett. “It’s got to be able to see outside. If you can cable outside to an outside antenna, that’s a great idea.”

And then there is the issue of how much an antenna design will cost.

“Everyone would like to be able to use a cheap Inverted F printed circuit board antenna to do everything in the world, but that’s just from the cost side of things,” he said. “From the engineering side, you’d love to have nice dipole antennas, cabled out to the most optimal location, end cabin use, facing the consumer. And for external vehicle, you want that facing outside the car in the proper direction. Those are the two extremes. The real world is somewhere in the middle.”

The considerations are many, driven by the complexity of different vehicle models, as well as complexity of use cases. Some systems primarily look outside the car (like an LTE modem), but WiFi and Bluetooth can look inside and outside of the car.

“Do you optimize for the passengers? Do you optimize to connect to an external access point? Are you trying to have a hybrid approach, which when you get to engineering tradeoffs means nothing is optimized? Those are huge challenges that you see across the market,” Barrett noted. “To try to address this, there are more cabled systems [available commercially], because at least if you cable out to an antenna then you’ve got some control over where that antenna sits relative to, say, passengers or external to the car. Although you can’t control the length of the cable many times, and know exactly what the RF is going to look, at least you’ve got a little more control there. You can clear the space between wherever your radio is radiating through its antenna to wherever its trying to radiate, and receive or transmit to/from.”

There are both in-cabin, and external considerations. Thankfully, performance in-cabin is not as critical. Antennas generally are close to each other, so the standard WiFi and Bluetooth technologies probably shouldn’t be too sensitive or transmitting at too high a power because it can be kept localized.

“However, there are so many applications now that also need to connect outside the vehicle, whether for an LTE modem connecting to a WiFi access point, you want to offload it to WiFi, the faster you can connect to that external WiFi as you’re in slow moving traffic, the more time there will be on that access point to dwell on it. The stronger signal there is, the better link budget from a receiver sensitivity or transmit power out standpoint the longer you’ll be able to maintain connection on that box,” he added.

Over-the-air upgrades are another area that can be impacted. For these, a vehicle may be sitting in a garage and the media server in the house is automatically downloading to the vehicle to upgrade the software, just like a smartphone. Because of varying sizes of homes, and depending on construction method, the RF environments have to be considered as well, so placement of antennas for external applications, and maximizing that link budget is critical. It all comes down to predictability of what can be controlled in the RF, what the vehicle looks like, and willingness to spend money on a decent antenna.

Algorithms, techniques
Then, when it comes to analyzing the system, there are a number of technologies that have existed for some time, Thompson said. “We keep finding better ways to calibrate them, and implementing them in a computer architecture so they run better, but a lot of those techniques have been around for many years. They do work quite well, but you have to be able to very accurately describe the physical reality that you’re going to be operating in to the simulator, and that often becomes part of the problem.”

Working out the best way to create that description, much like antenna design itself, depends on what is being analyzed, Thompson explained. “If I am looking at radiation issues, for example, not only from real antennas but just from the circuitry — EMI/EMC — those types of issues arise from the fact that the circuit is radiating. If you design an antenna at some particular frequency, and it’s working really great at that frequency, just because it was designed at that frequency doesn’t mean it won’t radiate at another frequency. The same holds true for the circuit board. From that standpoint, a lot of times, planar tools, techniques like Method of Moment, run faster and have the capacity, so you’d want to have something like that available to the designer. At the same time, you may be looking at bond wires, connectors, 3D effects so something more like a finite element method is more appropriate; an environment that could handle multiple electromagnetic techniques is really what the designer needs.”

ANSYS, CST, Antenna Magnus, and Mathworks, among others, all provide tools in this area. Cadence has indicated it is looking into this space with key partners. Tools from Cadence, Mentor Graphics, Synopsys, and other EDA providers have been used in the simulation of antenna design for some time.

Conclusion
Antenna design today is still part black art, but that is about to change given its role in the IoT and automotive arenas. Future design approaches will draw on knowledge about a design from many sources, not just the previous experience of a design team. That collection of data points will find its way into whatever tools are being used to design, verify and optimize antennas for each unique situation. Demand is rising along with the complexity of the antenna designs, and changes are coming quickly in this space.

Related Stories
Integration IP Helps IP Integration
The MIPI Alliance has been the plumber for the mobile industry and now hopes to migrate that success into the automotive and IoT spaces.
Waiting For 5G Technology
New wireless standard will significantly speed up communication, but dealing with mmWave technology isn’t going to be simple.
Interference In The Car
Security and wireless signal issues grow as vehicles become more connected.
RF GaN Gains Steam
Technology may be perfect for 5G, but there are still some challenges to overcome.