Evaluation Report Antenna Efficiency - 3DS...higher efficiency. See also [Rothammel 2013] chapters 6 to 8 or [Balanis 2016] chapter 9.8. The presented ring antenna on a button cell

Evaluation Report Antenna EfficiencyTalk 3D Experience Conference 2019 in DarmstadtAuthor / Presenter Georg RamschDate 2019 – NovemberFile name CST Vortrag Englisch.DOC

Anschrift: Plankstadter Straße 72, 68723 Oftersheim

Telefon: +49 (0) 6202 1263691

Email: [email protected]

Content Goal of Simulation 2 Description of the task 2 Literature References 2 Findings 3

Basis Monopole on GND with coplanar GND 3 Loop - Design 7 Button cell Battery – Ring Antenna 10

Conclusions and Recommendations 13

CST Vortrag Englisch.doc Seite 1

mailto:[email protected]

Goal of SimulationGoal of this talk / analysis is the investigation of parameters that have an impact on the performance of

small antennas like monopoles (lambda-quarter antennas) or even smaller for the use in IoT applications.

The target design requirement with the restrictions „as small as possible“ comes with a trade off

„performance reductions“ - which are presented here. From the conclusions recommendations are derived

as well as interpretations of the results for the use case.

Focus is on the structured approach for practical use – not a presentation of a „best design“ antenna.

The use case implicates much more parameters than could be presented here in a short talk.

The lambda-quarter antenna will be stated below as „monopole“ or „folded monopole“.

CST Microwave Studio version 2019 with time domain solver is used for simulation and results.

Description of the task

Following aspects are under investigation:1. Analysis of a reduced parameter set, that have an impact on the performance of small antennas

with construction - / real estate - restrictions. Exemplary model of a monopole.2. Approach of an alterantive design.3. Antenna design with severe real estate restrictions on a pcb – size of a button cell battery (e.g.

battery type LR44 with ca. 10mm diameter).4. Conclusions and recommendations from the findings.

Literature References

The more „practical guy“: [Rothammel 2013]Antennenbuch // Rothammel – Krischke // 13th edition // DARC Verlag 2013

The more „theoretical guy“: [Balanis 2016]Antenna Theory – Analysis and Design // Balanis // 4th edition // Wiley 2016


Findings Basis Monopole on GND with coplanar GNDPCB-integrated small antennas suffer often from the vicinity of GND within the reach of the reactive near field and the reduced size of the GND-reference plane. [Rothammel 2013] chapter 1.3.3.1 „Reactive Nearfield“ = 0,16 * Lambda. Internet: http://de.wikipedia.org/wiki/Nahfeld_und_Fernfeld_(Antennen)To get an idea of the impact of the near vicinity GND we look at a standard monopole with a varying size of a coplanar GND

Example in picture 2.1 und 2.2 : standard Monopole with 23,5mm length / 0,25mm width / on 0,5mm FR4-pcb-substrate orthogonal on a sufficient GND-plane ( radius around feed point > 0,25 Lambda). A standard 50 Ohm feed impedance is chosen. 50 Ohm is the most generally used impedance feed value – whereas the standard feed impedance of a monopole is lower. An infinetely thin Monopole on a infintely large GND-plane reaches its feed impedance @ 37,5 ohm (this value you will find via internet citations / For exact definitions see: [Balanis 2016] chapter 2.13 and [Rothammel 2013] chapter 19.

Image 2.1: Basis-Monopole orthogonal on a GND-plane without coplanar GND. ( = 0 mm)

Image 2.2: Basis-Monopole orthogonal on a GND-Plane with coplanar GND-plane of 20mm height.


http://de.wikipedia.org/wiki/Nahfeld_und_Fernfeld_(Antennen

Diagram 2.3: the optimum length of this monopole for 2450 MHz with 50 Ohm feed without koplanar GND for this model is 23,5mm. The reflection coefficient S11 is better -10 dB (standard value for S11). With increasing height of coplanar GND S11 decreases.

Diagram 2.4: Smith Diagramm of the monopole for 50 Ohm for varyiing coplanar GND height from 0mm up to 20mm. The real part of the input impedance of the monopole changes from 29 ohm (run no. 9 - no coplanar GND) down to 5.4 ohm (run no. 8 - 20mm coplanar GND).


Diagram 2.5: for the next parameter investigation the monopole length is adjusted to 21.5mm to get the best match for S11 @ 2,45 GHz. Dependancy of S11 matching of the monopole on the feed impedance. – at a constant height of the adjacent GND-plane of 20mm.

Diagram 2.6: Dependancy of the complex impedance – Smith diagram - of the monopole on the feed impedance. At a constant height of the adjacent GND-plane of 20mm. It becomes clear that the real part of the feed impedance is at a low of 3,5 – 3,8 Ohm. E.g. far from the standard 50 Ohm!


Diagram 2.7: Efficiency of the monopole - 23,5mm length - @ 50 Ohm feed impedance dependant on the height of coplanar GND. At 2450 MHz without coplanar GND - radiation: @ 95% and total efficiency at ca. 87%. The case with coplanar GND = 20mm reduces the total efficiency to 23%.

Diagram 2.8: Efficiency totally (green) of the monopole with 21,5mm length at a height of coplanar GND = 20mm dependant on the feed impedance. The maximum is at a feed impedance of 5 ohm => 54% and the minimum at a feed impedance of 50 ohm => 14%.


Loop - DesignModel: (naming vary) folded dipole or loop antenna (Schleifendipol) on a pcb with the size 27 * 42.5mm – 25% of a credit card size (54 * 85mm). The monople GND-plane requirement (=> 0,25*Lambda) is not given. Size restriction!

Image 2.9: Design-Study: 27 * 42,5mm pcb with a loop-antenna; metal size is 37 * 22mm.

Diagram 2.10: S11 of the loop-Antenna @ 25 Ohm feed impedance an an inductor in series for matching.. The impedance transformation 50:25 Ohm can be done with a balun.


Diagram 2.11: Smith chart of the loop-Antenna @ 25 Ohm.

Diagram 2.12: Efficiency of the loop-Antenna

With a 2:1 impedance transformation (inverse balun) from 50 ohm to 25 Ohm and a matching inductor an efficiency > 80% is achievable.


Image 2.13: Far field of the Loop-Antenne – direktivity is 3,1dBi.


Button cell Battery – Ring AntennaHere a design with strong space / real estate restriction is under investigation.

A ring antenna of 11mm diameter on a button cell battery similar to a LR44 type. The given restrictions make a matching difficult and a low efficiency is to be expected.

Two different feed impedances are compared a) 12.5 Ohm impedance ( 4:1 balun) and the 5 Ohm optimum.

Image 2.14: design of the ring antenna on a button cell battery.

Diagram 2.15: S11 of the ring antenna @ 12,5 Ohm and @ 5 Ohm feed impedance and a matching inductor (2 * 0,75 nH).


Diagram 2.16: Smithchart of the ring antenna with the 2 feed impedance values.

Diagram 2.17: Efficiency of the ring antenna. For 12,5 Ohm feed impedance total efficiency is 1,37% whereas the – difficult to achieve – optimum feed impedance @ 5 Ohm the total efficiency „climbs“ to 1,64%. What does this mean for the reach fo the radiation?Calculation for a typical IoT use case: 0 dBm * 0,01 (Efficiency) = 10 uW or –20 dBm radiated power. With an path loss of 40dB @ 2450MHz and a spacial orientation of transmit- and receive-antenna minimum– to maximum axis with anoth 20dB of isolation we get a total attenuation of -80 dB. Given the case the receive antenna has a similar gain of -10 bis -20dB, the effective receive power will be in the estimated range of -90dB bis -100dBm. This may cross the in the data sheets stated sensitivity limitsof receiver front ends. Transmission range would be in the range of 1m.


Image 2.18: Farfield of the ring antenna on a button cell battery. Antenna gain is equivalent to a Hertzian Dipole with 1,8 dBi.

http://de.wikipedia.org/wiki/Hertzscher_Dipol

[Rothammel 2013] page 110.[Balanis 2016] page 145.


http://de.wikipedia.org/wiki/Hertzscher_Dipol

CONCLUSIONS AND RECOMMENDATIONS Monopoles or small chip antennas within a pcb-circuit work only sufficiently, when an appropriateGND plane is present. The size of the GND-plane should be a radius of lambda/4 of the working frequency in case of a monopole or according the requirements of the data sheet, when a chip antenna is used.

The vicinity of a GND plane/edge to the radiating element of the antenna in the range of reactivenear field (0,16 * lambda) reduces the antenna matching (feed impedance in extreme cases down 3 ohm and S11 up to zero) and efficiency (down towards 1%).

The lower feed impedance can be partially compensated by baluns with a transformation ratio 2:1 or 4:1 – such exist as RF-wire wound transformers or as ceramic chip devices.

Optimization measures like Impedance Transformation and matching with LC-elements result in ahigher efficiency. See also [Rothammel 2013] chapters 6 to 8 or [Balanis 2016] chapter 9.8.

The presented ring antenna on a button cell battery can perform with 0 dBm RF-power of a transmitter in a „worst case scenario“ less than 1m transmit range and in the „best case scenario“ far beyond 10m.

Whereas the „worst case scenario“ may be a requirement for the use case. (=> It is a feature – not a bug: because of data privacy).

Calculation: 0 dBm * 0,01 (total efficiency) = 10 uW oder –20 dBm radiated power. With a path loss of 40dB @ 2450MHz and an orthogonal spatial orientation of transmit to receive antenna (orthogonal minimum- to maximum-axis results in an isolation of roughly 20dB for many cases) adds up to a total path loss of -80 dB. See also table 2.19.

Given the case the receive antenna has a similar „lousy“ efficiency as a transmit antenna with -10dB to -20dB then the receive power may be down in a range of -90dBm bis -100dBm. Such a low power level may be beyond the receive sensitivity limits stated in in the data sheets of the RF-devices..

Some RF-devices offer a low impedance differential Tx output in the range of 12 Ohm. With a 4:1 Balun impedances can be transformed down to 3 Ohm.

Table 2.19: Transmit range vs. Parameters.


Transmit power in dBm 0 0 0Antenna efficiency – Ref. Half wave dipole – Transmit – dB 0 0 -20Path loss @ frequency 2450 MHz – dB -40 -40 -40Spatial orientation loss – dB 0 -20 -20

0 0 -20Transmission distance – m 1 1 1Receive power - dBm -40 -60 -100Receiver sensitivity – dBm -90 -90 -90

Budget surplus – path loss vs. Sensitivity – db 50,00 30,00 -10,00Possible transmit range - m 316,23 31,62 0,32

Antenna efficiency – Ref. Half wave dipole – Receive – dB

Goal of SimulationDescription of the taskLiterature ReferencesFindingsBasis Monopole on GND with coplanar GND

Loop - DesignButton cell Battery – Ring AntennaConclusions and Recommendations

Documents

Evaluation Report Antenna Efficiency - 3DS...higher efficiency. See also [Rothammel 2013] chapters 6 to 8 or [Balanis 2016] chapter 9.8. The presented ring antenna on a button cell