5 FREQUENCY BANDS & PERFORMANCE

Table 3 gives the nominal frequency ranges for the receivers available on all or most VLBA antennas (Thompson 1995). Actual measured frequency ranges are broader than nominal; consult Hronek & Walker (1996) for details and http://www.vlba.nrao.edu/cgi-bin/wbd_dir.pl for updates on performance as a function of frequency across the VLBA bands. These actual frequency ranges may be especially important for avoiding radio frequency interference (RFI), and for programs involving extragalactic lines, rotation measures (Cotton 1995b; Kemball 1999), and multi-frequency synthesis (Conway & Sault 1995; Sault & Conway 1999).

Table 3: Receiver Frequency Ranges & Performance

Receiver	Nominal	Typical	Center	Typical	Baseline	Image
Band	Frequency	Zenith	Frequency	Peak	Sensitivity	Sensitivity
Designation	Range	SEFD	for SEFD	Gain	$\Delta S^{\rm 512, 2 m}$	$\Delta I^{\rm 512, 8 h}_{\rm m}$
	[GHz]	[Jy]	[GHz]	[K Jy$^{-1}$]	[mJy]	[ $\mu{\rm Jy~beam}^{-1}$]
90 cm	0.312 - 0.342	2227	0.326	0.097	(g) 29	(i) 196
50 cm (a)	0.596 - 0.626	2216	0.611	0.088	(g) 81	(j) 553
21 cm (b)	1.35 - 1.75	296	1.438	0.096	1.9	18
18 cm (b)	1.35 - 1.75	303	1.658	0.100	2.0	19
13 cm (c)	2.15 - 2.35	322	2.275	0.093	2.1	20
13 cm (c,d)	2.15 - 2.35	337	2.275	0.090	2.2	21
6 cm	4.6 - 5.1	312	4.999	0.130	2.0	19
4 cm	8.0 - 8.8	307	8.425	0.113	2.0	19
4 cm (d)	8.0 - 8.8	407	8.425	0.106	2.6	25
2 cm	12.0 - 15.4	550	15.369	0.104	3.6	34
1 cm (e)	21.7 - 24.1	502	22.236	0.107	3.3	31
1 cm (e)	21.7 - 24.1	441	23.799	0.107	2.9	28
7 mm	41.0 - 45.0	1436	43.174	0.078	(g) 13	90
3 mm (f)	80.0 - 90.0	4000	86.2	0.025	(h) 52	(k) 447

Notes:
(a) Optional filters restrict frequencies to 608.2-613.8 MHz.
(b) Different ranges within the same 20 cm receiver.
(c) Filters at NL, LA, and OV restrict frequencies to 2200-2400 MHz.
(d) Using 13/4 cm dichroic.
(e) Different ranges within the same 1 cm receiver. Continuum performance is better at 23.8 GHz, away from the water line.
(f) See Table 4 for individual station details.
(g) Fringe-fit interval 1 minute.         (h) Fringe-fit interval 30 seconds.
(i) Data rate 256 Mbps.                 (j) Data rate 32 Mbps.
(k) 8-station array; 4-hour integration.

Also shown in Table 3 are parameters characterizing the performance of a typical VLBA station for the various frequency bands. Columns [3] and [5] give typical VLBA system-equivalent-flux-density (SEFD) values at zenith and opacity-corrected peak gains, respectively. These are means over measurements in both polarization at all ten antennas, at the frequencies in column [4]. Performance shown for the 1-cm band includes the enhancement due to an upgrade funded by the Max-Planck-Institut für Radioastronomie in Bonn, Germany. See Walker et al. (2007a, 2008) for more details.

The typical zenith SEFD can be combined with the aggregate recorded data rate and appropriate integration times to estimate the root-mean-square (RMS) noise level on a single VLBA baseline ($\Delta S$; see Equation 3 in Section 9), and in a VLBA image ( $\Delta I_{\rm m}$; Equation 4 in Section 10). Characteristic values tabulated in columns [6] and [7] are computed assuming, for most cases, the VLBA's current 512-Mbps standard recording rate for continuum observations; a typical fringe-fit interval of 2 minutes; and a total on-source integration time of 8 hours. Exceptions, indicated in the table notes, apply to the fringe-fit intervals at the lowest and highest frequency bands, where shorter intervals are often required; for the recording rate limits imposed by the available RF bandwidth at the lowest frequency bands; and for most parameters at the extreme 3-mm band. Performance may be worse than the tabulated estimates on some baselines due to poor primary or subreflector surfaces or poor atmospheric conditions.

Table 4: Typical Performance Parameters at 86.2 GHz

Antenna	Nominal	Typical	Typical	Typical	Baseline
	Frequency	Zenith	Peak	Zenith	Sensitivity
	Range	SEFD	Gain	$T_{\rm sys}$	$\Delta S^{\rm 512, 30 s}$
	[GHz]	[Jy]	[K Jy$^{-1}$]	[K]	[mJy]
BR	80.0 - 90.0	3500	0.039	135	55.
NL	80.0 - 96.0	4900	0.055	270	65.
FD	80.0 - 96.0	3600	0.034	120	55.
LA	80.0 - 90.0	3100	0.051	160	$\cdots$
PT	80.0 - 96.0	4100	0.024	100	55.
KP	80.0 - 96.0	4600	0.025	110	60.
OV	80.0 - 96.0	5800	0.020	100	65.
MK	80.0 - 96.0	4100	0.023	100	55.

The 3 mm band extends beyond the design specification for the VLBA antenna, and is challenging for the panel-setting accuracy of the primary reflectors, the figure of the subreflectors, and the pointing of the antennas. In addition, performance in this band is highly dependent on weather conditions. Table 4 gives the approximate current performance at 86 GHz for each antenna, as well as the RMS noise in 30 seconds (at 512 Mbps) on a baseline to LA, which is one of the most sensitive 3 mm antennas.