Design of Compact Frequency-Tuned Microstrip Balun

In this letter, a compact frequency-tuned microstrip balun with a ring structure is proposed. By tuning the capacitance of varactor, the operating frequency of the balun can be tuned continuously. Also, by arranging the two loops on two layers, the difficulty in process caused by narrow gap can be avoided. Meanwhile, by introducing the lumped capacitors, the dimension of the balun is reduced greatly. The measured results indicate that the balun can operate at 620-1020 MHz when the voltage of varactor changes from 4 to 16 V. The phase difference of the two balanced ports is 180° ± 7°, and the amplitude difference is less than 0.8 dB. The realized balun has a dimension of 19.6 × 16.8 mm², equal to λ₀/25 × λ₀/29 (λ₀, the wavelength in vacuum at 620 MHz).