Designing a Circularly Polarized

Antenna for EagleSat

Dadija BliudziusEmbry-Riddle Aeronautical University

NASA Space Grant

Overview• Linear versus Circularly Polarized Antenna

o Circularly Polarized Antenna• Antenna Requirements • Theory• Wire Length Calculations• Equipment• Program• Testing• Goals• Summary & Conclusion

Linear versus Circular Antennas

• Linear Antennao Can only communicate with other antennas

who's waves oscillate in the same plane• Circularly Polarized Antenna

o Able to communicate with all antennaso No power loss due to polarization mismatch

Circularly Polarized Antenna

• Electromagnetic wave does not change strength, but rotates around a central axis

• Divided into two components; at right angles to each other o The vertical components in blue o The horizontal components in green. o The horizontal component leads the vertical component by one

quarter of a wavelength

Theory• Phase shifting

o Achieves circular polarizationo Wire lengthso Power splitters

• Antenna Length

o c = speed of lighto = frequency (436MHz )o = 0.687597 mo Quarter dipole

0 20 40 60 80 100 120 140-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

0 20 40 60 80 100 120 140-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

Antenna Requirements• Transmit and Receive on 436MHz• Fit within 10 x 10 x .16 cm • Weigh < 70g• Impedance of 50Ω• Connect to a communication board• Deploy antennas to a fixed position

Theory (cont.) • Impedance Matching

o Mismatch causes reflection (not all power reaches antenna)• Microstrip design

o Calculator to design for 50Ω impedanceo Transmission line on top and ground plane on bottom for directional signal

• Precise wire length o Phase shiftingo Designed for 50Ω impedanceo Permittivity

Inputs Measurements

Trace Thickness (T) 35 umSubstrate Height (H) 1.6 mmTrace Width (W) 3.31 mmSubstrate Dielectric (Er) 4.25

Wire Length Calculations

• Wavelength in air ≠ wavelength through PCB• Antenna wire length difference: ¼

o Power splitter used to minimize wire lengths

Wavelength

In Air (cm)

In FR4 PCB (m)

¼ 17.19 9.86½ 34.78 19.721 68.76 39.44

Equipment• Printed Circuit Board

o FR4 Materialo Low permittivity

• Antennao Tape measureo Flexible, metal, easy and cheap

to acquire• Power splitter

o Delays the signal by ¼ wavelength • Coax

o Connects between boards• SMA Connecter

o Test the phase shifting

Program: Diptrace• Designs the entire PCB • Pads for connectors• Pads for antennas• Mounting holes

Program: Matlab• Examines radiation pattern

o Directional dependence of the strength of the radio waves• Currently looking at an omnidirectional antenna• Plot the radiation pattern when finished building

Testing• Network analyzer

o Tests and maps out the phase shifting• GPS Antenna on a balloon launch

o April 10, 2014o Tests the clarity of signal

• HASP o August 2014o Examining the antenna’s transmissions and interactions with other

subsystems• Testing different scenarios

o Vacuum chamber

Conclusion & Summary

• Antenna development slow• Should be able to meet deadlines• Future work:

o Expand antenna knowledgeo Create a flight-ready antennao Become more familiar with radiation pattern

Acknowledgements• Jack Crabtree• Dr. Post• Dr. Yale• NASA SpaceGrant & interns