External antenna design for a stratospheric balloon
Designing an external antenna for a stratospheric balloon appears routine until one examines the actual operating environment. At 20–25 km, pressure drops to tens of hPa, the air is dry, and the temperature persistently hovers around −60 °C. UV radiation and day/night thermal cycling subject materials and adhesives to relentless stress. In that scenario, the antenna ceases to be a component and becomes a system. Radome, thermal management, sealing and cabling become part of the radio-frequency performance, not merely of the mechanical design. The choice of radome dielectric defines insertion loss and VSWR stability throughout the entire mission. Common plastics shift their dielectric constant and absorb moisture; PTFE/FEP, quartz composites or honeycomb sandwich constructions maintain low loss. The coating chemistry must withstand UV radiation and ozone without yellowing or cracking. Colour also matters: white finishes reduce temperature but promote condensation; dark finishes mitigate icing and raise the thermal balance. Tropopause crossing introduces water into any gap, followed by ice formation. ePTFE membranes and controlled vents prevent overpressure and condensation inside the radome. At low pressure, the Paschen curve requires that edges be rounded and RF transitions encapsulated to prevent partial discharge. In Ku/Ka band, every decibel counts. Long pigtails consume the link budget; the LNA must be placed close to the feed with a short semi-rigid coaxial cable. The gondola pendulates and vibrates, so the mounting must provide rigidity with minimum mass and mechanical decoupling. If the platform rotates, a high-gain link requires a gimbal or a phased array with tracking. Choosing omnidirectional antennas simplifies the design but entails accepting 10–20 dB less of link margin. Thermal control combines MLI, aerogel and closed-loop heaters to keep the electronics and the dielectric within their operating window. Reliability is verified through testing, not through a speed test. A climatic chamber at −70 °C, pressure of 10–70 hPa, accelerated UV exposure, vibration, and ice/water cycling must replicate the complete mission profile. Telemetry of temperature, RSSI, reflected power and heater current allows degradation to be anticipated before the link is lost. Conclusion: in the stratosphere, every dB and every watt has an owner. Closing the link budget and the thermal budget first, and then selecting low-loss, low-outgassing materials, is what distinguishes a demonstrator flight from a sustained operational campaign.
NASSAT - Network Satellite Systems