Long Wavelength Array (LWA) Concept

Figure 4: Angular resolution (arcseconds) as a function of frequency (MHz) for past and present low frequency imaging instruments in the 10-200 MHz range. The LWA will result in a dramatic improvement in angular resolution and frequency coverage.

Figure 5: Sensitivity (mJy) to a point source as a function of frequency for some of the same instruments shown in Figure 4. The sensitivities are estimates of the minimum detectable flux density provided by (past, present, and proposed) low frequency instruments. As in Figure 4, the LWA will make a dramatic advance in sensitivity at the lowest frequencies. The LWA sensitivity calculation is based on a $\lambda^2$ dependent collecting area assumed to be $10^6$ m${}^2$ at 15 MHz. A bandwidth of 3 MHz and integration time of 8 hours have also been assumed.

The dramatic successes with the narrow-band VLA system ([Kassim et al. 1993]) of modest collecting area ($\sim 10^3$ m${}^2$) and unprecedented angular resolution ( $\approx 20\hbox{$^{\prime\prime}$}$) provide strong incentives to develop a much larger ($\sim$ 500 km baselines) and more sensitive ( $A_{\mathrm{eff}} > 10^5$ m${}^2$) instrument. A completely electronic Long Wavelength Array (LWA) could explore the entire LW spectrum at unmatched levels of sensitivity (sub-mJy) and angular resolution (arc-second) ([Kassim & Erickson 1998]). Moreover, the ability to do this from the ground with intrinsically modest bandwidths and relatively inexpensive hardware permits the array to be developed at a fraction of the cost of higher frequency ground- or space-based systems of comparable size and sophistication. The approximately $25,000 total cost of the hardware required for the development of the 74 MHz NRL-NRAO system is dramatic testimony to this fact.

Figures 4 and 5 illustrate the landmark improvements the LWA, with its 400-km baselines, could achieve in the LW range over past or present instruments. Figure 4 shows that the LWA will surpass, in most cases by two orders of magnitude or more, the angular resolution of available low-frequency instruments. While a few instruments are edging towards improved angular resolution at one or two ``spot'' frequencies (e.g., the GMRT at 50 and 160 MHz, the VLA at 74 MHz), Figure 4 shows that the LWA can provide high angular resolution with a broad frequency coverage.

Figure 6: A possible Long Wavelength Array layout. For the array configuration, planned NMA stations are shown in red. The compact core will likely be located a few kilometers to the west of the VLA.

Figure 5 shows that the LWA achieves a parallel breakthrough in sensitivity, even over instruments with comparable collecting areas (e.g., UTR2 with $A_{\mathrm{eff}} > 10^5$ m${}^2$), reflecting the impact of improved angular resolution on limiting source confusion, a key limit to the sensitivity of low frequency instruments. The importance of reducing source confusion is demonstrated by the the impressive relative sensitivity of the 74 MHz VLA system which has a collecting area of only $A_{\mathrm{eff}} \sim 10^3$ m${}^2$.

Fortunately, collecting area is inexpensive at long wavelengths. This is because simple wire antennas can be mass produced and because system temperatures are dominated by the Galactic background, making low-cost preamplifiers entirely adequate. The modest intrinsic bandwidths also result in very low receiver and associated electronic costs.

Figure 6 shows a possible LWA layout with maximum baselines of order 400 km whose basic building block is a simple, linearly polarized crossed-dipole.