How do I design a direction finding antenna array for operation across a wide frequency range?

Designing a DF antenna array for wideband operation (e.g., 2-18 GHz, a 9:1 bandwidth) requires careful element selection, baseline optimization, and ambiguity resolution across the entire frequency range: (1) Element selection: each antenna element must cover the full frequency range. Cavity-backed spiral antennas are the standard choice: 10:1+ bandwidth (2-18 GHz), circular polarization (receives any polarization), stable phase center (critical for interferometer accuracy), and compact size (diameter ≈ λ_low/2 ≈ 75 mm at 2 GHz). (2) Baseline design: the fundamental challenge: at 2 GHz (λ = 150 mm), a baseline of λ/2 = 75 mm provides unambiguous but coarse resolution. At 18 GHz (λ = 16.7 mm), the same 75 mm baseline = 4.5λ, which is ambiguous. Solution: use multiple baselines of different lengths. Short baseline (e.g., 15 mm = λ/2 at 10 GHz): unambiguous across most of the band but low resolution. Medium baseline (e.g., 45 mm): moderate resolution, ambiguous at high frequencies. Long baseline (e.g., 150 mm): high resolution, ambiguous at most frequencies. The ambiguity resolution algorithm uses the short baseline to resolve the medium baseline, and the medium baseline to resolve the long baseline (cascaded unfolding). (3) Array geometry: for 360° coverage: circular array (5-9 elements around a circle). Each adjacent pair forms a baseline. Additional longer baselines are formed by non-adjacent pairs. For sector coverage (e.g., ±60° from broadside): linear or planar array (3-7 elements). (4) Calibration: the array must be calibrated across the full frequency range. Measure the phase difference between all element pairs at each frequency using a known reference source. Create a calibration table: Δφ_cal(f, baseline) for each baseline and frequency. During operation: subtract the calibration phase from the measured phase before computing the AOA.