On the Peculiarities of Electrically Small Antenna Design. J. Eichler, C.C.M. Junqueira, V. Sokol

Transcription

1 On the Peculiarities of Electrically Small Antenna Design J. Eichler, C.C.M. Junqueira, V. Sokol

2 Initial Simulation Results Measurement gives S11-20 db around 434 MHz!?

3 Outline 1. Initial Simulation Result 2. Sensitivity analysis 3. Environment influence, Preliminary SA measurement 4. VNA measurement including GND plate 5. Correction of the Epoxy Resin material properties 6. Non-destructive verification of the dielectric materials properties 7. Conclusion

4 Sensitivity Analysis Testing sensitivity of S11 on different parameters

5 Reference model Epoxy resin: Er=4, tgd=0.01 (original model: Er=5, NO losses!) Tuning Screw (L=18 mm) Teflon Er=2.08, tgd= Feeding Coaxial Line

6 Teflon: f res =f(e r ) f e r MHz

7 Tuning Screw Length: f res =f(myalt) f MHz / mm myalt f MHz / mm myalt

8 Epoxy resin: f res =f(e r ) f e r MHz f e r MHz

9 Epoxy resin: mag(s 11 )= f(tgd) S11 tgd

10 Conclusion Sensitivity & 434 MHz S11 is most sensitive to: Epoxy resin tgd S11 tgd Resonant frequency is most sensitive to: Epoxy resin e r Tuning screw position Teflon e r f e 32MHz r f e f myalt r 24MHz ( mean) 11.6MHz / mm ( mean) Most problematic issue is the uncertainty in the Epoxy resin material properties.

11 Preliminary Scalar Measurement Different scenarios Antenna and cable (without GND)

12 Scenario A quasi free space

13 Scenario B on the paper box

14 Scenario C on the table

15 Scenario D on the table (touching cable) Touching cable at 10cm distance!

16 Conclusion SA measurement Antenna resonant frequency as well as the S11 at resonance is very sensitive to the surrounding environment: On the table vs. in quasi free space means 23.8MHz difference Touching the cable means 10.2dB difference in S11 magnitude (at resonance) Antenna must be measured with a ground plate in order to make the structure well defined!

17 Vector Measurement VNA, anechoic chamber, GND plate

18 Measurement Configuration MHz, 3001freq. Points (0.1MHz step)

19 Measured S11 (Incl. GND Plate)

20 Model Correction (Incl. GND Plate) Reference plane of measurement 40 mm Reference plane of simulation 0.84 ohm Epoxy resin: e r =3.09, tgd= mm, 50 ohm

21 Corrected Model Meas. vs. Simul. Excellent agreement between simulated and measured S 11

22 Epoxy resin parameters verification Excluding the influence of the Epoxy resin by exciting the second resonant mode of the antenna

23 Modified Model and Tuning Screw Iron screw Air gap Epoxy Resin Surface polished by a sandpaper (1000) in order to make a good contact between the screw and the antenna body.

24 2 nd Resonant Mode &1125 MHz Tuning screw makes the short circuit exciting the 2 nd order antenna mode. It results in very weak E-field in the Epoxy region. E-field Surface current distribution shows clearly the second order resonant mode. Surface Current

25 Sensitivity to Teflon e r & 1125 MHz f e r 156MHz The resonant frequency is practically influenced only by the Teflon permittivity (170x more than Epoxy Resin). Therefore the testing model can be used for an additional correction of the Teflon permittivity.

26 Measured Permittivity of Teflon e r of Teflon: 2.104±0.009* *Only fine (polished) surface and normal to high torque assemblies were considered.

27 Final Material properties Teflon: e r =2.1 tgd= Epoxy resin: e r =3.03 tgd=0.035

28 Conclusions Antenna has to be measured and simulated with an appropriate ground metal plate The most critical issue is the uncertainty of the Epoxy resin material properties The estimated material properties of the Teflon and Epoxy resin have been verified by the non-destructive test at the second resonant mode of the antenna There seems to be an additional transition resistance at the antenna probe (in the range of 0.5 ohm) An excellent agreement between simulated and measured data has been achieved! Simulation time of the antenna in the band of 434 MHz (including the GND plate) is approximately 10 minutes using FD general solver.

29 Acknowledgement The antenna sample has been provided with courtesy of the Institute of Aeronautics and Space (IAE-Brazil) VNA measurements has been provided by the Czech Technical University in Prague and by the Institute of Aeronautics and Space in Brazil.