To the Moon and Back with the Apollo Transponder

Progress continues with our power up of the Apollo transponder. We achieved frequency locked turnaround transmission: from Earth to the Moon, then back. With Earth being simulated by the two antennas at the far end of the table attached to my growing pile of HP equipment, and Moon being the two antennas in the near corner attached to our Apollo S-band transponder. This setup is getting monstrously complicated, and we have hardly scratched the surface of the full S-Band system setup.

You can get a sense of how that works from the picture of a live transmission below, taken from the "Earth" end of the table.

The Earth tracking station transmits at 2.1064 GHz (first pile, synthesizer in the middle), sending a measly -93 dBm, simulating the faint signal arriving at the moon, using the blue conical antenna. The signal is received by a similar antenna at the at the transponder. The transponder locks on it (see previous post), translates the frequency up in the ratio 240/221, and re-transmits it at 2.2875 GHz using the white vertical antenna. 

Both frequency meters and my RF analyzer in the middle have a little black antenna at their inputs, and pick up the much stronger downlink signal which appears on the spectrum analyzer screen. You can read that it is at 2.2875GHz as indicated on the frequency meter. 

We receive it on earth with another white antenna, mix it with 2.3075 GHz from our local oscillator (HP instrument at the bottom of the pile), which creates an IF signal at 20 MHz at the output of the mixer. I then receive the 20 MHz IF with  my  ICOM IC7300 ham  HF radio, set to FM. The mixer is behind the frequency meter and my ICOM radio is not visible, on top of it. 

We use FM modulation and demodulation as a stand-in for the PM (phase modulation) that the transponder expects and transmits. FM and PM are somewhat related, so it sorta kinda works for the purpose of transmitting a tone.  I am in talks with Keysight to have them sponsor the channel, and arrange a loan of some modern PM-capable equipment. 

When we FM-modulate the uplink with an audible tone (coming from the HP 3325 sitting at the top of the pile), we can hear the tone re-transmitted to earth via the downlink on my ham radio . This works when the transponder is in the "ranging" mode. In this mode, the transponder repeats everything it hears from earth and turns it around, and re-transmits it back on the translated frequency. This is how ranging was accomplished, using clever pseudo random sequence correlation tricks. 

It is going to take quite a few videos to explain all of this!

Marc