The radiation pattern of a spiral antenna is fundamentally bidirectional, meaning it radiates electromagnetic energy equally in two opposite directions perpendicular to the plane of the spiral. This creates a broad, wide beamwidth that is nearly hemispherical on each side of the antenna. For a typical single-arm or two-arm Archimedean spiral, the pattern is characterized by a single main lobe with a beamwidth of approximately 60 to 80 degrees in the E-plane and H-plane when operating in its dominant mode. However, this is a simplified view; the actual pattern is highly dependent on the antenna’s design parameters, the operational frequency, and the specific mode of excitation. The most significant feature of a spiral antenna’s radiation is its frequency-independent behavior; the pattern remains relatively constant over a very wide bandwidth, often achieving a 10:1 or even 20:1 bandwidth ratio.
To truly understand this pattern, we need to look at the underlying physics. The spiral antenna operates based on the principle of a traveling wave. As the signal propagates outward along the spiral arms from the feed point at the center, it radiates most effectively from the region where the circumference of the spiral is approximately equal to one wavelength (the so-called “active region”). This region moves inward toward the center as the frequency increases, and outward toward the perimeter as the frequency decreases. This is the secret to its wide bandwidth. The radiation is predominantly circularly polarized, with the sense of polarization (right-hand or left-hand) determined by the direction of the spiral winding. A key metric for circular polarization is the Axial Ratio (AR), which measures the purity of the polarization. A well-designed spiral antenna can achieve an axial ratio of less than 3 dB across its entire operating band.
| Design Parameter | Impact on Radiation Pattern | Typical Values / Effect |
|---|---|---|
| Number of Arms (N) | Determines the possible modes of operation (Mode M). The fundamental mode is M=N/2. | 2-arm (most common): Mode 1. 4-arm: Enables Mode 2 for monopulse tracking. |
| Spiral Growth Rate (a) | Affects bandwidth and the sharpness of the active region. A slower growth rate supports wider bandwidth. | Archimedean (constant growth): Common for wideband. Logarithmic: Optimal frequency-independent performance. |
| Inner and Outer Radius | Sets the high-frequency and low-frequency cutoffs, respectively. | Outer Radius (Rout) ≈ λlow/π; Inner Radius (Rin) ≈ λhigh/π |
| Substrate Dielectric Constant (εr) | Higher εr reduces the size but can narrow the bandwidth and degrade the axial ratio. | Air core (εr≈1) for best performance. Rogers RO4003 (εr=3.55) for compact designs. |
| Cavity Backing / Reflector | Transforms the bidirectional pattern into a unidirectional pattern with significant gain increase. | Adds ~3-6 dB of gain. Cavity depth is typically λ/4 at the center frequency to avoid pattern distortion. |
The basic bidirectional pattern is ideal for applications where radiation is needed on both sides, such as in some direction-finding systems. However, for most communication and radar applications, a unidirectional pattern is desired. This is achieved by placing a cavity backing or a ground plane reflector behind the spiral. This cavity absorbs or reflects the backward radiation, creating a single, forward-directed main lobe. The introduction of a reflector has several critical effects. First, the gain typically increases by 3 to 6 dB. Second, the back lobe is suppressed, often to levels below -20 dB. Third, the front-to-back ratio becomes a key performance indicator. The depth of the cavity is crucial; a depth of approximately a quarter-wavelength (λ/4) is optimal to ensure the reflected wave adds constructively in the forward direction.
Let’s examine the pattern characteristics in more detail with and without a cavity. For a cavity-backed spiral, the radiation pattern in the principal planes can be summarized as follows:
- E-Plane Pattern: Exhibits a smooth, single-lobed pattern. The 3-dB beamwidth is typically between 70° and 90°. The side lobe levels are generally very low, often below -15 dB, due to the tapered illumination of the aperture.
- H-Plane Pattern: Very similar to the E-plane pattern, a characteristic of circularly symmetric antennas. The beamwidth is nearly identical, contributing to the symmetrical “pencil beam” appearance.
- Axial Ratio vs. Angle: The circular polarization is purest at the center of the beam (boresight). As you move away from boresight, the axial ratio degrades. The “3-dB beamwidth” for axial ratio—the angular range over which the AR remains below 3 dB—is a critical specification and is often around ±60° from boresight for a well-designed spiral.
The operational mode, determined by the phase progression between the arms, is another layer of complexity. While the fundamental mode (Mode 1) produces the broadside pattern described above, higher-order modes (e.g., Mode 2 on a 4-arm spiral) produce conical radiation patterns where the main lobe is tilted away from boresight. This is exploited in specialized applications like monopulse angle tracking systems, where the difference pattern (Mode 2) provides a sharp null at boresight for precise target location, while the sum pattern (Mode 1) is used for general tracking and communication. For engineers seeking robust and versatile designs, a Spiral antenna from a specialized manufacturer can provide these advanced multi-mode capabilities with guaranteed performance metrics.
Finally, the performance is not just about the antenna itself but its integration. The balun (balanced-to-unbalanced transformer) used to feed the symmetric spiral arms from an asymmetric coaxial line is a critical component. A poor balun can severely unbalance the currents, leading to pattern distortion, degraded axial ratio, and the excitation of unwanted modes. The choice of absorbing material within a cavity, or the use of a reflector, also influences cross-polarization levels and side lobe control. In array configurations, the mutual coupling between adjacent spiral elements can cause scan blindness and pattern grating lobes, which must be carefully managed through element spacing and arrangement.