Table 1 lists the operating bands and other characteristics of the VLA receivers. (The upper and lower frequency limits are those where the sensitivity has fallen to half its maximum value, except at Q Band where these are still being determined.)
| Frequency Range | Band Name | System Temperature | Antenna Efficiency |
| 0.298 to 0.345 GHz | P Band (90 cm) | 150-180 | 0.40 |
| 1.25 to 1.74 GHz | L Band (20 cm) | 37-75 | 0.51 |
| 4.25 to 5.10 GHz | C Band (6 cm) | 44 | 0.65 |
| 7.55 to 9.05 GHz | X Band (3.6 cm) | 34 | 0.63 |
| 14.25 to 15.7 GHz | U Band (2 cm) | 110 | 0.52 |
| 21.7 to 24.5 GHz | K Band (1.3 cm) | 160-190 | 0.42 |
 40 to 50 GHz | Q Band (7 mm) | 90-140 | 0.20 |
Note that a range of system temperatures is given for the
extreme bands. At P Band, the range includes the contribution of
nonthermal emission from the Milky Way--the lower value applies far
from the galactic plane. At L Band, the range includes pickup of
radiation from the ground at low elevation--the sensitivity decreases
by about one-third at 
elevation, one-half at 
elevation. At K and Q Bands, the system temperatures are increased at
low elevations by atmospheric emission, and are weather-sensitive (wet
conditions are worse than dry).
The VLA continuum system lets you observe at two independent
sky frequencies within each ``band''. This capability can be used to
increase sensitivity, to fill in the u-v plane more
densely by crude ``bandwidth synthesis'' or to study spectral or
Faraday depth changes in your source across a ``band'' (the latter
being especially worthwhile across the `L' Band because the Faraday
effect scales with 
).
Interference is rarely detected or suspected above 1.8 GHz at the VLA. It is a dominant factor in choosing a continuum observing frequency within L Band (1.25 to 1.74 GHz), particularly when using non-standard frequencies (e.g., when seeking to observe at the opposite edges of the band to determine Faraday rotation parameters).
As spectroscopists do not have freedom to choose center
frequencies for their projects, L Band interference may determine
whether a given HI or OH line experiment is possible. There is
self-generated interference throughout L Band at the VLA, mainly at
the harmonics of 50 MHz; this internal interference should be below
the noise in any continuum image made with an IF bandwidth 
6.25 MHz, but can be a serious problem for spectroscopy.
Extragalactic HI observations are also particularly vulnerable to
external interference because the red shift moves the line towards the
domain of mobile communications systems. Galactic OH observations
clash with satellite navigation systems. Before using any
non-standard frequency below 2 GHz, consult with VLA staff
(particularly Bill
Brundage, the VLA frequency coordinator)
for advice and lore based on recent observers' experiences.
Table 2 lists the more persistent L Band interfering
signals. An archive of recent L Band interference surveys is kept at
http://www.nrao.edu/doc/vla/interference/.
| Frequency | Avg. Flux | Pk. Flux | Source | Comments |
| 1277 MHz | 12 | 20 | Aerostat Radar | Sometimes absent |
| 1286 | 2 | 5 | Farmington Radar | Other weak lines nearby |
| 1300 | 2 | 5 | Internal RFI | |
| 1310 | 100 | 100 | ABQ Radar | |
| 1330 | 45 | 80 | ABQ Radar | Sometimes absent |
| 1381 | 3 | 100 | GPS L3 IONDS | On < 3% of the time |
| 1400 | 60 | 60 | Internal RFI | |
| 1429-1435 | 15 | 130 | Military | Four separate lines |
| 1444,1453 | 5 | > 100 | Hi altitude balloons | NASA/NSBF |
| 1465* | 6 | 8 | Alias of 1735 MHz | |
| 1486* | 15 | 20 | Alias of 1714 MHz | |
| 1500 | 2 | 5 | Internal RFI | |
| 1510* | 5 | 40 | Alias of 1690 | |
| 1515 | 15 | > 100 | Balloon, Alias of 1685 | |
| 1520* | 9 | 30 | Alias of 1680 | |
| 1525* | 6 | > 100 | Balloon, Alias of 1675 | |
| 1530-1544 | > 130 | > 200 | INMARSAT | Many `lines' |
| 1557-1567 | 10 | 20 | GPS Sidelobe? | Wide spectrum |
| 1570-1580 | > 500 | > 500 | GPS | Wide spectrum |
| 1584-1598* | 20 | 20 | Alias of GLONASS | |
| 1600 | 120 | 120 | Internal RFI | |
| 1602-1616 | > 500 | > 500 | GLONASS | Many separate `lines' |
| 1620 | 80 | 300 | ? | |
| 1625* | 15 | 20 | Aliased GPS carrier | |
| 1650 | 13 | 25 | Internal RFI | |
| 1678-1698 | 50 | 100 | Radiosondes, satellite | >6 variable `lines'. |
| 1710 | 10 | 10 | ? | |
| 1714 | > 500 | > 500 | Forest Service | |
| 1725 | 10 | 10 | " " | |
| 1730 | 25 | 25 | " " | |
| 1735 | > 100 | > 100 | Forest Service |
The asterisked signals are aliased, i.e. they originate from
RFI at another frequency but appear at the listed frequency via
defects in the VLA's frequency conversion system. A new frequency
conversion system has been prototyped and should be fully installed by
mid-1996. The signal strengths estimated in Table 2 are appropriate
for the D configuration, and should be considerably reduced in
the wider configurations--an attenuation of 100 may be expected
in the A configuration due to fringe phase winding.
Between 1220 and 1250 MHz, very strong broad-band RFI is always present (apparently due to satellite and radar transmissions). It may however be possible to observe in selected, narrow bandwidths in this region. Between 1435 and 1530 MHz, aeronautical telemetry from White Sands Missile Range will occasionally, and unpredictably, affect observing.
It may be possible to observe in the presence of RFI if (a) the RFI is not strong enough to cause serious gain compression in the front-end amplifiers, and (2) it is kept out of the correlator by a narrow back-end filter.
Table 3 may help you choose center frequencies and bandwidths that are likely to result in good data. There are very few good combinations for 50 and 25 MHz bandwidths. Table 4 shows what are ``statistically'' the best frequencies to use at L Band with the given bandwidths.
| BW | No RFI Expected | Weak/Occas'l RFI | ``Tolerable'' RFI |
| 50 MHz | none | 1365,1435,1465,1485 | 1335,1385,1415 |
| 25MHz | 1343 - 1347 | 1290 - 1297 | many |
| 1353 - 1387 | 1453 - 1470 | See Table 2 | |
| 1413 - 1417 | 1503 - 1517 | ||
| 1663 - 1665 | 1637.5 |
| Observe | AC | BD | ||
| Name | Center Frequency | Bandwidth | Center Frequency | Bandwidth |
| LL | 1464.9 | 50 | 1385.1 | 50 |
| L1 | 1364.9 | 50 | 1435.1 | 50 |
| L2 | 1515.9 | 25 | 1365.1 | 25 |
| L3 | 1515.9 | 25 | 1435.1 | 25 |