The S-Band (2400MHz) video transmitter generates a high quality, full
motion color video image. It utilizes a wide-band FM modulator, PA
(power amplifier), and video camera. Show below, is the flight
ready video link payload.
Video Link Payload
The payload includes the following major subsystems.
Board The controller handles commands from the UHF cross payload
receiver, sends commands to the camera controller, and controls the
operation of the PA.
Attenuator An in-line SMA attenuator is use between the modulator
and the PA. This reduces the power level from the modulator as well as
improving the impedance match between the devices. The in-line devices
allow for ease in adjusting power levels.
The final Power Amplifier is housed in a solid aluminum block for shielding
and as a heat sink. The device dissipates almost 1 watt of power in
the form of heat. This is still efficient for a class-C amplifier
operating in the S-Band. Because the payload operates in the thin
atmosphere of near space, a large heat sink must be utilized to draw heat
away from the device. Thanks to Louis, W7LMW, for
his efforts in machining the heat sink from a solid block of aluminum.
Cord The payload is mounted in the payload train using a 4 corner
harness. 4 attachment points on the top and bottom of the payload are
connected to the main payload string. The aluminum, 90° channel
enclosure provides high strength and rigidity.
Battery Pack This is the first payload designed to utilize a
non-primary (rechargeable) battery pack. Complete information on these
cells is available in the Power Sources
Antenna A quadrifilar helix antenna provides a spherical,
omni-directional pattern well suited for high altitude flights.
Because of the antenna's low gain (<2 dBi) and radiation pattern, nulls for
fading are minimized. The
antenna was a custom built unit from Directive Systems.
Although it is possible to build your quadrifilar helix, it takes a lot of
experience to build and tune it for a broad band application such as a video
link. The cost of the antenna (around $225) is very reasonable.
Although Directive Systems provide descent quality hardware, customer
service is very poor. Expect long lead times, unasnwered e-mail, and frequent toll calls
to check delivery status.
Camera The block camera is an OEM device that provides basic
camera functions in a bare bones device. Typically, these devices are
rated for operation at -20°C to +60°C and require minimal power. This
unit is a Sony
FCB-IX47 that includes S-video output, video overlay, RS-232 command
port, and special video feature. The video features include
freeze-frame, 0-lux operation (night vision), 16:9 wide screen, and full
remote zoom/focus/white balance control. With commanding, the camera can be placed in a mode
suitable for a balloon flight. For example, the white balance can be
fixed for outdoor use and the auto focus can be disable to prevent "hunting"
when pointed at a low contrast horizon. The camera also utilizes a 37mm
screw mount to accept wide angle or filter lens. For high altitude
use, a Tiffen circular
polarized filter was used. The filter greatly reduces UV light and
glare. When mounting the lens, care should be taken to
ensure moisture or debris isn't trapped behind the filter that may obscure
the image or cause condensation in the cold, dry air of the upper
atmosphere. Thanks to Ralph,
W0RPK, for providing the camera.