This article presents the design and tests of a closed‐loop controlled microwave beamformer for phased arrays. The proposed circuit owns an internal reference signal used to automatically set predefined amplitudes and phases of the beamformer branches. This signal is applied to the branches through directional couplers with high directivity and propagates in them similar to the actual signals coming from the antenna array. As a consequence of this architecture, we found that the impedance mismatch between the antennas and the beamformer as well as the mutual coupling in the array is taken into account during calibration, resulting in a more accurate adjustment of amplitudes and phases. In this work, the influence of the mutual coupling between the antennas and the directivity of the couplers on the beamforming calibration uncertainty is also analyzed. From the derived uncertainties, the corresponding pattern degradation is evaluated by performing Monte Carlo simulations. It is shown that using couplers with low directivity together with tightly coupled arrays can produce undesirable main lobe squints of up to 4.5°, main lobe level deviations of up to 4.2 dB, and side lobe level variations of up to 24 dB. On the other hand, the use of high directivity couplers can mitigate these problems. Lastly, the validation of the designed circuit is conducted in two stages: bench tests and measurements in an anechoic chamber with an array of six printed monopoles operating at 2.2 GHz. Amplitude and phase calibration errors less than 0.5 dB and 3°, respectively, were observed in the bench tests.

This study describes two stand‐alone iterative algorithms that fully solve the issue of generating non‐standard input impedances in probe‐fed circularly polarised rectangular microstrip antennas (CPRMA). Both were derived from a new cavity model‐based methodology that controls, at the frequency of operation (fnormalo), the input reactance and resistance levels without disturbing the circular polarisation (CP) characteristics of the antenna. The first algorithm finds the antenna dimensions and probe position for an arbitrarily selectable input impedance and the second algorithm determines the possible choices of input impedance for a given dielectric substrate and fnormalo. To highlight the applicability of both algorithms, two prototypes were designed and built: a right‐hand CP (RHCP) active antenna with input impedance ZnormalA=34.17+40.34iΩ, and an RHCP reference antenna (RA) with input impedance ZRA=50+0iΩ. These antennas were utilised in the design and characterisation of a co‐designed global positioning system active integrated antenna. The experimental and theoretical results show excellent agreement.

The difference between phase velocities of the odd and even modes in inhomogeneous microstrip coupled lines makes the microstrip directional coupler a poor device regarding directivity. In this work, this issue is successfully compensated by the addition of a matching network at the ports of coupled lines. Furthermore, the proposed matching network, composed of a single segment of microstrip line, was extensively studied providing significant insights for the design of a compact and broadband device. As result, simple design equations based on coupled lines S-parameters are obtained, as well as the achievable operation limits of the proposed matching network. Finally, in order to validate the proposed design methodology, a compact microstrip directional coupler is constructed to operate with directivity higher than 30 dB and coupling of 17 dB over a 600-MHz bandwidth.

No abstract

No abstract