Saudi-OSCAR 50

The Saudi-OSCAR 50 is a Saudi amateur radio satellite which was launched on December 12, 2002 by the King Abdulaziz City for Science and Technology. This satellite launched after a series of other amateur satellites, and is one of only two FM satellites along with AO-85.

Technical information

SO-50 carries several experiments, including a mode J FM amateur repeater experiment operating on 145.850 MHz uplink and 436.795 MHz downlink. The repeater is available to amateurs worldwide as power permits, using a 67.0 hertz PL tone on the uplink, for on-demand activation. SO-50 also has a 10-minute timer that must be armed before use. Thus, first transmit an initial carrier with a PL tone of 74.4 Hz to arm the timer.

The repeater consists of a miniature VHF receiver with sensitivity of -124 dBm, having an IF bandwidth of 15 kHz. The receive antenna is a 1/4 wave vertical mounted in the top corner of the spacecraft. The receive audio is filtered and conditioned then gated in the control electronics prior to feeding it to the 250 mW UHF transmitter. The downlink antenna is a 1/4 wave mounted in the bottom corner of the spacecraft and canted at 45 degrees inward.

Operating SO-50

SO-50 is easy to work with basic equipment; a hand-held of 5 W for the 2 m uplink is more than adequate for good reports. Use a second hand held tuned on the 70 cm band for reception. A 5-element quad for the uplink, and a 7-element quad for the downlink can be made from cheap timber and wire, and will give a strong working platform. A 3-element tape measure antenna for 2 m and 5-element for 70 cm will work, but the signal strength in both directions will be generally unreliable.

Contrary to internet angst, it is perfectly possible to transmit on 2m and simultaneously listen on 70 cm without the use of headphones without causing feedback, although headphones may be preferable.

Doppler shift means you typically begin the pass on 436.800 MHz, shifting down 2.5 or 5.0 kHz, according to your transceiver's increments, as the satellite approaches the closest point to you, shifting down again a step or two as it approaches the end of the pass. It is generally possible to keep a good input without shifting from 145.850 MHz.