Argus
The Next Generation Radio Telescope
STATUS REPORT ON ANTENNA DEVELOPMENT

The ElectroScience Laboratory (ESL) at The Ohio State University (OSU), with the support of a grant from the SETI Institute, has been conducting research on the design, construction and testing of antenna arrays for the Argus radio telescope system.

The goal of this research is to construct an array with 64 elements to operate over the wide frequency range of 400 - 2000 MHz (0.4 - 2.0 GHz). The array should have: (1) a highly uniform pattern over a hemisphere with low sensitivity both on the horizon and in the backward direction; (2) low-loss components; (3) good matching to the receiver; and (4) fabrication that is both inexpensive and easily done.

In a previous effort ESL used an antenna array comprised of closely-spaced bent dipoles arrayed over a conducting flat surface (ground plane). Since the elements were very close together, each dipole was significantly influenced by nearby elements (i.e., mutual coupling). Measurements on an array designed to operate around 1500 MHz showed some promise, but the behavior varied too rapidly with frequency and the array did not operate well over a wide frequency band. Hence, an investigation of alternative designs was begun.

A planar two-arm Archimedean spiral over a conducting ground plane was selected for study (see Figure 1 below). It is planar, meaning flat. It has two interwoven spirals, creating a balanced structure. Each spiral is an Archimedean spiral, meaning that the spiral expands outward linearly (at a uniform rate) rather than exponentially (as in a logarithmic spiral). A conducting ground plane is used to eliminate signals from being received in the backward direction. The way the spirals are wound means that this antenna receives radio waves having right circular polarization (winding the other way would allow reception of left circularly polarized waves).

Although the goal was to achieve a frequency range (bandwidth) of 10:1, the ground plane had the effect of reducing the bandwidth to only 5:1. However, by simply increasing the diameter of the spiral to twice its previous size, the 10:1 ratio can be achieved.

The prototype antenna shown in Figure 1 was designed for the frequency range of 400 - 2000 MHz. It was constructed with 3/8" (outside diameter) copper refrigeration tubing and has a diameter of roughly 60 cm (24"). The tubing is held to a dielectric substrate with inexpensive tie-wrap mounts. The antenna is placed about 5 cm (2") above a ground plane and is fed through a wideband tapered coaxial balun (i.e., balanced-to-unbalanced transformer). The total cost of the antenna components is about $30. GPS satellites were used to check the antenna pattern at 1575 MHz, and it was found to be close to uniform over much of the hemisphere. There was also good matching to the receiver over most of the 5:1 bandwidth, although with some need for improvement from 1700 - 2000 MHz. An alternative design for connecting a coaxial cable to the antenna, based on surface-mounted wideband transformers, has been developed and is now being fabricated. The cost of the components, circuit card, and connectors is estimated to be less than $10/antenna..

Although these antenna elements have been developed with a flexible array geometry in mind, there are plans to develop some useful array configurations and to verify the performance in such configurations. ESL is currently exploring dense array concepts, in which smaller replicas of the spiral are packed in voids between larger spirals.

Figure 1. The prototype planar spiral antenna. The antenna is operated over a ground plane with 5 cm spacing. The feed is attached from the back side via a wideband balun. (Click on photo to view larger image.)

Figure 2. VSWR measured for the planar spiral over a ground plane. (Click on photo to view larger image.)

Figure 3. Reception of GPS satellites by the planar spiral over a ground plane. The measurements were performed over a duration of roughly 3.5 hours. The blue circle indicates the horizon. (Click on photo to view larger image.)

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