You can’t have a radio without an antenna. These passive “mechanical” devices are the critical link between any two wireless devices, and the better they are, the better the communications link between them will be.
The antenna is the transducer that converts the signals from the transmitter into the electromagnetic radio waves that propagate through the “ether” to the receiver. Then, the antenna converts those electromagnetic waves back into the electrical signal containing the transmitted information.
A passive metal device would seem like a simple thing to implement. However, antennas are one of the most complex technologies in wireless communications. Many engineers call antenna design black magic and more of an art than a science. In fact, it’s all of the above.
Remember Apple’s problem with the iPhone 4? Designing the antenna into an external trim piece was clever and space-saving, but it created problems when it was held incorrectly and effectively shunted across the insulating slot in the trim. Calls were dropped, and service was lost. Even the new Verizon version of the iPhone 4 has this affliction. An insulated case solves the problem.
The antenna has become more of a challenge these days because many new wireless devices have put greater demands on it and have made wireless design more complex. Antennas are essentially resonant devices designed for a narrow range of frequencies. Today, they must serve a wider range of frequencies in form factors that present serious physical challenges:
• Wider frequency ranges: Cell phones must cover the low bands from 698 to 960 MHz as well as the higher bands from 1701 to 2170 MHz. This is hard to do with a single antenna. Impedance matching is a chore over such a wide range, so efficiency usually suffers.
• Multiple antennas per device: The average smart phone has up to seven radios. Besides the multiband cellular transceivers, smart phones include a Wi-Fi radio, Bluetooth, GPS, and possibly FM radio. TV is coming, too. All of these antennas have to fit inside a clean physical handset or a slim tablet package, and they can’t interfere with one another or be detuned by the user’s hand or face.
• New modulation demands more bandwidth: Long-Term Evolution (LTE) and other modern wireless devices have all adopted orthogonal frequency-division multiplexing (OFDM) as the main modulation/access method. This broadband scheme requires lots of bandwidth from as little as 5 MHz up to 40 MHz and beyond. The usual tradeoff is less efficiency for greater bandwidth, but is it worth it?
• MIMO on everything: The use of multiple-input multiple-output (MIMO) is a real breakthrough that is not only boosting data rates for a given bandwidth but also is making wireless links longer and more reliable despite multipath fading and other problems. The downside is that it takes multiple transceivers—each with its own antenna. Already, 2x2 MIMO is fairly common and will become more widely used in the future, and 4x4 MIMO is beginning to see more usage. MIMO means more antennas per device, so how do you get more antennas into a handset or tablet?
• Lower frequencies present problems: The higher the frequency, the smaller the antenna. Cellular frequencies from 800 MHz up to 2.3 GHz permit very small antennas. Wi-Fi and Bluetooth 2.4-GHz antennas are also small. But with the opening of the 700-MHz territory and the need to accommodate FM radio and TV in the lower VHF and UHF bands, antennas need to get much bigger if any kind of efficiency is to be achieved. Whip antennas and rabbit ears are out.
• Adaptive antennas and beamforming: Some new products and systems are beginning to adopt narrowbeam antennas that can be steered to improve link reliability and reduce interference. These antennas are desirable but complex and take up more space.
With the growing market for smart phones, e-books, tablets, and other high-volume wireless devices, more antenna research and design is needed. Fortunately, some unique approaches are underway to address these problems, including two main solutions.
First, designers can use multiple antennas for low and high bands and/or multiple feeds on single antennas. Antenna manufacturers have learned to take the basic antenna designs like a dipole, monopole, loop, patch, and slot and modify them in clever ways to widen bandwidth, increase gain, and improve efficiency.
Second, designers can use dynamic antenna tuning. This technique is not yet widely used but forthcoming as it overcomes many of the multiband and proximity problems associated with handsets and other handheld wireless devices. Dynamic tuning employs a variable impedance matching section that is tuned automatically to ensure a low voltage standing wave ratio (VSWR) at all times (Fig. 1).
A power monitor circuit using directional couplers to measure forward and reverse power allows the VSWR to be continuously monitored and calculated. An impedance matching network with inductors and capacitors is designed to be variable so it can be adjusted for lowest VSWR. If the frequency of operation changes or some object is brought into close proximity to the antenna, the VSWR will change. This change can be used to vary the values of the capacitors in the matching network to bring the antenna back into tune.
This technique is not new, as it has been used for years in military radios and the RF plasma etchers used in semiconductor manufacturing. Many modern amateur radio transceivers incorporate automatic antenna tuning to optimize power output and keep VSWR low.
Putting such a system in a cell phone is more of a challenge but doable. Power monitors are already part of most cell phones to control power output. Signals derived from these circuits can be fed to one of the existing processors, where an algorithm developed for the system is executed. The outputs are used to vary capacitor values in the matching network.
The biggest problem is finding suitable electrically variable capacitors. While varactors are available, they mostly use high dc tuning voltages that are difficult to implement in a portable device. Switchable CMOS capacitors are becoming available to make this approach practical. Peregrine Semiconductor has announced some digitally switched CMOS capacitors that are ideal for this application.
Innovative Antenna Designs
The industry’s literature offers an amazing number of unique designs to solve antenna problems. There are too many to list. But some recent antenna products and solutions show the range of possibilities today.
The Bell Labs division of Alcatel-Lucent recently announced a breakthrough called a wide-band active antenna array that’s part of the company’s lightRadio Cube (Fig. 2). This complete low-power basestation can be used to form small, light picocells and other configurations to expand the cellular system. The cube contains the complete software-defined basestation radio including baseband and the active antenna array based on the patch-type antenna.
While few details are available, the antenna appears tunable and can be adjusted for frequency of operation over the 700-MHz to 2.6-GHz range. The transmit power level is 1 to 5 W. By mounting multiple cubes in arrays of vertical or horizontal columns, directional beamforming and MIMO antennas can be created.
The overall benefits are smaller, lower-cost, energy-efficient basestations that can be mounted almost anywhere like on the sides of buildings, lamp posts, and other inconspicuous places. Such basestations meet the needs of the coming LTE 4G expansion, which will use a greater number of smaller basestations like femtocells in homes and picocells in a cloud-like system. With mobile devices closer to the basestation, 4G speeds can be more easily achieved. The lightRadio cube is in trials through the third quarter this year with production expected in 2012. It’s certainly one to watch.
One of the most innovative new designs for small antennas comes from DockOn, whose Compound PxM Loop (CPL) provides improved efficiency and gain over other electrically small antennas. The CPL is a composite of two antennas driven separately (Fig. 3).
One antenna generates the electric field and the other the magnetic field, which coalesce near the antenna to form the complete radio wave. The antennas must be driven orthogonally and simultaneously to ensure that they do not act independently, but that’s addressed by a special splitter/combiner that takes care of the phasing and feed impedances.
The resulting antenna boasts greater signal strength than other types of antennas. Such antennas are smaller than comparable types and have an efficiency often over 90%. They also have gain and a wide bandwidth of an octave or more. Best of all, the designs are easily implemented using microstrip technology on printed-circuit boards (PCBs) or by depositing patterns on other substrates.
Just recently, DockOn announced an omnidirectional circular polarized antenna using the CPL techniques. It has all of these benefits plus the advantage of circular polarization, which often improves reception or transmission when the exact orientation of other antennas in the system isn’t known.
Antennas work best when both transmitting and receiving antennas use the same polarization, usually either vertical or horizontal. But in many applications such as cell-phone use or RFID tagging, the orientation may be in between or opposite. Circular polarization allows signals of any polarization to be received or transmitted with efficiency. It also provides better performance in snow and rain and does a better job of penetrating obstructions like walls and trees.
DockOn has some standard products but also licenses its reference designs to others. Custom designs are possible as well. These antennas are applicable to almost any wireless product including ones that use MIMO or require arrays for beam forming or steering with greater directionality and gain.
A great deal of antenna development is focused on cellular basestations. As carriers upgrade their networks to the latest technologies like HSPA+ and LTE, new antennas are a must, mainly because of the MIMO requirement to get higher data rates and improved link reliability. Powerwave Technologies recently introduced an active array antenna for MIMO in basestations (Fig. 4).
The design puts the RF section of the radio on the tower with the antenna elements, including the power amplifiers that feed the antennas directly. Doing so prevents the horrific loss of power in the transmission lines that run up the tower from the power amps in the base to the antennas. This represents a huge power savings, boosting the efficiency of the basestation.
The MIMO Active Array Antenna is an integrated design with a 3- to 5-dB link budget that will double a basestation’s coverage radius compared to non-integrated antennas. The antenna is packaged in a single radome that houses all of the subsystems necessary to deploy a complete 3G or 4G basestation RF system on the tower.
The tower electronics includes the complete radio transceiver and the power amplifiers with digital pre-distortion and crest-factor reduction algorithms for improved amplifier efficiency. Also included are the filters and duplexer, the low-noise amplifier (LNA), and the antennas. The antennas themselves are cross-polarized aperture-coupled patches with gain.
Each pair of antennas forms a 2x2 MIMO system. The whole assembly weighs about 45 lb and only requires dc power and the fiber-optic cable for the common public radio interface (CPRI) data signals. The units are available to operate over the 700-MHz to 2.7-GHz range. A 3G unit can be upgraded to 4G later at minimal cost. The antenna is in carrier trials now but should be available in the third quarter of 2011.
SkyCross is another antenna innovator. Its products include meander line antennas (MLAs) and its flagship isolated mode antenna technology (iMAT). The MLAs combine a loop with meander lines for tuning. They have broadband versions with bandwidths covering multiple octaves without tuning.
While they’re very small, the MLAs have excellent efficiency. They come close to the Chu-Harrington limit, which is a measure of the bandwidth for a given volume or size of the antenna. A typical multimode MLA can cover the 800- to 2500-MHz range with a 0.4- by 0.25- by 1.25-in. unit. Units are available for most wireless applications.
A real achievement for SkyCross is its isolated mode antenna technology (iMAT), which enables a single antenna structure to act like multiple antennas by using multiple feed points. Each feed point accesses the antenna as if it consisted of multiple antennas, each with high isolation, low correlation, and high efficiency. Such an antenna is a great solution to the incorporation of MIMO into mobile wireless devices.
Most wireless technologies are moving to MIMO as a way to boost link data rate and to improve reliability in a multipath environment. However, it only works if the multiple antennas are isolated from one another by a sufficient distance. This works on basestations and other stationary equipment, but it is tough to achieve in a handset or other mobile device.
SkyCross discovered that you can feed or excite a single antenna structure in different places to get the effect of two or more antennas. Each feed uses the antenna to produce a different pattern of radiation with good isolation between them. The iMAT approach provides enough isolation to make the effect of unwanted coupling between antennas negligible. Thus, 2x2 MIMO in a handset is a done deal. The technique can be applied to any cellular wireless technology like HSPA+ or LTE, WiMAX, or 802.11a/g/n.
Now, SkyCross has created a version of this design called the T-Series that is optimized for tablet computers. The T-Series has a form factor created to fit the demands of ultra-thin tablets. These antennas combine the flexibility of connectorized notebook antennas with the sophistication and compact size of smart-phone antennas (Fig. 5).
The T-Series antennas can come in single-band or multiband forms from 700 MHz to 2.6 GHz. Standard models are available, but they can be customized to fit any multiprotocol, multiband configuration. The feed is unbalanced 50 Ω with coax. The gain is greater than 0 dBi with a VSWR of less than 2.5 to 1 over the bands of operation. The polarization is linear, and the radiation pattern is omnidirectional. For 4G versions requiring MIMO, the T-Series can be configured with the multi-feedpoint iMAT provisions.
SkyCross has been working closely with Verizon Wireless to develop and test the T-Series and iMAT designs. SkyCross demonstrated some of its new designs at the 2011 International CES in Las Vegas this January. The T-Series antennas are incorporated in new tablets from Acer and in Cisco’s Cius corporate tablet. An iMAT with MIMO was also presented in a product from Ionics EMS, a company offering a home automation control point that uses the Verizon 4G LTE network to remotely monitor and control home functions.
Also, SkyCross is working on the TV antenna problem for laptops, tablets, and other mobile devices. At least we won’t have to use rabbit ears. Small antennas to receive the lower over-the-air VHF channels (2-7) present a unique challenge to antenna makers. With some unique designs and a way to make the antenna tunable, it looks like the problem is getting solved.
Finally, Taoglas makes a wide range of antennas for all wireless applications, with new innovations in packaging and mounting. Its FXP.830 flexible antenna is designed for machine-to-machine (M2M) wireless applications in the 2.4/5.8-GHz bands, including Wi-Fi, ZigBee, and other standards (Fig. 6).
The antenna comes on a 42- by 7- by 0.1-mm flexible polymer material and has double-sided 3M tape on one side for easy peel and stick mounting. It works in devices where space is at a premium. Applications include telemedicine and telehealth where the device is worn on the body, remote monitoring, mesh sensor networks, security, smart metering, and other M2M projects.
The FXP.830 is basically a dipole design with linear polarization. It offers a peak gain of 3 dBi with an average gain of –2 to –3 dBi. The impedance is 50-Ω, and maximum input power is 5 W. The amazing spec is its 70% to 80% efficiency in the 2.4-GHz band and 65% to 80% efficiency in the 5-GHz bands. Feed is by coax cable.