Thermal loss in high-Q antennas
Tunable antennas are very promising for future generations of mobile communications, where antennas are required to cover a wide range of operating bands. This reported work was aimed at characterising the loss mechanism of tunable antennas. Tunable antennas typically exhibit a high quality factor (Q), which can lead to thermal loss due to the conductivity of the metal. The investigation shows that copper loss is non-negligible for high-Q values. In the proposed design, the copper loss is 2 dB,
... opper loss is 2 dB, for a Q of 260 at 700 MHz. Introduction: With the band proliferation that followed the standardisation of the fourth generation (4G) of mobile communications, active antennas have been investigated to enhance the operating bandwidth of mobile phone antennas while keeping a low profile. Active antennas can reconfigure their resonance frequency using microelectromechanical systems , pin diodes  or varactors  . These active components will add a varying reactance to the impedance of the antenna, thus modifying its resonance frequency. A recent overview of the tuning techniques is given in  . Independently of the tuning technique, when the antenna is forced into resonance at a lower frequency than its natural frequency, its bandwidth decreases and its quality factor, Q, increases inversely proportionally  . As the Q of the antenna increases, its efficiency decreases due to higher currents in the equivalent series resistance of the tuner. For high-Q values, the loss due to the tuner alone cannot explain the measured total loss . In the work reported in this Letter, the authors investigated the existence of a thermal loss in high-Q antennas, due to the conductivity of copper. For this investigation, the authors have designed a large patch antenna, naturally resonating at 700 MHz, as it is the lowest frequency to reach with 4G nowadays . Different widths of the patch result in different antenna Q, while maintaining the resonance frequency. The measured radiation efficiencies are compared to determine the influence of Q on the thermal loss of antennas.