(Christiansen et al., 1961; Bracewell and Swarup, 1961) except for the dispersive transmission lines instead of the branching system. If now the reception band ...
G. SWARUP and V. K. KAPAHI Radio Astronomy Centre, Tara Institute of Fundamental Research, Bombay
(Received 15 May, 1970)
Abstract. A simple image forming system using a multielement interferometer for obtaining rapid pictures of solar radio bursts is described. A dispersive transmission line is used to feed the elements in series through directional couplers. Truly instantaneous pictures of solar activity can be obtained by placing a number of narrow frequency filters at the end of the I. F. amplifier in the main receiver, located at one end of the array. The two dimensional extension of this principle is examined in some detail. Multibeaming in the two arrays of a crossed grating interferometer can be combined with fast phase-scanning in one of the arrays to produce rapid pencil beam pictures. If log-periodic antennas are used, observations can even be made at widely different frequencies simultaneously. For illustration, some important parameters for simultaneous observations at 60, 90 and 120 MHz are estimated for an interferometer assumed to be located at a latitude of 30~ N. The main advantage of the proposed system is that high-resolution rapid pictures of radio bursts can be obtained simultaneously at a number of frequencies with modest effort.
1. Introduction H i g h - r e s o l u t i o n fast-speed o b s e r v a t i o n s o f r a d i o b u r s t s are o f great i m p o r t a n c e in o u r u n d e r s t a n d i n g o f the variety a n d c o m p l e x i t y o f solar p h e n o m e n a , specially at m e t e r wavelengths. This has been well d e m o n s t r a t e d by o b s e r v a t i o n s with the C u l g o o r a r a d i o h e l i o g r a p h (Wild, 1967; W i l d et al., 1968; W i l d , 1968) at a frequency o f 80 M H z . W e describe b e l o w in Section 3, a simple i m a g e f o r m i n g technique b y which two d i m e n s i o n a l a l m o s t - i n s t a n t pictures o f b u r s t a c t i v i t y can be realised at a n u m b e r o f frequencies s i m u l t a n e o u s l y using a crossed-grating interferometer. The new m e t h o d o f i m a g e f o r m a t i o n is b a s e d on the simple principle o f frequency dispersion o f a transmission line feeding in series the a n t e n n a s o f a m u l t i e l e m e n t interferometer. A multielement i n t e r f e r o m e t e r w o r k i n g at a frequency o f 610 M H z a n d i n c o r p o r a t i n g a dispersive t r a n s m i s s i o n line has been successfully o p e r a t e d at K a l y a n near B o m b a y ( S w a r u p et al., 1966; K a p a h i a n d Isloor, 1968). This a r r a y is first briefly described in Section 2. T h e new m e t h o d o f i m a g e f o r m a t i o n is c o m p a r e d in Section 4 with the existing o r p r o p o s e d systems.
2. The 610 M H z Multi-element Interferometer at Kalyan T h i r t y - t w o e q u a t o r i a l l y m o u n t e d p a r a b o l i c dishes o f d i a m e t e r 1.7 m each are used to set u p two i n d e p e n d e n t arrays, one a l o n g the east-west direction a n d the o t h e r a l o n g the n o r t h s o u t h direction. A schematic o f the two interferometers is shown in F i g u r e 1. Solar Physics 14 (1970) 404-413. All Rights Reserved Copyright 9 1970 by D. Reidel Publishing Company, Dordrecht-Holland
A SIMPLE IMAGE FORMING TECHNIQUE
The east-west array (Figure 2) consists of 24 dishes spaced uniformly along a 631 m base line. This gives rise to fanbeams with half power beam width and separation of 2.3 and 61.4 rain of arc respectively. The north-south array employs 8 dishes spaced uniformly along a 256 m base line. GO'ft.
Fig. 1. Schematic of the feeder system for the east-west and north-south arrays at Kalyan.
Fig. 2. East-west array of 24 dishes of 1.7 m diameter at Kalyan.
a . SWARUP AND V. K. KAPAHI
The half power width and separation of fan beams for this arrangement are 5.2 and 45.8 rain of arc respectively. A novel feature of the arrays is the series feeder system shown in Figure 1. A single parallel wire transmission line is used to connect the 24 dishes of the east-west interferometer to a Dicke-switched receiver at the east end. The dishes are coupled to the transmission line through quarter wave directional couplers (Oliver, 1954) and the couplings suitably graded along the array in order to feed the dishes uniformly. If a is the loss factor of the transmission line between adjacent elements, the required power coupling at the nth element, K n (n = 1 corresponding to the element farthest from the receiver) is given by K, = ( f l - 1)/(fl- 1); where fl = 1/(1 - c 0. Each directional coupler is supported on a pair of teflon insulators mounted on wooden poles so as to be fixed with respect to ground. As the copper conductors (of diameter 7.6imm) of the parallel wire line expand or contract due to variations in ambient temperature, they can slide through the teflon insulators. The heavy weight at one end keeps the length of transmission line between adjacent elements constant. The phases along the array have been checked and adjusted by the modulated reflection method of Swarup and Yang (1961). The feeder system has been observed to be phase-stable over fairly long periods. Apart from its simplicity and much lower cost as compared to the conventional branching arrangement of transmission lines, this highly dispersive system of a single transmission line can be used with advantage to realize instantaneous pictures. A number of fan beams displaced with respect to each other are obtained by making observations at slightly different frequencies. At the different frequency, the phase gradient along the array is no longer the same. In the small angle approximation, this shifts the direction of beam maxima by an amount proportional to the difference between the two frequencies. To get instantaneous radio picture of the sun one could place a number of frequency-filters at the end of the I. F. amplifier of the main receiver in Figure 1. The detected outputs from the filters can be suitably displayed, say on an intensity modulated oscilloscope (Wild, 1967) or a solid state device (Vinokur, 1968) to record the pictures. A possible two dimensional extension of this multiple beaming principle which appears to be quite simple for making high resolution instant observations of solar bursts at meter wavelengths will now be described.
3. Proposed Two-Dimensional Array for High Speed Observations of Radio Bursts Figure 3 shows a schematic of two linear arrays of N elements each in the east-west and north-south directions. The elements of each array are coupled to a parallel wire transmission line through quarter wave directional couplers. The two arrays are combined through a phase switch to operate as a crossed grating interferometer. The system is essentially similar to the conventional crossed grating interferometers (Christiansen et al., 1961; Bracewell and Swarup, 1961) except for the dispersive transmission lines instead of the branching system. If now the reception band is
A SIMPLE IMAGE FORMING TECHNIQUE
sub-divided into a number of uniformly spaced frequency bands or channels, each array of the cross will give rise to a set of multiple fan-beams, which are angularly displaced in the sky with respect to each other for different channels. When the two arrays are correlated or phase-switched as in a Mills-cross, the direcdvity pattern of the system would consist of a number of closely spaced pencil beams at the intersection points of the series of fan beams of the two arrays. The set of pencil beams corresponding to the different frequency channels would lie along diagonals as shown