Antenna

Description of antenna performance.

Basic Performance Values

The antenna has a design consisting of a 2 x 2 array of square patches, which are fed by a network of microstrip splitters and matched to a 50-ohm impedance. Its proprietary design has been optimized to operate within the frequency range of 1400 to 1440 MHz. The directivity of the antenna is 14 dBi.

The design has been optimized to make use of FR4 circuit board material. The antenna is composed of a single-layer FR4 board that contains the patches and feeding network, and another single-layer FR-4 board that serves as the antenna ground plane. In between these two circuit boards, there is a 3D-printed grid that is made of PLA material. This grid provides mechanical stability and maintains an accurate spacing between the antenna's radiating element and its ground plane.

Through its two ports, the antenna can receive orthogonal hands of linear polarizations (horizontal and vertical). It achieves cross-polarization from one port to another (S21 parameter) of less than -25 dB, thus ensuring acceptable polarization purity.

The antenna beamwidth is the angular range covered by the main lobe of the antenna radiation pattern. This main lobe is the area of the pattern where the majority of the power is radiated. The beamwidth is usually measured at the angle between two points on either side of the main lobe, which is also known as the half-power points or -3dB point. At this point, the radiated power drops to half its maximum value. The beamwidth is measured in degrees and can be measured in either the horizontal or vertical planes. In the case of the drone sensor, the half-power beam width is 35 degrees.

Impedance Matching

S-parameters were measured using a nanoVNA V2 network analyzer[^1]. Tests were performed outdoors, with the main beam pointing at the sky.

  • Port 1 ("A")

    • Frequency range (S11 < -10dB): 1.35 - 1.53 GHz

    • S11 @ 1.42 GHz: -20 dB

    • S21 @ 1.42 GHz: -24.5 dB

  • Port 2 ("B")

    • Frequency range (S22 < -10dB): 1.33 - 1.44 GHz

    • S22 @ 1.41 GHz: -26 dB

    • S12 @ 1.41 GHz: -26.7 dB

Antenna Beam Pattern

The following is the simulated beam pattern at 1.4 GHz. As shown by the graph, the current design has a minor asymmetry in the main beam, which is most likely due to an interaction of the feed network with the radiating elements of the antenna. However, this asymmetry can be ignored in practical use.

The antenna beam was tested and verified using a TPI-1002-A signal generator and an RF level meter made and sold by RF Consultant[^2]. A linearly polarized dipole antenna was utilized as the transmitting antenna. The transmitting antenna and the sensor antenna were placed 5 meters apart, and the sensor antenna was rotated from -120 to 120 degrees azimuth angle. Tests were conducted for both antenna ports, and the results matched the simulation.

[^1] https://nanorfe.com/nanovna-v2.html

[^2] https://rf-consultant.com/products/tpi-1002-a-signal-generator

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