FREQUENCIES

 

Table 2 gives the nominal frequency ranges for the 9 receiver/feed combinations available on all 10 VLBA antennas (Thompson 1995). Passband-limiting filters are described by Thompson (1995). Measured frequency ranges are broader than nominal; consult Hronek & Walker (1996) for details. Measured frequency ranges may be especially important for avoiding radio frequency interference (RFI), and for projects involving extragalactic lines, rotation measures (Cotton 1995b), and multi-frequency synthesis (Conway & Sault 1995).

 

 

Receivers Nominal Typical Typical

!!!and Frequency Zenith Zenith

!!Feeds Range SEFD Gain

[GHz] [Jy] [mJy]

90 cm 0.312 - 0.342 2184 0.096

50 cm 0.596 - 0.626 2205 0.086

!1.35 - 1.75! !304 0.095 !4.9 !47

13 cm !2.15 - 2.35! !315 0.091 !5.1 !49

!2.15 - 2.35! !391 0.077 !6.3 !61

!6 cm !!4.6 - 5.1!! !289 0.132 !4.7 !45

!4 cm !!8.0 - 8.8!! !299 0.117 !4.8 !46

!!8.0 - 8.8!! !390 0.111 !6.3 !60

!2 cm !12.0 - 15.4! !513 0.111 !8.3 !80

!1 cm !21.7 - 24.1! !937 0.105 15.1 145

!7 mm !41.0 - 45.0! 1402 0.084 217

Notes: (a) Hronek & Walker (1996) describe additional antenna-specific filters not mentioned by Thompson (1995). (b) With 13/4 cm dichroic.

A 3 mm system is also available on the VLBA antenna at Pie Town. For more 3 mm information, consult the VLBA home page (see Section 26.3) or contact Vivek Dhawan (see Section 26.4).

Also appearing in Table 2 are parameters characterizing the performance of a typical VLBA antenna for the various receiver/feed combinations. Columns [3] and [4] give typical VLBA system equivalent flux densities (s) at zenith and opacity-corrected gains at zenith, respectively. These were obtained from averages of right circularly polarized (RCP) and left circularly polarized (LCP) values from 9 or 10 antennas, as measured by VLBA operations during regular pointing observations. The typical zenith s can be used to estimate root-mean-square (RMS) noise levels on a baseline between 2 VLBA antennas ( for a single polarization; see Equation 2) and in a VLBA image ( for a single polarization; see Equation 3). Characteristic values for assuming a fringe-fit interval of 2 minutes and for assuming a total integration time on source of 8 hours also appear in Table 2; both of these characteristic values assume an aggregate recording bit rate equal to the ``sustainable'' limit of 128 Mbits per second (Mbps) (see Section 5.16). No or entries are given for 90 cm and 50 cm because adequately wide bandwidths cannot be obtained. No entry is given for 7 mm since a 2-minute fringe-fit interval is unrealistically long.

Opacity-corrected zenith gains are needed for current continuum amplitude calibration techniques. These zenith gains vary from antenna to antenna, and will be monitored by VLBA operations and communicated to users (see Section 14). The typical values appearing in Table 2 are meant to be illustrative only.

RFI is known to be problematic at VLBA sites at 90, 50, 20, and 13 cm (Thompson 1995; Hronek & Walker 1996). The AOC frequency coordinator, Dan Mertely (see Section 26.4), can be consulted for details. Thompson (1995) discusses RFI levels harmful to VLBI.