The AM Forum

Another reflection from Walt:


This post deals with the dish antennas that accompanied the three Lunar Rovers (Moon Buggies) to the Moon during the Apollo Program.

The design and fabrication of those antennas was contracted out to RCA's Space Center, the RCA Astro-Electronic Div. Princeton, NJ. Three RCA engineers were assigned the task, of which I was one of the three. The principal task was the design of the dish feed. Along with other tasks during the design, mostly empirical, my main task was making all the measurements of the radiation patterns and gain during the design phase, as well as the final measurements to show proof of performance prior to shipping the completed antenna to NASA. The story below is a quote from Reflections 3, Sec. 28-4:

Sec. 28.5  The Lunar Rover (the Moon Buggy)

     The TV dish antenna in Photo 28-5.2 was used by the Apollo astronauts during their Moon ride in one of the Moon Buggies. It transmitted TV signals directly from the Moon Buggy to Earth and into the TV networks from a camera and a 5-watt S-band transmitter mounted on the buggy. When the buggy was stopped for a TV broadcast, the astronauts aimed the dish toward Earth, using an optical telescope attached to the dish mounting. To facilitate aiming with the high accuracy required, the optics in the telescope were designed so that Earth just filled the viewing area. The 28-inch diameter dish, constructed of gold-plated fine-mesh wire, has a gain of 26 dBic at 3600 MHz and folds up like an umbrella.

The dish is shown in Photo 28-5.1 at RCA on a test fixture designed by the Author, mounted atop the vertical stanchion of an antenna-positioner turntable (not shown). Here the dish received its RF measurements of radiation pattern and gain during both the development and the final proof-of-performance. It is situated in an anechoic chamber, or RF darkroom. The room eliminates reflections that would affect the accuracy of the measurements. For the pattern and gain measurements, the dish is illuminated with RF from a transmitting antenna positioned 900 feet downrange. The pattern measurement is made by rotating the dish in azimuth (through the rotation of the turntable supporting the stanchion), recording the signal level received by the dish during the rotation. A standard-gain reference horn is visible in the lower right in Photo 28-5.1. The horn is mounted on the same shaft as the dish, so as the dish rotates in azimuth, so does the horn. As the dish-horn combination rotates through the 180° position in azimuth, the open end of the horn becomes illuminated by the downrange transmitting antenna. As this occurs a coaxial switch just to the left of the horn is turned to switch the receiver from the dish to the horn recording the pattern of the horn as it rotates through the RF illumination. The gain of the dish antenna is then determined by comparing the recorded level of the signal received by the dish with that received by the reference horn. The uniformity of the radiation pattern around the axis of the dish is also measured. These measurements are performed by setting the dish at selected angles in azimuth relative to the down-range illumination while rotating the dish on its own axis, again recording the signal level. Measurements on all the antennas that went to the Moon on Lunar Rovers were performed by the Author. In the event that you saw TV pictures sent from the Lunar Rover by the Apollo astronauts, the antenna in Photo 28-5.1 is one of the three that transmitted the TV signals from the Moon. Fig 28-5.2 shows three astronauts aboard one of the three Moon buggies. Fig 28-5.3 shows one of the three Moon buggies parked in one of NASA’s used car parking lots on the Moon.

Hi Walt,

Another wonderful post! It must be nice to know that a piece of your work will be on the moon for many eons into the future.

A quick question for you, if I could. You described this hi-gain antenna as being used for the direct transmission of S-band television from the Lunar Rover to the Earth.

When the astronauts were engaged in an EVA (or moonwalk) on the lunar surface, how were their audio communications and portable life support system (PLSS) and biological telemetry relayed to Earth? I would have thought this circuit would have been via the low-power transceiver package contained within the PLSS, relayed to the communications equipment on the Lunar Rover, and then multiplexed with the S-band video feed and fed to the hi-gain antenna.

I doubt if the very low power transmitter in the PLSS and it's omni-directional antenna would have enabled communications directly with the Earth. Was the two-way communications circuit from the PLSS repeated instead thru the LM, for direct duplex and half-duplex communications with the earth link? If this was the case, I would assume the PLSS communications package operated at either VHF or UHF, as the astronauts would not always have a line-of-sight path with the LM, and the highly mobile nature of the astronauts would not permit the use of a directional antenna.

Thanks, and I look forward to your reply.

73,

Bruce

Sorry Ralph, I have no knowledge of the communication system used by the astronauts. My only knowledge concerned the transmission of the TV signal via the 5-watt S-Band tx driving the dish antenna.

Walt

Hey Bruce, forgive me for calling you Ralph--just wasn't thinkin' straight.

I goofed in not posting the pics mentioned in my post above. However, that post has no 'modify' feature allowing the addtion of the pics. Consequently, I'm adding them here: