A 151-MHz (~ 2m wavelength) radio telescope is presently being constructed at the Fenton Hill Observatory site. It will consist of a planar array of wire dipoles located approximately 1 meter above the ground, and will fill a rectangular area 8 lambda x 16 lambda, or an area of approximately 50' x 100'. As shown in figure 1, the overall array is divided into thirty-two primary elements of size 4 lambda x 1 lambda. Each of these primary elements will have an adjustable electronic phase delay, commanded by a control computer. By setting the appropriate phase delays in each of the elements before all the signals are summed, the array in effect can "point" to a given location on the sky to maximize its sensitivity to that location. Since the phasing of the array is accomplished through the use of rf PIN diodes, the time required to calculate phase delays and "point" to a given location is measured in milliseconds.
The primary purpose of this facility is to respond to gamma-ray burst triggers provided by the satellite-based Gamma-Ray Bursts Coordinate Network (GCN). In the same manner that our robotic optical telescopes respond to these triggers to search for prompt optical counterparts associated with GRBs, this array will search for prompt radio emission from GRBs. Although no prompt emission has ever been definitively identified as a GRB counterpart in other radio telescope facilities, "afterglow" radio emission first seen a few hours following a trigger and lasting many weeks has been observed by facilities such as the VLA. All of the efforts so far in looking for prompt emission have been hampered by the lack of precise early coordinates available to the GCN. With the launch last year of the HETE-2 satellite, and with the upcoming launch of SWIFT in a few years, observers can expect prompt coordinates (e.g., within a few 10s of seconds following gamma-ray detection) with fractional-degree accuracy. This array will take full advantage of these upgrades.
It is now widely believed that GRBs arrive from cosmological distances, and that they are associated with an early formation phase of the Universe. This has an important bearing on radio emission measurements, since a non-negligible dispersion effect comes into play from these distances and at these low frequencies. Lower frequency components arrive later than the higher frequency components, the difference in arrival time depending on the integrated free electron density along the intergalactic line of site. This effect can actually be helpful in radio measurements, as it provides discrimination against terrestrial radio interference that does not display such dispersion. An important feature in the receiver section of this array will be multiple spectral channels provided by a series of narrow-band rf filters within the 151 MHz radio astronomy band, and the signature of a broadband GRB radio counterpart would be a temporally dispersed signal across these channels.
Two radio astronomy antenna systems are presently in place near the central pad at Fenton Hill. One of them is a three-element rotatable beam antenna designed to resonate at 26 MHz, and to listen for the so-called "decametric" (ten-meter wavelength) radio emissions from Jupiter. These emissions are generated from the interaction of the magnetic field of Jupiter with its satellites, principally Io, and are quite predictable based on the relative orbital locations of Earth, Jupiter, and Io. Prediction tables are available from the University of Florida planetary radio astronomy website.
A second antenna system near the central pad consists of two crossed-Yagi beam antennas that were originally used for 2-meter wavelength (144 -148 MHz) amateur radio satellite communications. They operate quite well in the 151 MHz radio astronomy band. These two antennas are mounted on elevation-adjustable stands, and are separated from each other by approximately 15 wavelengths at 151 MHz, and so form a simple two-element interferometer which can generate fringes from strong radio sources such as Sagittarius A.