# What antenna is this in an Apollo 15 LM photo?

The video Apollo 15 landing from PDI to Touchdown (found in Why did the Apollo 15 LM violently shake & raise dust after the contact probes touched the lunar surface?) seems to pan across a still photo after landing, and at about 15:19 an antenna is shown. This answer mentions a "skew planar wheel antenna" or "cloverleaf antenna" that looks similar, though there also what looks like what might be a rod antenna in the LM photo as well.

What is this antenna (or these antennas) called, and what is it for?

Cropped screenshot from video. Note that the image is a composite of more than one photo and so the representation of the foreground may be distorted or incomplete the same way that Google street view shows strange things.

It's one of the VHF antennae.

Locations shown here.

1st image from here: http://www.ninfinger.org/karld/My%20Space%20Museum/pjlmpics.htm

2nd image from here: https://www.hq.nasa.gov/alsj/a11/LM5Structures.gif

This document https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090015392.pdf shows that VHF was used for comm between the CM and LEM, and between the LEM and EVA crewmembers.

VHF System –

• Voice and Data link between LM and CSM
• Voice and Data link between LM and EVA
• Ranging Data between LM and CSM
• Great! I'll read further this afternoon, it's hard to make out all of the details when displayed on bear skin. – uhoh Nov 19 '19 at 3:35
• Since the VHF antennas were on the "roof" of the LM, this picture can only have been taken during Dave Scott's Stand-up EVA. Found it among the pan taken from MET 106:53:45 onwards: AS15-85-11379 126k JPG . – GNiklasch Nov 19 '19 at 7:56
• @GNiklasch great find! A bit further along in the question video (15:53) you can see one of the steerable dish antennae, also on the "roof". I think the rndz radar but not sure. – Organic Marble Nov 19 '19 at 14:54
• Is that a quadrafilar antenna with... a ground plane? – William - Rem Nov 19 '19 at 22:51
• @William-Rem those are indeed labeled "ground plane". It's possible that without them the LM's irregular shape produced some nulls or low-gain directions, and the four radials helped to make the in-plane angular distribution more uniform, but that's just guessing. – uhoh Nov 19 '19 at 23:36

Is that a quadrafilar antenna with... a ground plane?"

and the answer appears to be yes; the four rods pointing radially below the quadrafilar "cloverleaf" are labeled as a ground plane in Figure 6.

As @OrganicMarble already points out, the VHF antennas were used for voice, data and ranging with the CSM (ranging up to 200 km) and voice to "extravehicular (EV) crewmen on the lunar surface". An electronic diplexer and an antenna switch allowed two VHF receivers and two transmitters to access either VHF Antenna A or B.

VHF A voice          1296.8 MHz
VHF A ranging        1296.8 MHz

VHF B voice backup   1259.7 MHz
VHF B data           1259.7 MHz
VHF B ranging        1259.7 MHz


I found NASA Technical Note NASA TN 0-6974 Apollo Experience Report - Lunar Module Communications System. Figure 6. Shows the various antennas on the LM and Figure 7. shows a rather complicated "simplified" block diagram

[

Two VHF inflight antennas were used for communications with the CSM. The antenna was designed to produce right-hand circular polarization. The radiation pattern provided approximately 47r -steradian coverage with a minimum gain of -6 decibels (with respect to the CSM linearly polarized VHF antenna) when the antenna assembly was terminated in a free-space environment. This requirement was accomplished by manual switching between the two antennas.

[...] Audio inputs from the commander or LM pilot were hardlined to the respective audio centers located in the SPA. If a VHF transmitter was selected, the audio center routed the voice input to the correct VHF modulator and provided keying of the transmitter. After amplification to a peak power of approximately 7.6 watts, the signal was routed through the VHF diplexer to the selected VHF antenna. Any received RF signals were routed through the VHF diplexer to the receiver section. The detected output of the receiver was routed to the PMP in the SPA, which further amplified and routed the signal to the audio centers. The transmit/receive selector switches and volume controls on the crewmen's control panels controlled headset audio signals. The VHF communication modes between the LM and the CSM are shown in figure 1.

Original:

Cropped, zoomed, sharpened and higher contrast:

National Archives Identifier: 16682061

Local Identifier: 255-AMP-AS10-34-5053

Creator(s): National Aeronautics and Space Administration. Lyndon B. Johnson Space Center. 2/17/1973- (Most Recent)

From: File Unit: Apollo 10 - AS10-27-3855 through AS10-35-5290, 11/9/1967 - 12/19/1972

Series: Photographs of the Apollo Space Program, 11/9/1967 - 12/19/1972

The original database describes this as:

Description: View of Very High Frequency (VHF) Antenna Array on Lunar Module (LM). Image was taken of translunar coast during the Apollo 10 mission. Film magazine was M,film type was SO-368 with 80mm lens. Film type was 70mm color.

Original: Film - 70MM CT

Interior_Exterior: Exterior

Ground_Orbit: On-orbit

Also, cropped from AS09-20-3069 Apollo 9, David R. Scott spacewalk:

and cropped from AS09-20-3064 Apollo 9, David R. Scott spacewalk:

• Great stuff! Can you show where you found that 'the VHF antennas were used for voice and data with the MSFN"? Everything I found said it was only for ship -to-ship or ship-to-EVA. – Organic Marble Nov 19 '19 at 23:37
• @OrganicMarble good catch, thanks! I misread item #5 on page 4 I'll fix that now. – uhoh Nov 19 '19 at 23:41
• The picture of the EVA guys shows it too. But I think it must go into the PMP (whatever that is) and then out to the ground on Sband, based on the "simplified" drawing. – Organic Marble Nov 19 '19 at 23:42