The Antennas of the DSN

The largest antennas in the figure above are quasi-parabolic reflector antennas, one with a diameter of 70 m. Smaller antennas used for deep-space mission support are 34 m in diameter, while 26- or 9-m antennas provide support to Earth-orbiting missions.

Each of the antennas has what is termed a Cassegrain configuration with a secondary reflector mounted on the center axis just below the focal point of the dish. The secondary reflector serves to relocate the focal point to near the surface of the main dish and establishes a more convenient location for the low-noise amplifiers, receivers, and powerful transmitters.

The earliest antennas in the DSN were of commercial design and were parabolic in shape. Then, as now, the exact efficiency of the antenna represented a compromise between gathering as much as possible of the radio signal from the distant source, and picking up little spill-over noise from the surrounding Earth.

By the 1970s, researchers of the DSN Advanced Systems Program recognized that the overall antenna performance could be substantially improved by what was termed a "dual-shape" design in which the shape of the secondary reflector could be modified to more uniformly illuminate the main reflector, while the main reflector was slightly reshaped into a quasi-parabolic form. It was not until the 34-m high-efficiency (HEF) antennas were being built in the early 1980s that the new design could be put into practice.

These antennas were optimized for performance at X-band (8.4 GHz) and were needed by the DSN for support of the Voyager spacecraft in their tour of the outer planets.

As the Voyager 2 spacecraft headed outward toward Neptune, it was recognized that increased signal collecting area was needed on Earth to effectively support this unique science opportunity. The DSN's largest antennas at the time were 64-m parabolas of the original design. Calculations showed that the best investment of scarce construction funds would be to modify these antennas using the dual-shape design and expanding their diameter to 70 m. It was also apparent that the upgraded large antennas would benefit the planned Galileo and Magellan missions as well.

The 70-m enhancement project was completed in time for support of Voyager 2 at Neptune, and represented a more than 60% increase in the effective collecting area of these antennas. Fully half of this is attributable to the dual-shape design, a product of the DSN Advanced Systems Program.

At the time of this writing, construction projects are underway that will build new 34-m antennas for the operational DSN, and will eventually eliminate the oldest ones. These new antennas incorporate the dual-shape design as well as a beam waveguide (BWG), which uses a series of additional secondary reflectors to relocate the focal point into a stationary room below the dish.

The BWG design feature had been used for years for communications satellite terminals where ease of service outweighed any added noise. Concern for noise from the additional mirrors kept such antennas out of contention for the DSN usage. In 1985, a team of JPL researchers from the DSN Advanced Systems Program worked in collaboration with Japan's Institute for Space and Astronautic Sciences (ISAS) to install one of JPL's experimental low-noise amplifiers into the ISAS 64-m BWG antenna in Usuda, Japan. The configuration was demonstrated with S-band (2 GHz) signals from the International Cometary Explorer (ICE) enroute to the comet Giacobini-Zinner, achieving surprisingly good performance with no detectable noise penalty.

Moving from this demonstration to application of the BWG technology in the DSN took the building of a prototype antenna, which was outfitted and evaluated by the DSN Advanced Systems Program. This antenna was in fact a replacement for the aging 26-m antenna that had been for many years the field laboratory for technology development. The antenna design was optimized using microwave optics analysis software, itself an evolving product of the Advanced Systems Program. The completed antenna has been demonstrated to operate effectively at S-, X-, and Ka-bands (2, 8, and 32 GHz, respectively).

Rotation of the single mirror at the center of the equipment room below the antenna can select among the various frequencies and modes of operation. Lessons learned by Advanced Systems Program personnel in the construction and evaluation of this antenna were incorporated into the design of the operational BWG antennas now in use and under construction, making their performance exceed that of the prototype, especially at the lower frequencies.