Satgen 422 A Mode L/S Challenge by GM4IHJ (BID SGEN422) 29 April 97 The announcement of a possible 1997 launch for a Uosat 12 minisat carrying a mode L/S transponder , introduces the possibility of communications and experiments in a combination of bands new to amateur satellite operations. This proposal is therefore very welcome , because it will provide a first class introduction to bands we must learn to use effectively, as we are forced to go ever higher in frequency by military, commercial and amateur inteference, and by massive terrestrial electronic polution of our lower VHF and UHF bands. There will be problems. The proposed 1.6MHz wide non inverting transponder will introduce doppler shifts of up to plus or minus 90 khz ,with peak doppler rates at closest approach of perhaps 18 khz/minute or more. Past experience with Oscar 13 mode S was not a suitable preparation for this sort of tuning problem, because that satellite had very little doppler rate during mode S apogee operation. A more suitable test of ones ability to cope with high doppler rates is available if you try tracking the CW beacons on the NOAA weather sats ( NOAAs 10 and 12 on 1698.0 MHz and NOAAs 11 and 14 on 1707.0 MHz plus or minus 50 khz or so of doppler shift). Equally useful, the NOAAs provide an excellent practical tutorial in antenna tracking of LEO sats operating around 2GHz. Indeed it will be interesting to see what operator aids develope to deal with these new problems, if as hoped Uosat 12 is a success. Certainly some sort of automatic doppler correction will be a must for either the digital or analogue modes , and a whole new family of antenna designs may be necessary. Noting that at IHJ the problem of really accurate tracking of the LEO NOAAs resulted in the abandonment of a single long helix with bearing and elevation motion, and the substitution of a set of individual,short, wide beam helixes, arranged so that they each covered adjacent, roughly 60 degree arcs, in both bearing and elevation . So when Uosat12 comes with its promise of lots of interesting antenna and doppler experiments, it will be most welcome. IHJ mode S reception is already available via the S band receive converter used for Ao13, and, a search of the spare room recently unearthed the 2 watt L band 144 /1268 transverter which was originally intended for the ill fated Ao10 mode L, which unfortunately turned out to require kilowatts rather than watts, for access. PS Oscar 10 mode B beacon 145.811 is FMing again as its solar cells are presently at a poor angle to the Sun. There is also deep fading roughly 3 times every minute as satellite spin causes the satellite body to eclipse the ground station view of the omni directional antenna.