Impact of mutual coupling between SKALA4.1 antennas to the spectral smoothness response

Abstract

One of the advantages of arrays with aperiodic distributed elements is their ability to mitigate the detrimental mutual coupling effects on the radiation pattern. However, we show that the mutual coupling inside a random array can still generate undesired structures in the frequency response although the single antenna features a spectral smooth response. For small subsets (a couple of SKALA4.1 antennas and a 16-element array) of a low-frequency instrument station of the Square Kilometre Array, the combination of large mutual coupling and antenna geometry creates systematic distortions in the element frequency responses. This phenomenon compromises the station spectral smoothness response versus frequency. However, we demonstrate that it is possible to partially mitigate these frequency structures by reconfiguring the antenna distribution based on exclusion zones.

Mutual Coupling Mitigation in Array Dense Regime by an Antenna Distribution Optimization in SKA1‐LOW

“…As already mentioned in the previous section, considering the length of the dipole at the bottom of the SKALA4.1 (1.6 m), we can infer that the spatial zone identifies the antenna couples with the extremities of the arms very close to each other. This result is furthermore consistent with the results reported in (Bolli, Bercigli, et al., 2022), where similar exclusion zones have been proposed to partially mitigate glitches in the spectral smoothness performance due to a large coupling between antennas.…”

“…In particular, at lower frequency, the antennas with stronger coupling are aligned to the North‐South direction ( y ‐axis), corresponding to the E‐plane (| dx | < 0.7 and 1 m ≤ | dy | ≤ 2 m, see also Figure 3a). This can be explained by the fact that the longer dipoles of nearby antennas almost touch in the E‐plane causing a strong capacitive coupling (Bolli, Bercigli, et al., 2022). On the other hand, at higher frequencies, the plane where the antennas exhibit a larger coupling is the H‐plane (| dy | < 0.7 and 1 m ≤ | dx | ≤ 2 m).…”

“…The averaged rECC $$\left(\overline{\mathrm{r}\mathrm{E}\mathrm{C}\mathrm{C}}\right)$$ values in the top panel indicate that, after the repositioning, a global decrease of the pattern variability of 6.1%, 2.7% and 5.9% is obtained at 50, 70 and 80 MHz, respectively. At 80 MHz, the $$\overline{\mathrm{r}\mathrm{E}\mathrm{C}\mathrm{C}}$$ shows a very low value probably due to the resonance effects that significantly deteriorates the antenna response (Bolli, Bercigli, et al., 2022). The standard deviations measuring the rECC variability are also shown in the bottom panel.…”

“…Numerical analyses showed a significant impact of mutual coupling on the antenna performance below ≃80 MHz, where the average of the minimum distances between the antennas $$\left({\overline{\mathrm{d}}}_{\mathrm{min}}\right)$$ is less than a half‐wavelength (i.e., dense regime). In (Bolli, Davidson, Bercigli, et al., 2022), a statistical analysis of the antenna patterns variability shows that the impact of mutual coupling between 150 and 350 MHz (i.e., sparse regime) is almost constant, while this quickly increases at lower frequencies, with also some narrow regions around 55 and 77 MHz with a more perturbed behavior (Bolli, Bercigli, et al., 2022). Nowadays, the SKA Organisation is assessing alternate layouts to mitigate the mutual coupling among antennas, one of them features a sunflower‐like distribution (Bolli, Davidson, Braun, et al., 2022).…”

Antenna Pattern Modeling Accuracy for a Very Large Aperture Array Radio Telescope With Strongly Coupled Elements

“…At these frequencies, errors can be an order of magnitude larger. These frequencies are known "problem" frequencies for simulation, due to mutual coupling being especially pronounced [17]. At the time of writing, EEPs have been computed at 5 MHz cadence over the entire operating band, and at 1 MHz intervals from 70-90 MHz.…”