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The initial design for the
onboard video system consists of a video board capable of accepting a composite
video source and transmitting that video on 439.25 MHz. (cable channel
60). The video board is mounted in
the electronics bay and powered by the power distribution unit (13.8Vdc
power for an optimal RF output of 1 watt).
This design is to be augmented with a pre-amp to allow for a greater
power output and thus greater distances.
The onboard antenna is a horizontally polarized dipole tuned to the
proper frequency.
The ground component of the video
system is represented in the following diagram. Various ground antennas have been
tested. A turnstile antenna gives
good performance and is made up of two phased dipoles, which allows us to
maintain good reception even as the onboard antenna rotates. Antenna rotation can yield nulls in the
received signal due to polarization issues.
Another useful antenna is the helix antenna which has the benefit of
high gain and is circularly polarized which alleviates the problem of
nulls. The antenna is feed into a
down-converter that will convert the signal to Channel 3 (or 4) and
includes a 20-dB amplifier. The
signal is recorded on VCRs and viewed on ground monitors.
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