VHF vs UHF

[ka9q]

Being a radio communications engineer I tend to get into minutiae like this, but I've long noticed that the Shuttle reused two of Apollo's VHF-AM frequencies while calling them "UHF". (A notable use was the Capcom's repeated call during the STS-107 entry: "Columbia, Houston. UHF comm check...")

The two frequencies are 259.7 and 296.8 MHz. The Apollo inter-PLSS link used 279.0, which I don't think the shuttle used, and the shuttle could use the military "guard" (emergency) frequency of 243.0 MHz, which I don't think Apollo had.

The VHF band is officially defined as 30-300 MHz while UHF is 300-3,000 MHz, making the Apollo usage the correct one.

The shuttle also reused the Apollo CSM S-band frequency pair (2287.5 MHz down, with the same uplink/downlink frequency ratio of 221/240), plus another I don't think Apollo used (2217.5 MHz down).

The term "S-band" originated in WW2 as one of a whole set of letter designations chosen to confuse the Germans about our radar developments. It runs from 2-4 GHz (2,000-4,000 MHz). So these S-band frequencies are also in the UHF range, making the shuttle's "UHF" reference doubly incorrect.

I wonder why they did this. Maybe everything having to do with the shuttle, which was supposed to be an improvement over Apollo, had to be "ultra" when Apollo was only "very"...

[Glom]

Being a radio communications engineer I tend to get into minutiae like this, but I've long noticed that the Shuttle reused two of Apollo's VHF-AM frequencies while calling them "UHF". (A notable use was the Capcom's repeated call during the STS-107 entry: "Columbia, Houston. UHF comm check...")

The two frequencies are 259.7 and 296.8 MHz. The Apollo inter-PLSS link used 279.0, which I don't think the shuttle used, and the shuttle could use the military "guard" (emergency) frequency of 243.0 MHz, which I don't think Apollo had.

Those frequencies are AM?

Maybe everything having to do with the shuttle, which was supposed to be an improvement over Apollo, had to be "ultra" when Apollo was only "very"...

Ha!

[cjameshuff]

Those frequencies are AM?

AM is a modulation technique, you can use it at whatever frequency you like.

[ka9q]

Yes, those two frequencies are AM, because standard aviation voice is AM. That's true for both the civilian and military aviation bands (these two frequencies are military).

I know of one straight NBFM voice link in Apollo: that in the PLSS carrying the LMP's voice and telemetry to the CDR, who then added it to his own and retransmitted it by AM to either the LM's or the rover's relay system. A ham named Larry Baysinger managed to receive Neil Armstrong's VHF AM transmitter and could hear both Armstrong and Aldrin, but not Capcom. An amazing achievement for a radio link being nominally used only to cover a distance of a few meters all the way on the moon.

The reason for this LMP->CDR->LM arrangement was because the LM was originally designed when only one astronaut was to go on the lunar surface at a time. (What were they thinking??) When this was changed to both, it was easier to set up this relay arrangement than to modify the LM's comm system.

Apollo did use FM elsewhere: for video at S-band, and on the Unified S-Band 1.25 MHz voice subcarrier in both PM and FM modes. But this relay link described above is the only one I know of where FM voice directly modulates the carrier.