Satgen347 Operating Fo20 Mode JA Pt2 by GM4IHJ 18 Nov 95 Most satellite users start out on Mode A satellites 145 up 29 down. But sooner or later they want to move up to Mode J 145 up 437 down , in order to try the digital satellites. This can be an ego damaging experience. Mode J , either Analog or Digital is a far cry from simple mode A. But the change has to be faced at some time , so how best to make it ? Mode J analog on Fo20 offers a simpler but by no means easy route , allowing the operator to become familiar with the mode before attempting the daunting shift to digital. The difficulties of mode J are two fold - Fierce Doppler shift and general reception problems with antennas and tracking. Considering the Doppler problem first. Mode A sats experience max doppler shift between start and finish of a near overhead low earth orbit pass , of about 5 KHz total, ie in most passes you only lose the signal out of your receiver bandwidth once . Mode J is much less forgiving. The mode J beacon on 435.795 MHz dopplers nearly 18 KHz in a nearby pass , ie the signal leaves your Rx bandwidth at least 9 times per orbit pass. Worse still it does so much quicker in the middle of the pass than at the start or finish of pass. The situation is not quite so bad when you work through the transponder , because Fo20 uses subtractive mixing , 145 up mixes with transponder osc 580 Mhz to produce 435 down, a switch which subtracts uplink doppler from downlink doppler. So a transponder pass doppler is about 12 Khz, still enough to leave your RX bandwith nnarly every 2 minutes or so. Please note that subtractive mixing means you must transmit LSB up to get USB down. Unfortunately doppler shift does not proceed at a steady rate, it is slow at the start of a pass and at the end of a pass, but changes very quickly in the middle of a pass. Even more frustrating if you at the end of a pass talk to a station who is so placed that he is in the middle of his pass, his doppler rate of change is much higher than yours and it is difficult to keep your two downlinks on the same frequency. Fortunately the best DX occurm when one station is at the end of the pass and the other is at the beginning, when doppler rates at both ends are very slow. On Phase 3 high altitude sats mode J is by contrast very easy because the sat is moving very slowly and doppler is minimal. But to come back to low orbit Fo20. What can a mode J analog operator on a LEO sat do to improve the situation. The crudest , simplest doppler aid uses a grid :- FUJI 20 Mode JA Tx/Rx Tuning Aid 145900 . v 920 . v 940 .t vg960 . v 980 . v 146000 OH | | | | | | 145900 . v 920 . v 940 . v 960 . v 980 . v 000 far | | | | | | 435900 880 860 840 820 435800 CPA|-----v-----|-----v-----|-----v-----|-----v-----|-----v-----| close 145900 920 940 960 980 000 | | | | | | 145900 v . 920 v . 940 v . 960 v . 980 v . 000 far | | | | | | 145900 v . 920 v . 940 v . 960 v . 980 v . 000 OH end Sloping lines of v's trace transponded dop change as orbit pass progresses Centre line shows uplink for downlink at closest approach .Top and bottom show start/finish of an overhead pass .2nd and 4th show ends of a distant pass. Eg OH pass begins abou v= 145916 , centrepass 145910 ,end pass at 145904 for a constant 435890 down. But distant pass starts 145913 , ends 145907 for 435890 downlink.