LRV Antenna

[Jeff Raven]

Apologies if this has already been covered elsewhere (I did a search and didn't see it, but of course might have missed it). On another site I was having a back and forth with someone who doubted the landings (they perhaps seem to have come around at least a bit) who asked a question I didn't have an answer for: 

" it always puzzled me why a directional dish antenna would be mounted on the rover bouncing around.   Why wouldn't it have been just a whip antenna and back to the lander to repeat back to earth with a direction antenna on the lander?"

My initial (and totally spit-balled) thought was that perhaps it had to do with power issues, or even bandwidth, but I really have no idea. If anyone can point me in the right direction, that would be appreciated. I'm also curious how they kept it pointed correctly. I saw on another site (https://moonhoaxdebunked.blogspot.com/2019/02/818-how-could-astronauts-still-send-tv.html)  that it was pretty tolerant of not being exactly spot-on, but was wondering about the details.

As always, I appreciate your allowing me to pick the brains of those who know a lot more about this stuff than I do.

[Peter B]

I'll leave it to the experts to confirm, but the LRV was certainly out of line-of-sight of the LM for much of each traverse. Would that affect the signal between the LRV and LM?

The LRV antenna was AFAIK only used for the TV, and because it needed to be pointed directly at Earth, the TV could only be used when the LRV was stationary. Hence there was only TV when the LRV was stationary - at each geology stop. TV while driving would have been amazing, but wasn't technically practical. Obviously, when the LRV was stationary the antenna would stay pointed in the one direction. I understand there was a simple viewing device attached to the antenna to allow the astronauts to re-aim the antenna at Earth each time they stopped.

As for the need to point the antenna at the Earth, there is a piece of video from Apollo 17 where the TV camera remained on while Cernan drove the rover a short distance in a straight line at a slow speed. The picture quality was pretty terrible.

As for more detailed answers, my guess is that the Apollo Lunar Surface Journal website would have a decent amount of information, and the video mentioned above was on the Apollo Archive website.

[onebigmonkey]

Looking at Earl Swift's 'Across the airless wilds' and the 'Haynes manual' for the LRV, it seems that the choice of the HGA was because TV needed a lot of bandwidth, more than the a simple whip antenna could have dealt with, and the dish allowed a more focussed signal beam to be pointed at Earth. Through the Lunar Communications Relay Unit it could also handle voice and biomedical data if line of sight to the LM wasn't possible with the low gain antenna, and receive instructions for TV control.
 

[smartcooky]

I thought the Dish antenna was also used to communicate with mission control to allow the vertical panning of the LRV camera to capture the LM ascent stage lifting off.

Dwight would probably have a better idea about that.