Description and requirements

• Initially, the task was to be able to use open Wi-Fi networks that are placed far away and have weak signal levels.
• The antenna either should have been small or portable or could be assembled/disassembled.
• F=2.45 GHz but possibly also F=5..6 GHz. Wi-Fi and Bluetooth should be covered.
• The receiver should not be placed so far away from the antenna for practical use. Focus distance r=50 cm for 2.45 GHz. r=120 cm for 5.9 GHz.
• The diameter of the antenna should be no more than 70cm for practical use. D=70cm.
• We assume that the source is placed at an infinite distance.
• The antenna design is a metal ring with an external radius = of 0.35m and an internal radius = of 0.247m.
• Not tested with a real Wi-Fi signal yet, only spectrum analyzer result so far.

Fresnel zone antenna

• During simulation, it found that for focus distance f=50 cm and diameter of antenna D=70cm, the optimal gain is achieved using only shielding (shading) second Fresnel zone.
• Thus, all zones are open except the second one.
• Such configuration can give an amplitude gain 3.0 (9.5 dB).   

Vector diagram for gain calculation:

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Calculation

For 2.45 Ghz

For 5.5GHz

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Simulation

Setup

• OptiFDTD simulation SW, https://optiwave.com/resources/academia/free-fdtd-download/
• OptiFDTD gives the result in DFT (digital Fourier transform).

Simulation setup in optiFDTD:

Simulation setup in optiFDTD with plane wave source:

Results – plane incident wave

F=2.45GHz

Result at receiver:

Result along the Z-axis:

Field magnitude along the Z-axis:

Peak position around f=46mm (96mm-50mm):

Conclusion

• A gain of around 2.8 (9.0 dB) can be expected for F=2.45GHz.
• For F=2.45GHz focus distance f=46 mm where the field is maximum.

F=2.45GHz with the presence of a tablet pc or smartphone at the focus

Setup with the presence of a tablet pc at the focus:

Simulation with the presence of a smartphone or tablet pc at the focus:

Conclusion

• Seems like presence of smartphone/tablet pc does not affect the gain a lot.
• Some standing waves can be seen at the antenna of the receiver but the field strength is still high.
• Should be tested with the spectrum analyzer.   

F=5.9 GHz

Result along the Z-axis:

Field magnitude along the Z-axis:

Conclusion

• A gain of around 2.8 (9.0 dB) can be expected for F=5.9GHz.
• For F=5.9 GHz focus distance f=120+ mm where the field is maximum.

Results – simulation with the point source at the far field

Setup

Setup with points source:

F=2.45 Ghz

Comparison at a far distance and point source simulation:

Far-field comparison without and with antenna:

Far-field comparison without and with antenna:

Conclusion

• A gain of around 2.8 (9.0 dB) can also be seen with far-field simulation.
• It also proves that the antenna will work in both modes: receiver and transmitter. It is also theoretically proved by the reciprocity principle.

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PCB design

• Thus, the antenna design is a metal ring with an external radius = 0.35m and an internal radius = 0.247m.
• For portability, the ring is split into 8 pieces, so the antenna can be assembled and disassembled.
• PCB design has only one metal layer (TOP).

PCB design:

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Assembly

• Parts should be connected using interleaving – one part normal, one part upside down,
• In this case, the metal layers (TOP) touch each other directly.
• Such a connection also helps to compensate for the curvature of each PCB.   

Two parts connection:

Antenna assembled:

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Test results

Setup

• Beacon with spread spectrum as a source at the distance of about 40m.
• Loop H near field probe as a receiver.
• FLP1003 spectrum analyzer.

Measurement at receiver point:

Measurement results

• The following pictures show difference with and without the antenna presented.
• Case with antenna shows a higher signal level (5...10...

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