ELFIN is a 20 rpm spinner, with its spin axis orbit normal (as dictated by science requirements). The antennas are positioned in between the spin axis and the spin plane to mitigate spin fading in all orientations. ELFIN-A is IARU coordinated within the amateur satellite service’s 435-438 MHz band so that we may beacon globally. We use an AstroDev Helium radio to provide a 19200 baud GFSK AX.25 UI downlink. Beacons will be transmitted at 9600 baud via the same radio & frequency
Frequencies, TLEs and Beacons
ELFIN-A transmits 9600 baud beacons on 437.450 MHz with the FCC experimental call sign - WJ2XNX.
ELFIN-B transmits 9600 baud beacons on a 437.475 MHz with the FCC experimental call sign - WJ2XOX.
TLEs can be found here after launch.
For determining our link budget, we used the method of taking the difference between our required Bit Energy to Noise Ratio and our determined Bit Energy to Noise Ratio.
Link Budget coming soon.
Antenna radiation patterns
Simulations were done using 4NEC2, with separate models for the uplink and downlink, and validated through long range testing & radiation pattern measurement. You can download the models here.
The downlink (UHF) antenna is a bent dipole, which for the purposes of these simulations is assumed to be tuned to 436.5 MHz. Results show a gain of 1.1dBi and a null of 8.1 dB.
We have developed a field test procedure, software and set-up to verify these results. This setup uses a pair of RFM 22 radios, coupled with a LSM303 to plot out antenna radiation patterns in real time using MATLAB. Once validated we will make these tools available to the community.
The uplink (VHF) antenna is also a bent dipole, tuned to 144.4 MHz. Results show a gain of 3.05dBi, with a null of 6.6 dB.
Antennas consist of bistable fiberglass tape springs with embedded beryllium copper strips, made by the AFRL for their VPM Cubesat. This material is useful for a variety of CubeSat-scale deployables, however for ELFIN the most important feature is their magnetic cleanliness. This non-magnetic requirement is what prevents us from using the prevalent spring steel carpenter tape elements or NiTi (Nitinol) wire.
The tape springs are not held in their secondary stable state, allowing them to provide their own kick force, and are retained with Spectra fishing line, burned with a conventional 1/2 W axial resistor.
All antenna elements are stowed in the new bonus volume offered by the new 3U+ specification (often referred to as the tuna can).