Extended band coverage on EVLA antennas

New frequencies

EVLA receivers often are sensitive over a wider frequency range than their VLA predecessors, and observations in these until now unattainable frequencies have become possible.

Those interested in observing outside the traditional VLA bands should be aware that only data from EVLA antennas can be used - VLA antennas will not produce useful data at these frequencies. JObserve does not support the creation of the required observe files; see the the Observe File Preparation section below for more information.

Note that EVLA observations in IFs A/C below the traditional lower VLA frequency limit suffer from saturation issues caused by too large a frequency shift in the signal path. IFs B/D will give useful data since their lack of automatic attenuation prevents the saturation seen in IFs A/C. We see this behavior in L-band below 1250 MHz and in C-band below 4400 MHz.

Maximum IF separation

Whereas for the old VLA LO system the maximum IF separation is 500 MHz (between extreme band edges), for the EVLA this goes up dramatically.

Currently the maximum IF difference for EVLA antennas is 4 GHz, obviously subject to the restrictions of the receiver bandwidth. This is still an area of development, however, and we recommend observers to monitor this section for the latest news. This new IF separation has been demonstrated to work for C-band; for now, we recommend observers planning K-band and higher to contact us prior to observing.

Eventually, for the higher frequencies, the maximum difference between IFs is expected to go up to 12 GHz; we will announce this as soon as this is the case.


Test observations below 1200 MHz have recently been carried out with the EVLA to assess the feasibility of pursuing spectral-line observations for red-shifted HI 21cm or other spectral-line transitions at frequencies where the VLA has severe limitations and/or simply cannot observe.

These test observations show that successful observations are possible with the EVLA at frequencies that are believed to be dominated by RFI due to aeronautical navigation transponders, at least in A and B configurations. As this RFI is more evident on the shorter spacings, as well as at day-time hours, more tests will be carried out at various hours of the day and in C and D configurations to fully assess the limitations on astronomical observing due to these transmissions. We expect that daytime observations at these frequencies may be much more difficult.

Note that, for reasons explained above, below 1250 MHz only B/D data are usable.

This new 1.0-1.2 GHz capability on the EVLA is now available for users, and NRAO will consider exploratory proposals to use it in the current configuration through the end of D configuration, 15 September 2008. Click here for more information.


Most EVLA antennas currently in the array (see the table in the relevant section for the current status) have the new wide-band C-band receivers installed, which can be tuned outside the traditional VLA C-band range (4.5 - 5.0 GHz). Tests show that meaningful results can now be obtained from 4.2 - 4.5 GHz and from 5.0 - 7.7 GHz as well. The rms (in arbitrary units) as a function of frequency between 5.5 and 8.0 GHz is plotted here. The features at 6.0, 6.4, and 7.9 GHz are resonances which we expect to go away once the Ortho Mode Transducers (OMTs) are installed some time in 2008.

A plot showing the K value against frequency is shown here at higher frequency resolution than the figure referred to above. This plot covers in detail the range from 5900 to 6700 MHz. This range was chosen since it is not only there that the resonances occur, but it also contains the familiar OH lines at 6031 and 6035 MHz and the Methanol line at 6668 MHz. See the Observational Status Summary for a definition of the K-value.

Note that at outside the traditional C-band range polarization information is lost, as illustrated by this figure showing the ratio RL/RR for two baselines for an unpolarized source. Note that only between 4.5 and 5.0 GHz this ratio is close to zero, as expected for an unpolarized source. Polarization observations won't be able to exploit the extended frequency range until the installation of the OMTs.

To illustrate the opportunities this new frequency range offers, we observed a number of maser lines in W3OH. In figures 1 - 3 we show their scalar cross-power spectra on one of the EVLA - EVLA baselines. Figure 1 shows the 6030.747 MHz OH line, figure 2 the 6035.092 MHz OH line, and figure 3 the 6668.5192 MHz Methanol line. The flux density scale is indicated below the plot; because of a lack of accurate calibrator flux densities at these frequencies we did not perform any flux calibration; we expect true flux densities to be about 40% higher than the y-axes indicate.

Test observations at frequencies between 3800 and 8200 MHz show that the sensitivity of IF pair AC degrades significantly at frequencies below 4400 MHz. During the transition period, observers interested in using the EVLA with the current VLA correlator to observe at C-band frequencies between 3800 MHz and 4400 MHz need to utilize IF pair BD. If all four IFs are needed, the automated power adjustment for IF pair AC needs to be disabled in the observing script. For the latter, contact local staff. Please direct technical questions about observing at these frequencies to Emmanuel Momjian.

Figure 1: Effective system temperatures for all four IFs of EVLA antenna 2 between 3800 and 5600 MHz


Most EVLA antennas currently in the array (see the table in the relevant section for the current status) have the new wide-band K-band receivers installed, which can be tuned outside the traditional VLA K-band range (21.2 - 25.2 GHz). Recent tests show that meaningful results can be obtained from 18.1 - 26.5 GHz, with a small penalty in performance at the extreme ends, as shown in this figure, which plots the rms (in arbitrary units) averaged over the four IFs as a function of frequency at the band edges. Note that at 18.1 and 18.3 GHz not all IFs led to a solution. Between these edges, the rms rises linearly with frequency, with a slope largely determined by the frequency dependent antenna efficiency.

Observe file preparation specifics

Since observations at the new L, C, and K-band frequencies can only be done with EVLA antennas, there is no danger of introducing phase jumps caused by the small fluke changes while Doppler tracking, on VLA - EVLA baselines. After a temporary 'ban' on all Doppler tracking we now can recommend Doppler tracking again for EVLA - EVLA only observations (see here for further details).

If you don't want Doppler tracking, you will need to specify the appropriate VLA LO settings directly in the observe file. Note that these are 'illegal' LO settings as far as the VLA is concerned, and JObserve will not accept them - they are only used to set the appropriate values in the EVLA LO chain. In the specific case of L-band observing between 1000 and 1200 MHz we have made an online version of the LOSER program available.

For general help on finding LO values in order to arrive at the sky frequency of your choice, please contact Gustaaf van Moorsel

Modified on Tuesday, 29-Jan-2013 13:57:48 MST by Gustaaf van Moorsel