Tuning a PIFA antenna using a zero-loss tunable capacitor opens up new design windows for antenna architectures.
Wireless designers continue to make tradeoffs among power, cost, size, and performance specifications. Engineers working in the consumer mobile market have difficult challenges due to its extremely competitive nature, as well as the time pressures to produce designs that work in some very difficult environments.
Irresistible Market Forces
Several trends are converging into the perfect storm for smartphone antenna design. The amount of data being transferred per phone user is anticipated to grow at 25 percent per year between 2012 and 2018. According to Ericsson [1] the total data traffic on wireless networks is growing at 50 percent per year. This trend puts pressure on wireless operators to increase spectral efficiency to serve their users. In turn, there is a need for more efficient RF performance in smartphones – beyond the 25 percent annualized growth per user. One tool for carriers is to move users to LTE/4G networks, since the higher spectral efficiency of these networks means networks can support more users, even with higher volumes of data.
Unfortunately, LTE deployment is in frequency bands outside the 2G/3G spectrum, extending the frequencies needing support at both below and above the frequency bands. 4GAmericas points out that simply having a roaming-capable international system in the 700 MHz band (part of the “Digital Dividend”) could be worth almost $1 trillion in additional GDP in the Asia-Pacific region. For developing economies, widely-available broadband access would result in $300 to $420 billion in combined GDP, and would add 10 to 14 million jobs [2,3].
This means that the economic drivers to create efficient mobile terminals amount to an irresistible force that antenna designers cannot avoid.
The Laws of Physics Can’t Be Broken
Smartphone antenna designers face requirements of size constraints, band coverage, and efficiency. In one of the most referenced antenna papers in history, Harold Wheeler introduced the concept of an electrically-small antenna [4]. The topic was further examined by Lan Jen Chu, who explored the physical limitations of small antennas [5], leading to what is referred to as the Chu Small-Antenna Radiation Limit.
The idea is that antennas have a limit in bandwidth constrained by antenna size, or antenna volume, which includes the antenna radiation sphere. Antenna volume is measured in wavelength, so lower frequencies make the antenna appear electrically smaller, thus less efficient. This concept is embodied in the following equation:
∆f ∝ (a)3
λ
f η
Where:
f = center frequency
∆f = antenna bandwidth
a = radius of sphere containing the antenna
λ = wavelength
η = antenna efficiency
Antenna bandwidth is proportional to the cube of the size, and inversely proportional to the cube of the wavelength and the efficiency. For a given frequency and size, you can have a narrowband, high-efficiency antenna, but not a wide bandwidth, high-efficiency antenna. Many studies have reaffirmed these concepts and explored physical implementations of electrically small antennas. Sievenpiper [6] used measured data from 112 published antenna designs and compared it to the Wheeler and Chu limits. He found excellent agreement between the theoretical limits and the published data.
The Planar Inverted F Antenna
Smartphone antenna designers have adopted the Planar Inverted-F Antenna (PIFA) for the majority of phones in production. The PIFA is a class of microstrip antennas that uses a shorting pin to compensate for the capacitance induced by the long parallel section. See Figure 1.
To enable a wider frequency range and/or a smaller size, the PIFA antenna can be equipped with a tuning element, a digital variable capacitor (DVC), attached to the radiating element (Figure 1). The location of the DVC on the element is a key design parameter and changes the way the variable load affects the antenna. We can see that one end of the antenna is shorted and one end is open, so the impedance varies across the antenna, as the voltage and currents are forced to conform to the boundary conditions (open versus short). This means that the DVC does not see a 50-Ω load and the impedance will vary at different frequencies for a fixed antenna. This fact is important when considering the losses the tuning element might induce on the antenna. The combination of PIFA and ground design techniques has been explored to push the PIFA envelope even further. Cabedo [7] shows improved performance by using ground plane slots in conjunction with a PIFA.
It’s key to note the difference between tuning in the impedance-matching circuit of the antenna and tuning the resonance of the antenna itself. Tuning the match improves the power the antenna has available to radiate, since less power is reflected back into the source. However, impedance tuning does nothing to change the radiation efficiency of the antenna. The implication is that an antenna using only impedance-match tuning to cover a broad band of frequencies must have a fractional bandwidth, ∆ f/f, wide enough to cover the required frequencies. From the previous equation we can see that the radiation efficiency must decline as the fractional bandwidth increases. Figure 2 illustrates the improved efficiencies of tuning a narrowband antenna compared to a broadband antenna.
With a narrowband antenna achieving higher efficiencies, why hasn’t this approach to covering multiple bands been adopted before? The answer is that the resistive losses in the tuning element can degrade the performance of the antenna such that all the gains are lost. Previous tuning solutions have used PIN diodes, GaAs switches, or CMOS/SOS switches. Many have low losses in a 50-Ω system, but much higher in a non-50-Ω environment, such as an antenna. RF switches specify insertion loss, which assumes a 50-Ω impedance, so the IL metric is not very useful when evaluating suitability in an antenna tuner.
A better metric is Equivalent Series Resistance (ESR), since symmetric impedance is not assumed. The impact of ESR on radiation efficiency is significant, as shown in Figure 3. Even 1 Ω of ESR can generate 2 to 4 dB of radiation loss, which could overwhelm gains made by tuning a narrowband antenna. High Q passive components have an ESR in the range of 0.2 to 0.3 Ω and produce around 1 dB of loss. However, those devices are fixed, not tunable.
Zero-Loss Tunable Components
Newly available devices based on RF MEMS technology have an ESR in the range of passive devices and up to 32 different capacitance values. The equivalent circuit is a lossless 1P32T switch connected to 32 high-Q capacitors.
The impact of this level of device performance is significant. Antenna designers have new design space available to address the issues of shrinking antenna size and wider frequency coverage. Designs using this technique have shown 2 to 4 dB of net improvement in radiation efficiency. With the flexibility of placing the tuning element on the aperture, a large tuning range can be generated with relatively modest change in capacitance. Typically, a tuning range of 250 MHz for the low bands (700 to 950 MHz) can be attained using only 1 to 2 pF of capacitance range. This amounts to a ∆f/f of over 30 percent, compared to a typical fractional bandwidth of 10 to 20 percent for untuned antennas with equivalent efficiency. A tunable PIFA antenna can improve efficiency up to 100 percent percent over a fixed PIFA in the 700 MHz band, as illustrated in Figure 2.
Measured Results
Meaningful performance data can only be measured on a complete smartphone, since the industrial design can have a significant effect on the radiation performance of the antenna system. The challenge in this design was to cover bands from 704 to 844 MHz, and improve the efficiency levels for total radiated power (TRP). Fixed antennas could not meet the minimum requirements in the low band because of the small volume allocated to the antenna. A reduced antenna volume has a significant impact on the instantaneous fractional bandwidth. By trading off the instantaneous bandwidth for higher efficiency, the tuned antenna meets the size constraints and delivers improved efficiency over all the low-band frequencies. Table 1 shows the measured results.
The data shows significant performance improvement in radiation efficiency. A weighted average of +2.3 dB improvement in LTE Band 12 translates to nearly 70 percent improvement in radiation efficiency. In the data-hungry LTE band, this level of improvement will result in doubling the effective data rate at the same power levels.
Irresistible market forces are causing antenna volumes to shrink, even as performance requirements are increasing and frequency bands are being added to smart phone platforms. Tuning a PIFA antenna using a zero-loss tunable capacitor opens up new design windows for antenna architectures. The Cavendish DVC provides tuning capability without inducing resistive losses so that 2 to 3 dB of efficiency gain is possible, even in the difficult low frequency bands.
REFERENCES
[1] Ericsson Mobility Report, June 2013, [Online] Available at
http://www.ericsson.com/res/docs/2013/ericsson-mobility-report-june-2013.pdf
[2]4GAmericas, “The Benefit of Digital Dividend,” [Online] Available at : http://www.4gamericas.org/documents/4G%20Americas-Benefits%20of%20Digital%20Dividend-September_2012.pdf, September, 2012
[3]4GAmericas, “The benefits of using LTE in Digital Dividend Spectrum,” [Online] Available at:
http://www.4gamericas.org/documents/Benefits%20of%20LTE%20in%20Digital%20Dividend_11.08.11.pdf, November, 2011
[4] H.A. Wheeler, “Fundamental Limitations of Small Antennas,” Proceedings of the. IRE, vol. 35, pp. 1497-1484, December 1947.
[5] L.J. Chu, Physical Limitations of Omnidirectional Antennas, Journal of Applied Physics, 19, December 1948, pp 1163-1175
[6 ]D. Sievenpiper, D. Dawson, M. Jacob, T. Kanar, S. Kim, J. Long, R. Quarfoth, “Experimental Validation of Performance Limits and Design Guidelines for Small Antennas,” IEEE Transactions on Antennas and Propagation, vol. 60, issue 1, pp. 8-19
[7] A. Cabedo, J. Anguera, C.Picher, M. Ribó, and C. Puente, “Multiband Handset Antenna Combining a PIFA, Slots, and Ground Plane Modes,” IEEE Transactions on Antennas and Propagation, vol. 57, No. 9, pp 2526-2533, September 2009
