[chris (OP)]

This question dropped into our Naked Scientists inbox, and I thought it would be perfect fodder for us to dissect and discuss here:

Can you help me explain this to my friend? here's what he said:

I do believe we had the technology to get to the moon but I do not believe we had the technology to transmit an analog TV signal from the moon to the earth. I looked it up myself with no moon context. I looked up how much power is needed to transmit an analog TV signal. Turns out about 50,000 watts per 25-50 miles for VHF. Analog VHF TV signals dissipate quickly, they spread out, they don't travel like a beam. Even with 50,000 watts and a very tall aentena matched to the power (about 100 feet) the signal would be too weak to lock onto after 50 miles. I'm supposed to believe that six foot TV antenna on top of The Eagle could transmit at a high enough power for the signal to reach earth capable of being locked onto? The entire lander was operating on a 12 volt car battery!

What does everyone think?

[Bored chemist]

Analog VHF TV signals dissipate quickly, they spread out, they don't travel like a beam.

That's because the antennae are designed to do that-  they need to spread the signal to everybody.
But the ones they took to the moon were directional antennae which do form a beam.

I do not believe we had the technology to transmit an analog TV signal from the moon to the earth.

Well, the Russians received them too...
You seem to imagine they were receiving these transmissions with the sort of antenna you have on the roof of your house.
That's nonsense. They used large dish antennae- one of the more notable ones was Jodrell Bank.
https://en.wikipedia.org/wiki/Jodrell_Bank_Observatory
It's huge, so obviously, it intercepts much more of the signal than you would get from the few metal rods on the roof.

[chris (OP)]

Out of interest, what transmitter power did they use, and what bandwidth? What was the data rate for those pictures coming in from the lunar surface?

[evan_au]

One of the antennas used to receive the first TV signals from the Moon starred in the movie "The Dish".
- At 64 meters diameter, this dish can pick up a lot more signal than the 1m-wide antenna on your roof
- The reception was actually divided between two parabolic dishes at Parkes and Tidbinbilla, in Australia.
- These dishes are very directional, so they can eliminate a lot of competing noise which is picked up by a simpler dipole antenna as used for terrestrial TV.
See: https://en.wikipedia.org/wiki/The_Dish

The initial broadcast was a slow-scan TV black-and-white signal at 10 frames per second, which used about 500kHz bandwidth.
- The signal-to-noise ratio was improved by using FM modulation (terrestrial analog TV broadcasts used AM modulation)
- The transmitter was S-Band, at 2.3 GHz. This has around 1/10 of the wavelength of commercial terrestrial TV broadcasts, allowing  the signal to be concentrated in a narrow beam from the high-gain steerable antenna (0.65m diameter)
- The transmit power from the Moon was 20 Watts (commercial TV broacasts use an omnidirectional antenna to reach the maximum number of subscribers, which is why they need 50kW of transmit power to reach perhaps 30km)
- Terrestrial TV broadcasts are effectively "line-of-sight"; the range of TV reception is limited by the curvature of the Earth. This is not a problem for broadcasts from the near side of the Moon, since the Earth is permanently in line-of-sight, from 300,000km away.
- On the ground, the slow-scan images were converted to commercial TV standard (30 frames per second, 5MHz bandwidth) by pointing a TV camera at the slow-scan TV image.

Later missions used color cameras with a higher bandwidth, via a separate transmitter and an umbrella-style antenna 3m in diameter.

Some sources Google found for me that had relevant information:
TV Cameras: https://www.hq.nasa.gov/alsj/ApolloTV-Acrobat5.pdf
https://www.hq.nasa.gov/alsj/LM12_Communications_ppC1-10.pdf
The steerable S-Band antenna on the lunar module is shown here. Unfortunately it doesn't clearly indicate the dish diameter (which is important to calculate antenna gain).
https://airandspace.si.edu/collection-objects/antenna-steerable-s-band-apollo/nasm_A19770614000
https://www.hq.nasa.gov/alsj/LM12_Communications_ppC1-10.pdf
http://ed-thelen.org/pics4/apollo-s-band.html

[Bored chemist]

Unfortunately it doesn't clearly indicate the dish diameter (which is important to calculate antenna gain).

If the packing peanuts at the bottom of the crate are "normal" than it's about 2 feet across.
Does that help?

For what it's worth, there are amateur TV transmitters- the equivalent of "ham radio" who get  large ranges on low powers.
It's perfectly possible.

[Petrochemicals]

Id guess atmospheric interferance plus environmental interferance are not something you have to contend with across empty space, all you have to do then is penetrate a few kms of thick atmosphere. Mobile phones have difficulty when you are moving fast or the wind is strong. There is also the old gag about police radios being heard halfway through a record whilst listening to your portable radio stereo.

https://www.ofcom.org.uk/tv-radio-and-on-demand/advice-for-consumers/tv-or-radio-interference-or-reception-problems

[Janus]

Unfortunately it doesn't clearly indicate the dish diameter (which is important to calculate antenna gain).

If the packing peanuts at the bottom of the crate are "normal" than it's about 2 feet across.
Does that help?

For what it's worth, there are amateur TV transmitters- the equivalent of "ham radio" who get  large ranges on low powers.
It's perfectly possible.

Those ham radios took advantage of "skip" bouncing the signal between surface and ionosphere, which allows the signal to "bend" over the horizon. 
I remember late nights, after the Sun had long set,  slowing tuning an old AM radio through the dial.  On a good night I could pick up a station from all the way on the other side of the country as its signal bounced off the Heaviside layer. (I lived way out in the country, so there weren't a lot of local stations cluttering up the bandwidth.)

[evan_au]

"skip" bouncing the signal between surface and ionosphere

Ionospheric skip propagation works with "shortwave" radio bands, up to around 30 MHz.

The old analog terrestrial TV used frequencies above 50MHz, which pass through the ionosphere out into space. They don't propagate past the curvature of the Earth,

[Janus]

"skip" bouncing the signal between surface and ionosphere

Ionospheric skip propagation works with "shortwave" radio bands, up to around 30 MHz.

The old analog terrestrial TV used frequencies above 50MHz, which pass through the ionosphere out into space. They don't propagate past the curvature of the Earth,

Which is why you had to use a AM radio (550-1720 khz) rather than FM( 88 - 108 mhz)* on those late nights.

*Analog television used 54-88 mhz (ch2-ch6) and 174-216 mhz(ch7-ch13)
The bottom of the FM radio dial just overlapped into the Ch6 TV range, and as television audio was in FM ( Video in AM),  you could pick up the Channel  TV broadcast audio with an FM radio.