Minimizing the MeerKAT system noise temperature

Robert Lehmensiek*1 and Isak P. Theron2

EMSS Antennas (Pty) Ltd, 18 Techno Avenue, Technopark, Stellenbosch, 7600, South Africa. tel: +27 21 880 1188, fax: +27 21 880 1982. 1lehmensk@ieee.org, 2iptheron@emss.co.za

Abstract

This paper highlights some of the design choices made in maximising the sensitivity of the MeerKAT interferometer. In particular special care is taken in minimizing the system noise temperature in each component of the receiver front end. The receiver’s system noise temperature is below 7 K. The radiation patterns are stable over frequency with an antenna efficiency above 74 % and antenna noise temperature at zenith is below 11 K.

1. Introduction MeerKAT is a 64 element astronomical interferometer, which is part of the Karoo Array Telescope project in South Africa and is a precursor for the Square Kilometre Array (SKA) [1]. It will operate between 580 MHz and 14.5 GHz with initial emphasis on band 1 (580 MHz to 1015 MHz) and band 2 (900 MHz to 1670 MHz). For radio telescopes the most important performance metric is the sensitivity which is proportional to the antenna efficiency (or effective aperture area, Ae) and inversely proportional to the system noise temperature, Tsys. The system noise temperature is due mostly to the receiver noise, TRx, and the antenna noise (spillover, cosmic microwave background and atmosphere), Ta. This paper focuses on aspects of the design that minimized the system noise temperature. The results are given for band 2 with band 1 having similar results.

2 The Dish

The MeerKAT dishes are offset Gregorian dishes with a main reflector size of 13.5 m x 15.85 m (long and short axes in the plane of the rim) and a sub-reflector width of 3.6 m. The equivalent F/D is 0.55 resulting in a 48.9° feed subtended half angle. The choice for these dimensions was presented in [2] and was driven by cost and mechanical constraints such as size limits for road transport. The ground radiates thermal noise and any (spillover) illumination thereof will add to the system temperature, reducing the system sensitivity. The sub-reflector was thus designed to be slightly over-sized at the bottom by an angle of 20° which is measured from the secondary focus of the Gregorian system between the bottom edge of the sub-reflector and the edge of the extension, as shown in Fig. 1. This extension reduces the noise temperature contribution from the ground as demonstrated in Fig. 2 for an ideal Gaussian feed with varying edge taper on a 15 m aperture diameter offset Gregorian dish. It shows that the sub-reflector extension has a flat sensitivity performance over a wide tipping angle when feed down tipping is used [3]. For feed down tipping the dish’s optical axis is lowered in the symmetry plane from zenith towards the horizon in the direction of the sub-reflector. In Fig. 2 feed down tipping is defined with θp positive and for feed up tipping θp is negative. Feed down tipping has the added logistical advantage that the feeds are accessible closer to ground level.

Fig. 1: Dish Geometry showing the extension.

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Fig. 2: Normalised sensitivity (Ae/Tsys) of a Gaussian feed illuminating an offset Gregorian dish with an aperture diameter of 15 m without (left) and with (right) a sub-reflector extension assuming Tsys = Ta + 10 K.

3 The Feed Horn

The choice for the horn was a wide flare angle axially corrugated conical horn, as illustrated in Fig. 3 [4]. This horn can be designed to have good pattern symmetry and cross-polar performance [5]. Two steps in the circular waveguide are used to match the TE11 circular waveguide mode feeding the horn to free space. The horn diameter is larger than required to obtain a circular Gaussian beam. This allows a flat top feed pattern with sharper cut-off of the pattern past the sub-reflector edge for lower spillover noise. The optimization goal was to maximize the antenna’s sensitivity while simultaneously minimizing the TE11 circular waveguide mode reflection coefficient and limiting the sidelobe and Jones cross-polarization levels. The second sidelobe level (the first is accounted for in the astronomy calibration) had to be below -25 dB with a gradual roll-off to the -40 dB level for the far-out sidelobe levels. The cross-polarization levels had to be below -25 dB and -30 dB inside the -1 dB and -3 dB contours of the main beam. A weighted sum of these parameters provided the objective function. A hybrid optimization algorithm that combined the Population-Based Incremental Learning (PBIL) global search algorithm with a downhill simplex method was used as optimizer. The aperture efficiency is shown in Fig. 3 with the sidelobe and cross-polarization levels within specifications. The circular waveguide reflection coefficient was well below -20 dB. The spillover noise contribution and the sensitivity evaluated over the tipping angles are given in Fig. 4, where the brightness temperature distribution model in [6] and the average polarization reflection coefficient approximation were used. As the horn has no concentration of electric fields the loss is very small, in the order of 0.005 dB.

Fig. 3. Model of the wide flare angle axially symmetric corrugated horn (left) and aperture efficiency (right) when illuminating the offset Gregorian dish. The environmental radome, three corrugations, two matching steps and the TE11 mode circular waveguide port are shown in the horn model.

Fig. 4. Antenna noise (left) and sensitivity (right) over dish pointing angles at the frequencies 900 MHz, 1285 MHz and 1670 MHz. The results are given for both linear polarizations (vertical: solid lines; horizontal: dashed lines).

4 The OMT and directional coupler

A mechanically robust low-loss orthogonal mode transducer (OMT) as presented in [7] was used as the OMT. The structure is shown in Fig. 5 and consists of a pair of orthogonal conical dipoles placed in the circular waveguide feeding the horn at approximately a quarter wavelength (at center frequency) from a fixed backshort. The balanced dipole excitations are converted to unbalanced coaxial transmission lines using a folded Marchand balun [8]. A thermal break is realized by a gap between the warm (room temperature) horn and circular waveguide feeding the horn, and the cold (≈ 70 K) OMT’s dipole and balun structure. The cryostat’s barrier between vacuum and the atmosphere is realized by a high-density polyethylene dome. The parameters of the OMT were optimized using a Nelder-Mead numerical optimization technique. It is important to note that the combination of this horn and OMT do not give suck-outs in the frequency band. The S-parameters of the OMT and horn are also given in Fig. 5. The predicted loss of this compact OMT is of the order of a quarter wavelength air-filled coaxial transmission line, i.e. less than 0.03 dB.

Fig. 5. Model of the OMT showing the vacuum dome, crossed-dipoles with the integrated Marchand baluns and thermal break. The predicted (dashed) and measured (solid) S-parameters of a prototype OMT and horn. Calibration of the receiver requires a known noise signal to be coupled into both the signal paths. Commercial-off-the-shelf couplers can be used, but they typically have high losses. In this design a symmetrical quarter wave directional coupler was integrated as part of the OMT and realized by air-filled transmission lines which minimize the loss in the signal path. A single coupling line positioned between these signal lines couples the noise signal into the signal paths and requires only one noise source. In addition, it allows for full phase calibration between the two channels. The specified coupling level was -35 dB. The coupler has a measured directivity above 24 dB and a loss less than 0.03 dB. The feed’s noise contribution to the system temperature, given that the OMT and coupler are cooled to 70 K and the vacuum dome and horn are at 300 K, is given in Fig. 6, and is below 1.4 K across the frequency band.

Fig. 6. Predicted noise contributions of the feed for the two OMT ports.

5 The LNA and System Temperature Breakdown

A short aluminium semi-rigid cable connects the directional coupler to the LNA and provides thermal isolation. The cable’s loss contribution is about 1 K to 1.5 K. The LNA was designed by the Herzberg Institute of Astrophysics (HIA). When cooled to a physical temperature of 20 K it has a system noise contribution of less than 3 K. The system temperature breakdown is given below. This is in good agreement with the measured receiver noise which increases from 6 K at the low frequency to 7 K at the high frequency.

900 MHz 1670 MHz

Antenna noise (zenith pointing) 10.9 K 7.4 K Dish (transmission for a panelled dish) 1.0 K 1.0 K Horn, radome, vacuum window, OMT and coupler 1.2 K 1.4 K Coupler to LNA thermal isolation 1.0 K 1.5 K LNA 3.0 K 3.0 K Post-LNA RF and digitiser 1.0 K 1.0 K

Total Tsys 18.1 K 15.3 K

6. Acknowledgments

The authors acknowledge the Department of Science and Technology (DST) of South Africa for their financial support.

7. References

1. MeerKAT Array, South Africa. [Online]. Available: http://www.ska.ac.za/meerkat/. 2. I. P. Theron, R. Lehmensiek, and D. I. L. de Villiers, “The design of the MeerKAT dish optics,” Int. Conf. on Electromagnetics in Advanced Applications (ICEAA), Cape Town, South Africa, Sep. 2012. 3. D. I. L. de Villiers and R. Lehmensiek, “Sub-reflector extensions for reduced noise temperature in low-side sub-reflector offset Gregorian systems,” European Conf. on Antennas and Propagation (EUCAP), Praque, Czech Republic, Mar. 2012. 4. R. Lehmensiek and D. I. L. de Villiers, “Wide flare angle axially corrugated conical horn design for a classical offset dual-reflector antenna,” European Conf. on Antennas and Propagation (EUCAP), Praque, Czech Republic, Mar. 2012. 5. R. Lehmensiek and I. P. Theron, “The design of the MeerKAT L-band feed,” Int. Conf. on Electromagnetics in Advanced Applications (ICEAA), Cape Town, South Africa, Sep. 2012. 6. G. C. Medellín, “Antenna noise temperature calculation,” SKA Memo 95, Jul. 2007. 7. R. Lehmensiek and I. P. Theron, “Compact low loss L-band orthomode transducer,” Int. Conf. on Electromagnetics in Advanced Applications (ICEAA), Sydney, Australia, Sep. 2010. 8. W. K. Roberts, “A new wide-band balun,” Proc. IRE, 45, Dec. 1957, pp. 1628–1631.