The 6 dB gain and 80 degree illumination angle were chosen so that the horn would adequately (but not overly) illuminate the 150-foot parabolic primary reflector. Known affectionately on campus as The Dish, this antenna is a Stanford icon and has a long and storied history. Tamara tested the feed horn in an antenna measurement lab at SRI International in Menlo Park, California and measured the VSWR (1.26), impedance (42.7+1.6j Ohms), optimum frequency (437.4 MHz) and antenna pattern. The pattern was determined using a 400 MHz helix antenna as a reference transmitter.
After the testing was completed at SRI, the horn was installed at the primary focus of the Stanford reflector. Due to unforeseen physical constraints, the horn had to be installed parallel to the center of the primary and outfitted with a 90 degree reflector to properly guide the incoming waves. The Dish was then programmed to scan the Sun in both azimuth and elevation, and profiles of the antenna pattern (smoothed by the solar disk) were obtained in those directions. At 437 MHz, the solar signal produced a 20 dB increase in power over the background level.
Tamara, Ivan and the Horn
The photograph above was taken as the feed horn was being installed at the prime focus of the Stanford Dish. To ease the installation and maintenance of such feed antennas, the tripod of the Dish may be lowered to the ground by winch and cable. In the photo, the tripod is resting just above the azimuth rail which permits the Dish to be steered over a full 360 degree circle. The 400 MHz horn operating frequency was chosen to allow a UHF radio-frequency interference (RFI) survey to be conducted this summer. Later in the fall, the Stanford Dish will be used in a test of the Mars Relay beacon aboard the Mars Global Surveyor spacecraft. The transmit frequency of the Mars Relay is 437.1 MHz and the receive frequencies are 401.5 and 405.6 MHz. During the Mars Relay test, the Dish will transmit to the Relay and then receive the return signal from the Relay. The RFI survey will be conducted in support of this test.
For more information, Tamara may be reached by e-mail at firstname.lastname@example.org.