Varactor tunable helical antenna

Abstract

In this paper, a varactor tunable helical antenna for DVB-H (Digital Video Broadcasting for Handheld devices) at 470 - 702 MHz is presented. The frequency tuning is implemented by using a varactor which capacitance value is changed with a reverse bias voltage. The varactor is placed as a series component to the feed of the helical antenna. An inductor of 270 nH is used to separate the bias DC voltage and RF frequencies. The measurements were done with two finite size ground planes in vertical polarisation. The centre frequency of the antenna with a 50 mm ground plane is 474 MHz when the varactor is biased by 0 V. When the reverse bias voltage over the varactor is -20 V, the frequency shift is 26.6 % up to 600 MHz. For a 260 mm ground plane the centre frequencies at same bias voltages are 468 MHz and 596 MHz with a frequency shift of 27.4 %, respectively. The total volume of the tunable helical antenna is very small compared to the wavelength, like the radius of the helix, and thus the shape of the radiation pattern corresponds to the helical antenna radiating in the normal mode; the nulls of the radiation patterns are in directions of 0° and 180°. The maximum gains are between -10.9...-7.3 dBi and is close to the DVB-H standard at tuning frequency. The effect of the varactor on IMD (Intermodulation Distortion) is presented at different reversed bias-voltages with fixed power levels delivered to the component. In the worst case when the bias voltage is -1 V, the power levels of modulated signals are approximately 30 dB down on the peak signals. (6 pages)