مرکز منطقه ای اطلاع رساني علوم و فناوري - Dispersion measurements of one-element short backfire (SBF) antennas

Abstract :

Three one-element short backfire (SBF) antennas at 40 MHz, 140 MHz, and 350 MHz were built in the institute for the use with the ATS-6 radio beacon experiment (RBE). Ehrenspeck\´s data experimentally measured in the GHz range were transformed into the VHF range. Since there were no relevant data available concerning the dipoles and feeder systems, various types were tested in combination with differently shaped SBF reflectors. The antennas were optimized with respect to gain, sidelobe suppression (ss), and forward-backward ratio (fbr). For all antennas ss was better than 20 dB and fbr better than 30 dB. The data, measured with the two identical antenna methods, are presented and compared with Ehrenspeck\´s data obtained with the pattern integration method. Ehrenspeck\´s gain seems to be about 0.8 dB higher than that measured at the antenna test range at Lindau, very likely due to the different measuring methods. In all other aspects the agreement of the data seems to be fairly good. The ATS-6 RBE yields the opportunity to measure the differential group delay of the RF signals. For this purpose the relative dispersion-phase shift $\\Delta _{D}$ -between the 40-MHz, 140-MHz, and 360-MHz carriers and their $\\pm1$ -MHz sidebands has to be known fairly accurately. The following data were obtained: $|\\Delta _{D40/41}| \\leq 4\\deg$ /MHz; $|\\Delta _{140/141}| \\leq 1.03\\deg$ /MHz; $|\\Delta _{D360/361}| \\leq 0.4\\deg$ /MHz. Furthermore the total delay with respect to vacuum of the 360-MHz, 140-MHz, and 40-MHz SBF antennas was coarsely measured by using the two identical antennas method. We got the following figures $52.5 \\times 10^{-9}$ s at 40 MHz, $15 \\times 10^{-9}$ s at 140 MHz, and $5.8 \\times 10^{-9}$ s at 360 MHz. These data are practically independent from azimuth and elevation angles of the antennas and from the conductivity of the ground as long as the elevation angle is larger than $15\\deg$ . Similar measurements with Yagi antennas showed a strong variation of the aforementioned parameters when installing them at different locations. Thus the SBF antennas seem to be the best possible compromise of any antenna type for the specific purpose of the differential group delay measurements.