Antenna Deployment Testing
Author: Sean McAuliffe
One of the primary deployable components of the ORCASat spacecraft is its dipole antenna. The dipole must remain stowed during its journey to the ISS, after ORCASat is released into space however we need to deploy the antenna in order to establish contact. The antenna deployment system on ORCASat is one which is commonly found on small satellite projects. We use two lengths of spring loaded steel, found in common tape measures, as the two arms of the dipole antenna. You can see these arms in their deployed state, protruding from the antenna board in the image below.
Spring loaded steel is used for this role because it is easy to stow, and easy to deploy (the steel naturally wants to settle into a flat configuration), with few moving mechanical parts, which would add complexity and risk of failure. The steel has electrical characteristics sufficient to function as a dipole antenna.
When stowed the two antenna arms are rolled as they would be inside the body of a tape measure. The rolled up arms are held in place by two retaining arms - seen in black plastic - which pivot about a single hinge. When closed, the retaining arms are held in place with plastic fishing line, which is pulled taught, wrapping around the exterior of the arm. This configuration can be seen in the videos below.
On each retaining arm, the fishing lines path is positioned so that it makes contact with two resistors (seen on the green circuit board attached to the interior face of the retaining arms). The resistors act as resistance heaters. When it is time to deploy the antenna a current is passed through the resistors, which covert the electrical energy into thermal energy; heating the fishing line until it melts.
When the line melts the tension holding the retaining arms closed is released, and the natural tendency of the spring loaded steel to straighten forces the arms open, and the antenna springs into shape. The design choice to use steel repurposed from tape measures is common among small satellites like ORCASat, because it is easy to implement, and the steel is easily purchased for very little money. To order spring loaded steel directly from a manufacturer is much more expensive. The steel we use is identical in its electrical and mechanical properties. Plus, people find the yellow tape measure look entertaining.
We recently tested the antenna deployment mechanism, with great success. We were able to reliably deploy the antennas by powering the resistance heater circuit.
The antenna board contains the resistance heating deployment circuitry, and a direct coaxial connection to the board containing our radio transceiver and modem. The performance of this setup as an RF antenna was successfully tested, and the results of that testing can be seen in a previous blog post.