Test Transmissions

A series of tests were carried out in conjunction with the SMA and the Department of Communication and the Arts (DCA). The DCA mobile test transmitting equipment was set up, over 8 days in 1996 between July 22 and August 28, at four different elevated sites. The test sites, chosen to be representative of the future locations of transmitters, were situated to the west and north of the MOST at distances 30km. The details are given in Table 1.

After preliminary tests the transmitter frequency was set within the MOST's bandpass at 844MHz (vertically polarised carrier, 20% AM modulated with a 1kHz tone). The effective isotropic radiated power (EIRP) was adjusted to avoid saturation of the MOST receiver and was switched on/off at 2 (or 5) minute intervals. The SMA/DCA team set up a standard antenna and calibrated receiver to measure the irradiance at the MOST. Both the receiver and transmitter antenna heights were set at 5m.

 

Table 1: Transmission Site Details

site	Mt. Taylor	Red Hill Lookout	Mt. Ainslie	St. George Hill
bearing from MOST ()	271	281	296	354
distance (km)	31.7	28.4	26.6	36.6
measured path loss (dB)	142	140	153	142

Measurements of the Remote Sidelobes of the MOST

The purpose of these tests was to determine the typical sensitivity of the MOST to interference from the selected sites. For each transmitter location the MOST was steered to 5 tilts (, & ) and 5 meridian distances (, & ) making up a grid of 25 pointings.

The signals received by MOST during these tests were recorded in two ways:

1. The signal from one fan beam was recorded on a chart recorder with a 0.5s time constant.
2. The usual MOST data acquisition system was used to calculate the rms signal across all 64 fan beams using a 24s integration time.

On each day of testing a strong unresolved celestial radio source was observed in order to calibrate both the analogue chart records and the digital data acquisition system.

The chart recorder measurements of a single beam showed large short-term amplitude variations even though the signal from the calibrated test antenna was steady. These fluctuations are thought to be caused by time variable distortions of the incoming wave front over the 1.6km length of the MOST.

As the digital data acquisition system integrated the signal over 24s, we were concerned that some of the fine structure in the interference response might be smoothed out. A detailed comparison of the chart records and the digital results showed that the interference peaks were rarely more than 2 or 3 times the rms signal measured digitally. Accordingly we have used the digital data for subsequent analysis.

The data for all sites and telescope positions are presented not as the observed signal strength but as corresponding sidelobe gains of the MOST. These gains are calculated using the known equivalent flux density of the transmitter and the gain (1.0 10) of the MOST main beam. Figure 3 illustrates the different gain levels observed for the four interference sites. The mean gains for the sites are given in Table 2.

  
Figure 3: The sidelobe gain of the MOST shown as a function of tilt and meridian distance for the four test sites. The darker shadings indicate a higher gain value according to the key shown. The dotted line on the Red Hill site shows the track of the MOST in tilt and MD during the synthesis observations.

The average gain over all sites and telescope positions is 0.1. Examination of the detailed results shows that the gain varies from <_ 0.01 to >_ 1. The scatter among the individual pointings may arise because we have grossly undersampled the complex sidelobe structure of the telescope, as discussed in Section 3. For the three westerly transmitters there is a slight tendency for the gain to increase with MD. This is not surprising as at high MD the main beam (or the left-hand polarised beam) is directed towards the west. The average gain for the Mt.Taylor transmitter, situated almost due west, appears to be lower than the other three sites, but this is barely significant in view of the scatter. Overall the gain for St.Georges Hill (north) is slightly higher than for the other sites and shows little variation with MD. It is perhaps surprising that the gain remains relatively high even when the MOST is tilted to the south where one would expect the reflector to screen the line feed from interference. In the next section we describe the results of interference measurements made continuously during a 12 hour synthesis observation.

Synthesis Observations

Two tests were made to determine the susceptibility of the MOST to interference during a normal synthesis observation. The first was a reference observation taken on 1996 August 4 with no interference transmissions. The other taken on August 28 had the test transmitter located at Red Hill Lookout. Both observations were made in the 70 arcminute mode with the field centred on R.A. 10 26 25.0 Dec. -30 46 56 (B1950). The transmitter had a 5 minute on - 5 minute off cycle with an irradiance at the MOST of 5.6 10Wm, equivalent to a flux density of 1.9 10Jy in the MOST bandwidth.

Figure 4 shows the rms signal across the 64 fan beams during the 12 hour synthesis observation taken on August 28. The transmitter shows strongly in the first and last 30 minutes of the observation. The strong broad feature in the middle of the observation is due to the Sun being recorded in a sidelobe. Short duration spikes are due to nearby out-of-band mobile phone interference.

  
Figure 4: Data from the 1996 August 28 synthesis observation plotted as flux density (rms mJy) versus sample number. The response to the test transmitter's on/off cycle is obvious at the start of the observation (MD near -60) and the end of the observation (MD near +60). Solar and mobile phone interference are also visible.

The interference seen strongly at the beginning and the end of the observation corresponds to a telescope gain g, which appears inconsistent with the measurements of the gain at fixed pointings (see Figure 3). The track of the MOST, shown as a dotted line in Figure 3, has a corresponding gain of the order 0.02 for most of the fixed pointings. In fact the strong response in the synthesis observation is the result of a known (near end-fire) grating lobe scanning through azimuth 281, the bearing of the transmitter located at Red Hill Lookout. Figures 5(a) and 5(b) show the effect of interference on an image. These have been prepared from the two observations using standard MOST reduction software.

  
Figure 5: Images made from synthesis observations during August 1996.

The image constructed from data taken on August 28 shows the effects of interference from the test transmitter as well as interference from the Sun, the strongest radio source in the sky. The horizontal structure is caused by the transmitter and the vertical structure by solar interference. It can be seen in Figures 4 and 5(b) that the transmitter and the Sun are contributing about equally to the degradation of the image. Observations with the MOST are usually made at night and when daytime observations are required the usual practice is to schedule them to avoid the Sun in known sidelobes as much as possible. Scheduling of the August 28 observation was determined by the availibilty of the transmitter and hence we could not avoid the Sun showing in a sidelobe.

© Copyright Astronomical Society of Australia 1997