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CST – Computer Simulation Technology

Simulation of a Dual-Band Antenna for a Sounding Rocket

Sounding rockets are relatively small rockets developed primarily for high-altitude experiments, and are used in many academic and low-cost research missions. Antennas for sounding rockets need to be compact, lightweight, omnidirectional and have a form factor that allows them to be integrated on the rocket without causing drag, which can make their design challenging. A group at ENSEIRB-MATMECA, Bordeaux Institute of Technology, used CST STUDIO SUITE to develop a low cost antenna that can be integrated into the nose cone of a rocket and transmit payload and telemetry data in the 869.4–869.65 MHz and 2.4–2.4835 GHz frequency bands.

In the course of flight, the maximum distance between the rocket and the ground base station is 2.5 km, and the elevation relative to the base station varies between 90° and 160° (Figure 1). However, to minimise the risk from malfunctions such as parachute failures, the antenna was designed to have maximal elevation coverage. Full specifications can be found in [1] – the most critical is that in the upper band, the maximum allowed gain variation is 8.7 dB....



Figure 1: A diagram of the rocket’s flight. Over the course of the flight, the angle between the rocket and base station varies considerably.

To avoid having any part of the antenna protrude out of the rocket and cause drag, the entire thing is integrated inside the fiber-epoxy nose cone. Although a broadband monopole would be one possible solution, it has few degrees of freedom and this makes it difficult to minimize coupling to the rocket fuselage. A bicone antenna has more degrees of freedom, and can be constructed in a cross-pattern that achieves a low weight but omnidirectional performance (Figure 2).



Figure 2: The antenna design. Full dimensions are available in [1].

In order to find a design that meets the specifications, the group optimized the antenna parameters using the time domain solver in CST STUDIO SUITE®. Sweeping the parameter g  (the gap between the upper and lower halves) allowed the group to impedance match the antenna in both bands (Figure 3 and 4), chosing g = 1mm as the optimum.



Figure 3: (a) Real and (b) imaginary part of antenna impedance Z11 versus g.


Figure 4: Antenna |S11| versus g.

In designing the length of the taper (L11), there needs to be a compromise between two competing effects. Simulation revealed that increasing L11 reduces the coupling between the antenna and surface waves on the rocket body at 2.45 GHz, and therefore improves the coverage of the antenna. However, this also has the effect of reducing the gain of the antenna. A parameter sweep of L11 from 20 mm to 50 mm found that the best compromise was found at L11 = 40 mm, where the 8.7 dB gain variation range at 2.45 GHz is ? = 18° to 177° (Figure 5).



Figure 5: Simulated elevation radiation patterns, for ?=0°, for varying L11 on a 1.8-m-long fuselage at (left) 869.5 MHz and (right) 2.45 GHz.

A prototype was constructed (Figure 6), and good agreement was found between the measured and simulated results (Figure 7) – simulation also allowed the performance of the antenna to be analysed as installed on the full-size rocket, which would not fit in the measurement chamber. The antenna met the requirements and was integrated into the Project Artemis rocket, which subsequently won the 2015 Prix Espace et Industrie (Space and Industry Prize) from the French government space agency CNES.



Figure 6: The antenna under test, on a section of rocket fuselage. Students would like to thank the Poly-Grames research center, Montréal, Canada, for the measurement.


Figure 7: Radiation patterns at (a) 869.5 MHz and (b) 2.45 GHz at ?=0 – measured and simulated on a 0.28 m section of fuselage, and simulated on the full 1.8 m fuselage.

References

[1] J. Prades, A. Ghiotto, E. Kerhervé and K. Wu, "Broadband Sounding Rocket Antenna for Dual-Band Telemetric and Payload Data Transmission," in IEEE Antennas and Wireless Propagation Letters, vol. 15, pp. 540-543, 2016. doi: 10.1109/LAWP.2015.2457338

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