ApolloHoax.net
Apollo Discussions => The Reality of Apollo => Topic started by: Trebor on February 07, 2014, 03:07:16 PM
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I have unfortunately entered into one of 'those' debates where some individual is claiming the TV Signal from the rover could not have been picked up by the technology of the time.
Me I think a large radio telescope, like many that existed at the time, should have few problems.
But what was the power output of the transmitter? And is there a good reference on the gain of the receivers?
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I did the link budgets myself a while ago and I will go check them, but off the top of my head I believe the power output of the rover in FM (TV) mode was about 10 watts. The LM was about 20 watts, but the feedline losses were much larger, especially when using that large deployable dish.
The frequency (for path loss calculations) is S-band, just below 2290 MHz.
The LRV had two antennas, a medium gain antenna that could support only voice and telemetry, and a high gain antenna for TV. The medium gain antenna only needed approximate pointing so it could be used while driving. The high gain antenna had to be manually pointed at earth during each stop.
Like the LM, the rover transmitter modulation was switchable. PM (phase modulation) was used for voice/telemetry (no video) and FM (frequency modulation) was used for video/voice/telemetry. The PM signal, particularly its strong carrier component, was much easier to detect with a small antenna on earth.
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Here are the numbers from one of my old spreadsheets for the TV mode on the LRV.
Transmitter output power, 10 W
Transmit losses: 1.5 dB
Power at antenna: +8.5 dBW
Transmit antenna diameter and gain: 76 cm, 23.5 dBi
Transmit effective isotropic power: +32 dBW
Path loss (384,000 km, 2265 MHz): 211.24 dB
Receive flux: -179.24 dBW
Receive antenna diameter and gain: 25.9 m, 53 dBi
Receive losses: 1 dB
Receive power: -127.25 dBW (-97.25 dBm)
System temperature: 210 K
Signal-to-noise ratio, 1 Hz bandwidth: +78.13 dB-Hz
FM bandwidth (not sure, will have to check): 5 MHz (67 dB-Hz)
IF SNR: ~ 11 dB
which is above capture; a typical FM demodulator "quiets" at around 8 or 9 dB.
Note that the receiver has two critical performance figures: noise as well as gain. And when you point a high gain dish at the moon, you also pick up noise from the moon itself that depends on its surface temperature. The 210K figure above includes the moon noise for that antenna.
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I have unfortunately entered into one of 'those' debates where some individual is claiming the TV Signal from the rover could not have been picked up by the technology of the time...
This web page...
http://legacy.jefferson.kctcs.edu/observatory/apollo11/
might contain a surprise for your opponent as it tells how a Kentuckian, Larry Baysinger, used a home-made set-up to record the output of the transmitter on Neil Armstrong's back during the Apollo 11 EVA.
The story is fascinating not only because of what Baysinger accomplished, but also because he was interested in UFOs back in 1969 ('the "brain" behind the activities of the Louisville UFO Investigations Committee'), and was keen to catch Nasa out should they have broadcast an edited version of the astronauts' conversations.
(With thanks to ApolloGnomon for providing the link in this post http://www.apollohoax.net/forum/index.php?topic=515.msg17420#msg17420)
Edit: Deleted "-- IIRC --" because I did RC.
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Yes, Baysinger's feat really should get more publicity. His setup was very marginal, and he lucked out by having the EVA begin as the moon was setting. The slowly changing direct and ground reflections caused alternating enhancement and concellation of the signal, and at the enhancement peaks he heard it better than had the moon been high overhead. This is a trick often used by ham moonbouncers.
He was also wise to go for the VHF signal from Armstrong's backpack even though it was only intended to reach the short distance to the LM. Had he gone for the S-band signal that was actually intended to reach earth, he would have failed because it was wideband FM at the time to carry video, and it was far below threshold on anything smaller than a very large dish.
The other hams who heard Apollo direct from the moon did listen to the S-band signal, but only in the PM mode.
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This web page...
http://legacy.jefferson.kctcs.edu/observatory/apollo11/
might contain a surprise for your opponent as it tells how a Kentuckian, Larry Baysinger, used a home-made set-up to record the output of the transmitter on Neil Armstrong's back during the Apollo 11 EVA.
That's a very cool story! Next time someone spouts that nonsense about the censored Apollo 11 transcript that has Neil and Buzz talking about flying saucers lined up on the edge of a crater and watching them, I'll be able to point them to an independent source that completely refutes it.
Not that I think they'll accept it, but hey!
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Trebor, where is this debate taking place? May I join in if its on an open forum?