Array Bandwidth
Understanding Array Bandwidth
A phased array antenna achieves its steering ability by introducing precise phase shifts between elements. At the design frequency, these phase shifts create exactly the right wave interference pattern to point the beam in the desired direction. But at a different frequency, the same phase shifts produce a slightly different electrical length — and the beam points in a slightly different direction. This fundamental limitation is beam squint, and it defines the usable array bandwidth for steered applications.
The Phase vs. True-Time-Delay Problem
Steering a beam to angle $\theta$ requires a time delay of $\tau = d\sin(\theta)/c$ between adjacent elements (where $d$ is element spacing and $c$ is the speed of light). At frequency $f_0$, this time delay corresponds to a phase shift of $\phi_0 = 2\pi f_0 \tau$. A phase shifter implements this exact phase shift. At a different frequency $f$, the same phase shifter still produces the same phase shift $\phi_0$, but the correct phase shift should be $\phi = 2\pi f\tau = \phi_0 (f/f_0)$. The error causes the beam to squint by an angle proportional to $(f-f_0)/f_0$ and the steering angle $\theta$.
True-Time-Delay as the Solution
True-Time-Delay (TTD) networks insert actual physical or switched delay lines between array elements. A delay line introduces the same time delay across all frequencies, correctly steering the beam at every frequency simultaneously. TTD networks are significantly more complex and expensive than simple phase shifters, but are mandatory for wideband phased arrays in modern multifunction radar and broadband EW receivers.
Key Equations
Array Bandwidth in the context of phased array antennas describes the frequency range over which the array maintains its intended radiation pattern performance — specifically...
Key specifications:
500 MHz | 3 dB | 2.15 dB | 8 dB | 5 % | 25 dB
Gain: G = ηap×4πA/λ²
Comparison
| Band | Range | Wavelength | Application | Standard |
|---|---|---|---|---|
| Array Bandwidth | 1 GHz region | 300.0 mm | Primary use | ITU allocation |
| Adjacent lower | 0.9 GHz | 333.3 mm | Related band | Shared spectrum |
| Adjacent upper | 1.1 GHz | 272.7 mm | Related band | Guard band |
| Harmonic 2f | 2.0 GHz | 150.0 mm | Spurious | Filter required |
| Sub-harmonic | 0.5 GHz | 600.0 mm | LO option | Mixer design |
Frequently Asked Questions
How much beam squint is acceptable?
The acceptable beam squint depends on the application. For a narrowband communications array with a 1% fractional bandwidth, squint is negligible. For a 20% fractional bandwidth radar searching for targets, a beam squint of several beamwidths across the operating band would cause the array to miss targets at band edges. The rule of thumb is that squint becomes significant when $\Delta f / f_0 \times \sin(\theta)$ exceeds roughly half the antenna's 3 dB beamwidth.
How does element spacing relate to array bandwidth?
Element spacing is typically set to $d = \lambda/2$ at the highest operating frequency to prevent grating lobes when steering. This means that at lower frequencies, $d < \lambda/2$, which is electrically closer spacing and limits grating lobe formation. Wider-bandwidth arrays must use element spacings that prevent grating lobes across the full bandwidth at the maximum steering angle simultaneously — a constraint that typically requires custom element spacing algorithms for arrays steering beyond ±45°.
What is Squint-Free bandwidth?
The squint-free bandwidth is the range of frequencies over which the beam pointing error remains within the half-power beamwidth of the array, guaranteeing that the target remains within the main beam across the full operating band. For a narrowly steered array (small $\theta$), the squint-free bandwidth is relatively wide. For a widely steered array ($\theta$ near 45° or 60°), the squint-free bandwidth shrinks dramatically, potentially requiring TTD steering for even modest bandwidth applications.