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I'm assuming many of you are familiar with Larry Baysinger. A radio amateur who independently detected and recorded the transmissions from the Apollo 11 EVA using a homemade antenna.

Larry Baysinger recorded the VHF (Very High Frequency) signals coming from the antenna mounted on their PLSS (the "backpack"). Larry Baysinger did not record the Unified S-Band signals (which consisted of PM and wide band FM) coming from the LM's antenna or from the erectable S-Band antenna. This is because the wide band FM signals carried audio and video, and required a very large antenna to detect which is not possible to do in a backyard with everyday home items. As for PM, that was used only during flight. This excerpt can be found here.

The S-band downlink had two modes: PM and wide band FM. In the PM mode, used during flight, voice was on a narrow band FM sub carrier that phase modulated the main carrier along with other information. This could be received by relatively small dishes. The wide band FM mode was necessary for TV. It is a "one for all and all for one" mode that needed a very large antenna to receive anything at all; in FM it was not possible to use a small antenna to pick up just the audio.

For this reason, Larry Baysinger chose to detect VHF signals with a frequency of 259.7 MHz coming from their PLSS.

Now my question is: what was the transmitting power of those VHF signals? Different websites have different results. For example, this website claims that the radio transmitting power was 12 Watts.

The NASA astronauts didn’t know it, but WHAS radio technician Larry Baysinger and his journalist friend Glenn Rutherford had tapped the frequency of the 12-watt radios — walkie talkie-type communicators — used by Edwin “Buzz” Aldrin and Armstrong on the moon’s surface.

On the other hand, this Quora answer (the second one with the brown-ish photo) states that the radio on their PLSS transmitted 259.7 MHz with 0.5 Watts.

This was even more remarkable when you think that the astronauts were only using about 500 milliwatts RF power.

Now I know that a Quora answer may not be the best place to receive information as it is not credible, but I want to see an authentic source (preferably a NASA document or something) that displays the parameters of the PLSS, including the transmit power of their VHF radio signals.

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  • $\begingroup$ just fyi Baysinger's work is discussed and some photos and links are included in this answer $\endgroup$ – uhoh Dec 31 '19 at 22:45
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This is a bit of a breadcrumb trail, but page 2-99 of NASA MSC-01372-1 "Apollo Operations Handbook Extra Vehicular Mobility Unit" (March 1971) discusses the power used for Apollo 15-17:

The dual mode is the normal operating position of the switch. In this mode, the EVC-2 transmits a 0.3- to 2.3-kHz voice signal and two interrange instrument group (IRIG) subcarriers (3.9 and 7.35 kHz) via a 279-MHz FM transmitter. The trans­mitter has an unmodulated output in excess of 500 mW. The composite signal from the EVC-2 is received at EVC-1 , mixed with an additional 0. 3- to 2.3-kHz voice signal and two ad­ditional IRIG subcarriers (5.4 and 10.5 kHz), and transmitted to the LM on a 259.7-MHz AM link.

(Emphasis added; EVC-1 and EVC-2 are the individual astronaut's units as part of the overall "extravehicular communications system" EVCS) The "in excess of" allows some uncertainty, but there are references to having to select components to get the overall system working, so it's probably not a lot over.

The LM transmitter is described in NASA LRTM-SY-1, Rev. D "Crew Training Manual Lunar Communications Relay Unit" 30 March 1991. Page 4 says (discussing transmission to the astronauts on the surface):

The VHF transmitter is amplitude modulated 70% and VOX operated by the ground con­troller's voice. The transmitter delivers 320 milliwatts CW at a frequency of 296.8 MHz.

(Emphasis added) That's comparable to the astronaut transmitters, which seems reasonable.

But was that astronaut radio what was used for Apollo 11? Here, the record is less clear.

NASA TN D-6723 "Apollo Experience Report - S-Band System Signal Design and Analysis" March 1972 discusses (pages 63-65) various early problems and how they were resolved:

With the solution of these and other, smaller problems, the EVCS was made to operate successfully for Apollo 11 and subsequent missions.

(The predecessor system was described on page 26 of TN D-6974 "Apollo Experience Report - Lunar Module Communications System" (September 1972) as having 75mW RF output; it was replaced by the EVCS after use on Apollo 11)

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