Fall 2005 Issue

Here WC5Z describes a simple and inexpensive flight payload system for experimental high-altitude balloon flights.

By Michael Helm, WC5Z

High-altitude experimental balloon flights have been described elsewhere in the literature. This article covers the design of a small multi-purpose payload, which is useful to fly as a standalone payload or in conjunction with other experimental payloads. This payload includes a low-power 2-meter beacon transmitter for tracking purposes and some limited status telemetry, along with an inexpensive digital camera which will take pictures at timed intervals (photo A).

The total cost of duplicating this payload including camera and even the batteries should be less than $100. The entire system is designed to operate from a 6-volt DC power source in the interest of keeping battery weight low.

The 2-meter Beacon Transmitter

The low-power 2-meter beacon transmitter is designed around a conventional oscillator-multiplier chain1 with the circuit as shown in figure 1. The design is based around proven technology rather than a leading-edge approach, since reliability is the most significant performance requirement. This beacon transmitter provides an FM-tone modulated signal that is on/off keyed with the CW ID/telemetry message. A simpler transmitter design has been flown that does not include the FM audio tone, but many of the casual balloon trackers only have FM equipment and the signal is more pleasant for those trackers if the FM tone modulation is included. The FM tone modulation does not significantly hinder those who are using SSB/CW receivers for tracking.

The transmitter starts with a 12.288-MHz surplus crystal that can be obtained for very low cost. The first stage is an oscillator-tripler; the next stage doubles to the 72-MHz region. Following is a doubler stage to 147.456 MHz which feeds a single amplifier stage. Double-tuned circuits are used in most stages to provide for good signal purity. Although it is possible to eliminate some of the double-tuned circuits, I have found this often results in close-in spurious signals plus or minus the fundamental crystal frequency from the desired final carrier frequency. Double-tuned circuits add little to the total cost and weight and are very desirable from a clean-signal standpoint. Even though this transmitter only produces about 25 mW

Photo A. Controller, two separate beacon transmitters, and the modified digital camera prior to final packaging before flight.

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