Satgen 561  Mode J Operating  by GM4IHJ  (BID SGEN561)  1999-12 25

Recent correspondence suggests that several radio amateurs are trying
mode J satellites for the first time. So perhaps a review of the
peculiarities of the Mode may prove useful, noting that it is a quite
considerable step up from, mode A satellites and, FM single shout
repeaters in the sky. 
The first problem is with the 2m uplink frequency . Oscar 7 used mode
B 70cm up 2m down, but gradual saturation on two metre frequencies in
some countries made it almost impossible to read the mode B 2m
downlink in among the rampant terrestrial FMers. So Oscar 8 was
designed as the first mode J satellite . Going up at 2m its uplinks
could ignore the FM all around while the 70 cms came down on what was
the ultimate in quiet bands in those pre Pave Paws days. However it
soon became obvious that all was not simple. 
Firstly the Faraday rotation of the uplink ( much worse on 2m than 70
cms) meant your signal could lose the satellite 3 or 4 times in a
pass. The solution was to use two antennas. Here at IHJ this consisted
of two 5 element yagis on the same centre pole with their elements
offset one from the other at 90 degrees. Two separate downleads then
lead to a simple antenna switch and when the uplink faded the switch
was thrown. Several variations of this theme exist . Some operators
who already had an ordinary 2m yagi bought another , placed it
alongside the first with its elements 90 degrees offset feeding a
second downlead to the changeover switch in the shack. Other more
sophisticated types used twin or XY yagis as above but fed the
downlink leads to a phasing unit which allowed a choice of 0, 45 and
90 phase switching. 
These antenna solutions solved not only the Faraday problem but also
the problem where satellite aspect favoured one dimension or another.
While on the 70 cms downlink no complication was really necessary.
Faraday was much less on 70 cms and could usually be discounted. So a
single yagi sufficed.
But that was not the end of the shocks for the new Mode J user. Almost
all beginners to mode J experience break through of their 2m uplink
into their 70 cms receiver. The problem is the sheer power of the
adjacent uplink signal which if it gets into the receiver front end,
will overload it , any non linearity encouraging the developement of
the signals third harmonic at a fequency near to that to which the
receiver is tuned. So that the unsuspecting user thinks the satellite
is repeating his own signal back to him indicating that he has access
to the satellite for communication with others, When in fact all he
has is a loop through his own equipment and probably no contact at all
with the satellite.
The solution to this is a cavity resonant filter tuned to 70 cma in
the receiver antenna down lead, in front of the receiver , providing
passage for any 70 cm downlink but massively attenuating any
transmitter break through.
Last but not least , the unsuspecting new user may find that he seems
to be on a different sideband to every else, as his own signals are
returned as LSB despite his USB uplink. This is because the satellite
receiver first mixer uses subtractive mixing, in order to reduce
doppler shift by in effect subtracting uplink doppler from downlink
doppler . So if you use mode J on SSB, remember it is LSB up for USB
down.