The Very Small Array
Pending publiciation in QST, September 2002
Used by permission of the American Radio Relay League.
ABSTRACT
The nonprofit SETI League began conceptual design work in 1999 on Array2k, a planned phased array of satellite TV dishes, to be used as an amateur radio telescope of unique flexibility. Although the funding required to implement this design still eludes us, The SETI League has amassed, through a multitude of grants and small contributions, the resources necessary to permit us to begin construction of a small-scale prototype. Thus, an eight-dish Very Small Array (VSA) is now taking shape in the backyard of the author's rural Pennsylvania home. This paper shows how donated dishes, student labor, and ham ingenuity are being combined to test a high-tech concept on a shoestring budget.
The VSA project is funded in part by generous grants from the American Astronomical Society and the ARRL Foundation.
INTRODUCTION
The SETI League, Inc. launched its Project Argus sky survey in April 1996, with the ambitious goal of real-time all-sky coverage (Shuch, 1997). Our experience in implementing a global network of small radio telescopes (Shuch, 2000) has underscored the importance of developing larger scale telescopes with improved sensitivity. Due to negative economies of scale, we early decided to explore the arraying of a quantity of the very type of antennas used in the current Project Argus network -- that is, extrapolating from our area of greatest expertise.
Array2k (Shuch, 2001, October) is an array of small satellite dish antennas all interconnected to accomplish specific beam patterns. As presently envisioned, the array comprises 32 individual parabolic dish antennas on az-el mounts. Four sub-arrays, each comprised of eight individual antennas, are established in a cross-like formation, with one sub-array each running north, south, west and east of the array's phase center. See Figure 1 for an artist's conception of a sixteen-dish implementation of Array2k, and Figure 2 for its anticipated beam patterns.
FISCAL REALITY
The SETI League, Inc. set a preliminary hardware budget of $160,000 for Array2k. Overhead and infrastructure needs can be expected to raise the total cost to around $250,000. Though roughly one percent of the cost of professional radio telescopes, this is still a substantial sum, beyond the reach of amateur radio clubs like The SETI League. Nevertheless, fundraising efforts were initiated in May, 2001, resulting in a handful of modest (but hopelessly inadequate) contributions. The generosity of our 1300 members in 62 countries around the world notwithstanding, significant grant monies or major corporate sponsorship will be required to bring Array2k to fruition.
A small-scale prototype to test the technologies proposed for Array2k, on the other hand, is within both amateur capabilities and existing budget, and may help us to attract major donors. Thus, I began in the Spring of 2002 to construct the Very Small Array (VSA) in the backyard of my rural Pennsylvania home.
To facilitate completion of this prototype, the American Astronomical Society very generously provided a NASA Small Research Grant in the amount of $2000, and the ARRL Foundation kindly kicked in an additional $3000. Thanks to matching funds contributed by more than fifty radio amateurs, $10,000 has now been allocated to implementing the eight-dish VSA. A quantity of donated 1.8 meter dishes and azimuth-elevation mounts (see Figure 3) should allow us to complete the VSA prototype within budget, hopefully by early next Spring, about one year after the project was begun.
MOUNTING THE MASTS
The VSA requires that its antennas be laid out in true North-South and true East-West baselines. Establishing these baselines requires precision surveying. Fortunately, I spent a few years as a Professor of Electronics at the Pennsylvania College of Technology, Penn State University. That institution boasts an exemplary Civil Engineering program, whose students take courses in surveying. Through the good graces of my former colleague Prof. William Sprinsky, CE student Timothy Wentz was recruited to sight the Sun with a theodolite. Using solar ephemeris tables and an accurate clock, he was thus able to establish precise baselines (Figure 4), and to stake out the locations of the individual antennas (Figure 5). A 24-inch auger on a Bobcat track vehicle (Figure 6) was rented, and eight holes of 42-inch depth were dug, centered on the individual antenna locations. Contrary to appearances, the eight holes in the ground depicted in Figure 7 were not the work of a giant race of alien gophers!
Concrete block was laid in the bottom of each of the antenna holes, to support the antenna masts. Each mast was cut from 4 inch OD, 3 ½ inch ID Schedule 40 galvanized steel pipe. A hole was drilled near the bottom of each mast section, and a length of steel reinforcing bar placed through it, to prevent twisting of the mast under wind loading (see Figure 8).
The terrain at the VSA site is quite uneven (Figure 9). Leveling the masts was accomplished with two bubble-levels with magnetic backings, stuck to each mast 90 degrees apart (Figure 10). Temporary guys (Figure 11) held the masts in place, while 0.4 cubic yards of concrete were poured into each of the eight holes. Curing time for the concrete was two days, after which time the guy wires could be removed. Because of the VSA site's rolling terrain, and the fact that proper phasing of the array elements requires that the antennas all be at approximately the same absolute altitude, a string level was used to mark trim lines on all eight masts, which were then adjusted to proper height with a cutoff wheel (see Figure 12). The result is eight masts that appear to the eye not to be level (Figure 13), but in fact are.
AESTHETIC MITIGATION AND PRB-1
As is good practice in residentially zoned neighborhoods, the author received approval from his Township Board of Supervisors prior to commencing construction of the VSA. However, with eight masts planted, the neighbors immediately began protesting the project to that same Township Board.
To any radio amateur planning an antenna project which might impact on his or her neighbors, I can highly recommend K1VR's fine book on Antenna Zoning, available through the ARRL (Hopengarten, 2001). Had I bothered to consult it before I started working on the VSA, I could have spared myself considerable grief. Legalities notwithstanding, one must take the concerns of one's neighbors very seriously. We radio amateurs in the United States enjoy a degree of legal protection which our counterparts in other countries well may envy. As the holder of a US Amateur Radio license, constructing an antenna to be used under the rules of the Amateur Radio Service in the allocated ham bands, my antennas fall under the protection of PRB-1, the FCC's federal pre-emption of local zoning regulation over ham radio antennas. Since the VSA is designed to operate within the 23 cm amateur radio band, for reception tests in connection with our W2ETI moonbounce beacon (clearly a ham radio educational and scientific activity), I invoked PRB-1 to my local Township Supervisors. In brief, PRB-1 recognizes the value to the community of the Amateur Radio Service, acknowledges the importance of antennas to achieve effective ham radio communications, and prohibits local governments from unrealistically restricting ham antennas. And, to my surprise and delight, the local township Solicitor informed my Supervisors at a local Township meeting that PRB-1 did indeed apply, protecting the VSA from zoning restrictions and local regulation.
If you think that ruling allayed my neighbors' concerns, you overestimate the power of reason. Federal regulations aside, they argued to our Township Supervisors, they moved onto our scenic hilltop to enjoy the wonders of nature, not the terrors of technology. Since membership on the Township Board is an elected position, whose voice do you suppose carries best, that of one lone ham, or a dozen of his voter/neighbors?
Without belaboring the ensuing legalities, suffice it to say that compromise carried the day. Since an amateur radio telescope points generally 'up', and since moonbounce activities can be conducted when the Moon is relatively high in the sky, it was practical to mount the dishes of the VSA relatively close to the ground, pointing up. This permitted me to plant a ring of trees (see Figure 14) around the dishes, shielding them from the view of my neighbors. The sad irony is that the cost of the shrubbery ended up exceeding the cost of the sheet metal. But I have to admit that the forty arbor vitae recently planted in my backyard are attractive -- almost as pretty to my eye as the dishes they mask!
The bottom line is that the Township solicitor sent me a letter, stating "that in the Township's view your backyard antennae and related facilities are within the protection of certain FCC Rulings, and do comply with all applicable Township regulations such as screening and setback requirements. Your continuing effort to deal openly and above-board with the Township about your property and your projects has been very much appreciated."
In summary, we each can choose between confrontation and conciliation. Our legal rights notwithstanding, as good neighbors it behooves us, and benefits our hobby, to choose the latter.
MOVING AHEAD
Trees planted, the first dish appeared within a day (see Figure 15). It flies the same Flag of Earth that graces SETI facilities worldwide. The Flag symbolizes the fact that SETI is carried out on behalf of humankind as a whole. The individual people, organizations, and nations involved are immaterial, since any signal received will belong to all of humanity, and represent Earth's entry into the Galactic community. I can think of no symbol more appropriate for an Amateur Radio educational and scientific project.
Next, each of the VSA masts was topped with an azimuth-elevation mount (Figure 16), and the mounts began to receive their dishes (Figure 17). Offset feedhorn-support tripods (Figure 18) were then affixed to the reflectors. As these dishes are designed for non-blocked apertures, the feed points appear significantly off-center.
Feeding an offset dish is a bit trickier than it would be to feed a standard prime-focus parabola, such as the one seen in the background of Figure 19. With offset dishes, the reflector is only a sector of a paraboloid, rather than a full parabola. Thus, the effective focal length to diameter ratio (F/D) is greater than it would be for the equivalent prime-focus dish, and the required beamwidth of the feedhorn is correspondingly narrower (in the case of the VSA antennas, 41 degrees). Design considerations for the required high-gain feedhorns have been well documented by W1GHZ (Wade, 1999). But I chose a different approach.
At the left in Figure 19 is the commercial feed supplied with the Prodelin dishes donated for the VSA. It was built for the 3.7 to 4.2 GHz C-band satellite TV allocation. Since we intend to use the VSA at L-band, on a frequency of 1296 MHz, one can scale the dimensions of the standard C-band feed by a factor of three. The result is shown on the right of Figure 19. As is the amateur tradition, and should be evident in the photos, this feedhorn is constructed from low-cost materials!
At press time, 1296 MHz feedhorns had just been installed on the eight dishes of the VSA. As Figure 20 shows, the VSA has begun to look like a real antenna array. Over the coming months, those eight dishes will begin to acquire the necessary electronics to recover echoes off the Moon from the W2ETI 1296 MHz EME beacon (Shuch, 2001, November). The continuous availability of EME calibration signals will enable us to optimize the array's receive circuitry. We plan to try out new ways to phase and combine signals from multiple dishes, providing limited electronic beam-steering. Then, it will be on to radio astronomy, and SETI research, and (just maybe) a shot at the ultimate DX.
CONCLUSION
Once the Very Small Array becomes operational, I hope its success will enable the grassroots SETI League to attract major corporate funding for its much more complex Array2k, a massive radio telescope array first contemplated in 1999. The bargain-basement VSA will be used to test engineering concepts for the planned $250,000 Array2k, which is itself a hundred times cheaper than conventional radio telescope designs. Thus, we hope to help bring radio astronomy and SETI research to the masses. Already, several SETI League members in the Third World have used amateur techniques to build the first radio telescopes in their respective countries. But whether the VSA paves the way for more ambitious projects or not, it shows the world how donated dishes, student labor, and ham ingenuity can combine to test a high-tech concept on a shoestring budget.
Hopengarten, Fred, Antenna Zoning for the Radio Amateur, ARRL, Newington CT, 2001.Shuch, H. Paul, Project Argus and the challenge of real-time all-sky SETI, in Astronomical and biochemical origins and the search for life in the universe, IAA Colloquium 161, 693 - 700, 1997.
Shuch, H. Paul, Project Argus: one hundred up, 4900 to go! IAA-00-IAA.9.1.04, Oct. 2000.
Shuch, H. Paul, Array2k: multiple dishes, multiple modes. IAA-01-IAA.9.1.02, Oct. 2001.
Shuch, H. Paul, 2001: A moonbounce odyssey. QST, 85(11): 38-43, November 2001.
Wade, Paul, 1999: W1GHZ Microwave Antenna Book Online, http://www.qsl.net/n1bwt/preface.htm.
