Satgen274  Satellite Profile No3  Fuji Oscar 20 (FO20) by GM4IHJ  25June94
BID of this msg is SGEN274  Please use this BID if you retransmit this msg

The first mode J transponder 145 up/435 down was made in Japan and flew
very successfully aboard Oscar 8J. Subsequent mode J birds FO12 and FO20
have continued to provide this popular service with FO20 being operational
in June 94. With its high downlink fequency ( 435 rather than 145 ) , mode
J gives greatly improved reception particularly in those areas where 145
MHz is plagued by FM, computers and other electronic noise. But it has to
be said that mode J operation is not as easy as Mode B ( old Oscar 7 or
present RS14). The doppler shift can be fierce compared with say RS10, and
the stringent reception requirements on 435 MHz mean that you cannot get
by with poor coax cable and simple antennas if you want good weak signal
long range communications. So newcomers are advised to cut their teeth on
RS10 before trying FO20. That said mode J can give endless fun ,
particularly on trans Atlantic contacts where the large downlink bandwidth
reduces adjacent signal interference and the low doppler shift of near
horizon signals makes tuning easy, whilst the low elevation angle allows
use of an ordinary azimuth only trained antennas.
50 watts of EIRP from your 145.9 to 146 MHz CW/LSB transmitter will get
you good access via a 5 element horizontal yagi except when the sat is
high overhead. But you do need a pre amp directly on say a 12 element
horizontal 435.9 to 435.8 MHz downlink antenna, and do not forget to bring
your coax feeder out of the back of the antenna, not forwards or downwards
through the antenna field of fire. For reception you can chose any of
several 435 MHz receivers but IHJ prefers a 435/29 MHz converter feeding a
smooth tuning CW/USB HF receiver fitted with good communications filters .
In the Oscar 8 days IHJ used a 2m azimuth rotated antenna on the chimney ,
and hid the 435 antenna out of sight of the 2m antenna behind the bungalow
wall. The reason for this is that the 3rd harmonic of the transmitter
uplink is too close to the downlink frequency for adequate rejection if
the antennas "see" one another. Later however a home made 435 cavity
filter in the downlink feeder did a better job of protection from this
nasty harmonic.
FO20 mode J has a beacon centred on 435.795 MHz plus or minus 12 kHz of
doppler. Be warned this beacon can sometimes also be heard at several
places in the downlink bandwidth. Also please note that FO20 uses
subtractive mixing so you transmit on Lower Side Band but you read the
downlink on Upper sideband due to satellite transponder frequency
inversion. This latter is a neat trick which ensures that communications
have 30% less doppler than the beacon
FO20's orbit period is about 112 minutes at an inclination of 99 degrees
and its orbit is elliptical not circular , which means it can sometimes
give spectacular DX at apogee. Because it is higher it stays in range
longer than RS10,12,14, but you see less orbits. UK stations see a
sequence of 3 or 4 early morning passes, followed by a break of about 6 or
7 hours , after which there is another sequence of 5 orbits ( or 7 for
northern stations).
At present in June 94, FO20 is permanently in the Analog JA communications
mode. It can operate in the digital store and forward mode JD, but this is
not presently available because of on board computer problems.  73 de
GM4IHJ @ GB7SAN