Wideband feed for a tracking mode and array applications at RATAN-600

In cooperation with St.Petersburg Electrotechnical University “LETI”

Wideband feed is being developed now for RATAN-600 radio telescope in a tracking mode in the band of 0.5-5 GHz. The offered design consists of 2 (linear polarization) or 4 (dual linear polarization) Vivaldi radiators (A.Golovkov, V.Khaikin, M.Sugak et al. Wideband Vivaldi feed for array applications at RATAN-600. (In Proceed. of XXVII URSI GA, Maastricht, 2002). This solution provides growth of effective aperture at longer wavelengths, stability of beam pattern in the band and acceptable beam pattern symmetry in E- and H- planes. An expected HPBW is 70-110 degrees. To improve band properties at law frequencies traditional topology of Vivaldi radiator and feeding system were modified. As a result good matching in the band (VSWR < 1.8) was reached in the prototype. We expect low enough insertion losses (0.5 dB) in the feed with FLAN-2.8 substrate material.

In Fig1. calculated beam patterns of a single Vivaldi feed at 2 GHz and 3 GHz are presented.

Fig1.Calculated beam pattern of a single wideband Vivaldi radiator in E and H planes at 2 and 3 GHz

Measured co- and cross- polarized amplitude beam pattern of a single wideband Vivaldi feed prototype in the range of 1.5-5 GHz with a step of 100 MHz are presented in Fig.2. Measured phase beam patterns will allow us to reconstruct behaviour of a wideband feed phase center in the range.

Fig.2. Co-(up) and cross-(down) amplitude beam pattern of a first Vivaldi feed prototype in the range of 1.5-5 GHz with a step of 100 MH measured in HUT (Espoo, Finland) anechoic chamber

Fig.3. A single Vivaldi feed prototype under test in HUT(Espoo, Finland) anechoic chamber(left), testing broadband horn(right)

A phased array in RATAN-600 focus gives us a chance to track cosmic sources during one hour with an unmovable main reflector without aberrations. A non equidistant linear phased array with a videband Vivaldi feeds and delay lines (DL) may be applied to correct electrical path lengths along the radio telescope focal line. Such an array allows us to use a normal (not reduced) feed size with high enough beam efficiency of each radiator in an array and provides small enough grating lobes.

Fig.4. Simulated beam pattern of RATAN-600 at 4 cm with phased array for different RMS phase errors and 100 m (left) / 200 m (right) apertures.

Simulated beam patterns of RATAN-600 at 4 cm with phased array in a tracking mode for different RMS phase errors along the focal line and 100 m (left)/200 m(right) apertures are shown in Fig.4.