A new study from Danish researchers suggests the ongoing push towards global 4G roaming and high-quality, frequency-reconfigurable antennas will need to account for new loss capacities not considered by previous technology.
The worldwide demand for 4G technology and higher data rates in cell phones has led to a new, urgent need for frequency-reconfigurable or tunable antennas. Fourty distinct 4G bands have been defined around the world, and to support 4G in more places, more bands must be defined.
However, the large (and growing) number of 4G bands used around the globe has made it difficult for a single cell phone – and a single on-the-go person – to get 4G coverage everywhere. Due to an antenna’s fundamental relationships between weight, bandwidth, and efficiency, an antenna that could effectively cover all 40+ 4G bands would be larger than the cell phone it supported.
“To keep the antenna size acceptable for phone manufacturers and maintain good efficiency, we need to reduce the antenna bandwidth. We need to be able to reconfigure the operating bandwidth to different bands,” Samantha Caporal Del Barrio, Ph.D., from Aalborg University’s Antennas, Propagation and Radio Networking (APNet) section in Denmark, said in a Institute of Engineering & Technology (IET) press release. “The number of frequency bands is likely to keep increasing, if so, frequency-reconfigurable antennas will be the only way to address future communication standards.”
However, the main problem with tuning antennas to operate at different frequencies is that doing so reduces efficiency. Two reasons for this problem include tuner loss and thermal loss.
Because tuning forces an antenna to resonate at a different frequency than its natural resonant frequency, high-strength fields concentrate around the antenna structure; the more varied the frequency, the stronger the fields. The strong field strength of these antennas, known as high-quality factor (Q) antennas, translate into large currents on the antenna and high voltages between the antenna element and the ground plane.
While all tuners have some kind of lossy element, the stronger the antenna fields are, the higher the currents passed to the lossy element will be, which means more energy loss in the tuner, as well as radiation power loss. Thus, antenna efficiency lessens the further the antenna is tuned away from its original resonance frequency, making tuner loss the most damaging loss mechanism.
Thermal loss, in contrast, occurs because the antenna material, usually copper, is not a perfect conductor. While copper has good conductivity, very high field concentration over a small area causes heat (and thus power) dissipation. However, because high field values are needed for this to occur, such heat loss is only significant in very high-quality antennas. Most mobile antennas are not greatly affected by heat loss, as most mobile antennas are built to prioritize wide bandwidth, resulting in low-quality antennas.
“It is only now, because the bandwidth required for worldwide 4G has become too large for traditional designs, that interest in narrow-band tunable antennas is rising,” Caporal Del Barrio explained. “Therefore the loss mechanism of these antennas hasn’t been extensively investigated in the literature.”
In a paper titled “Thermal Loss in High-Q Antennas,” recently published in IET’s Electronics Letters, a team of scientists from APNet and Intel Mobile Communication in Denmark have shown that while thermal loss was not a huge limiting factor for low-Q mobile antennas, it will become an important consideration for engineers when they design tunable, high-Q antennas.
“It is essential to know this mechanism because antenna efficiency dictates over-the-air performance, and so, the feasibility of a design,” Caporal Del Barrio said. “Thermal loss can be a limiting factor to the achievable tuning range. Moreover, while tuner loss can be improved over time and can hopefully become negligible, thermal loss is an intrinsic property of the antenna material.”
Though thermal loss is a known scientific phenomenon, its previous irrelevance to low-Q mobile antennas has resulted in scant research on the subject, and poor handling using simulation tools, with long calculation times and inaccurate results. This lack of proper attention means that thermal loss is usually only discovered very late in the design process, when fabricated antennas are subjected to performance measurements.
However, the new information found out by Danish scientists – specifically, that in high-Q antennas, thermal loss can be more than 1 dB – signals that loss is something that designers need to take seriously, before more ineffective antennas are fabricated.
– Melanie Abeygunawardana
