Understanding Broadside Antenna Arrays

Broadside arrays are a type of antenna array used to achieve a directional radiation pattern. They are designed such that the main beam of radiation is perpendicular (broadside) to the line or plane along which the individual antenna elements are arranged.

Let's analyze the characteristics listed in the options in the context of a typical broadside array, particularly a linear broadside array which is a common configuration.

Analyzing Each Statement for Broadside Arrays

• A. Number of dipoles of unequal size: In most standard antenna arrays, including broadside arrays, the individual radiating elements (like dipoles) are identical in size and characteristics. Using elements of unequal size is less common for simple broadside arrays and is usually employed for specialized purposes like pattern synthesis (e.g., Dolph-Chebyshev arrays, which are often endfire, not broadside). Therefore, this statement is generally false for a typical broadside array.
• B. Number of dipoles equally spaced: Uniform spacing between the elements is a common feature of linear and planar arrays, including broadside arrays. Equal spacing simplifies design and analysis and helps in controlling the radiation pattern, such as minimizing sidelobes. This statement is true.
• C. Collinear dipoles: For a linear broadside array, the antenna elements (often half-wave dipoles) are typically arranged along a straight line or axis. While the dipoles themselves might be oriented parallel to each other and perpendicular to the line of the array, the arrangement of the elements themselves is along a line, hence "collinear" referring to the array structure. In contrast, a collinear array in a stricter sense might mean elements are placed end-to-end, which is often associated with endfire radiation. However, in the context of array *arrangement* along an axis, "collinear" can describe the linear structure of the array. Given the other correct options, this is the likely intended meaning: the elements are arranged along a common line. This statement is true in the sense of a linear array configuration.
• D. Dipoles in phase: This is a crucial characteristic for achieving a broadside radiation pattern. To maximize radiation perpendicular to the array axis, all the elements must be fed with currents or voltages that are in phase (zero phase difference between elements). A progressive phase shift is used for beam scanning or endfire patterns, not broadside. This statement is true ($\alpha = 0$).
• E. Dipoles are 90 out of phase: A progressive phase difference of 90 degrees ($\pi/2$ radians) between adjacent elements typically results in an endfire radiation pattern (radiation along the array axis), or a scanned beam, depending on the element spacing. It does not produce a broadside pattern. This statement is false.

Summary of Findings

• Statement A: False (Typically equal size dipoles)
• Statement B: True (Equally spaced dipoles)
• Statement C: True (Dipoles arranged collinearly along a line)
• Statement D: True (Dipoles in phase)
• Statement E: False (Dipoles 90° out of phase gives endfire or scanned beam)

Confirming the Correct Option

• Option 1: A and B only (Incorrect - A is false, C and D are true)
• Option 2: A, C and E only (Incorrect - A and E are false, B and D are true)
• Option 3: B, C and E only (Incorrect - E is false, D is true)
• Option 4: B, C and D only (Correct - B, C, and D are true)

Revision Table: Broadside Array Properties Recap

Additional Information on Antenna Array Types

Antenna arrays are broadly classified based on their radiation pattern relative to the array axis. The two fundamental types for linear arrays with uniform element excitation are broadside and endfire arrays.

• Broadside Array:
  • Main radiation lobe is perpendicular to the array axis.
  • Achieved when all elements are fed in phase ($\alpha = 0$).
  • Elements are typically equally spaced.
• Endfire Array:
  • Main radiation lobe is along the array axis.
  • Achieved with a specific progressive phase shift between elements ($\alpha = \mp kd$, where k is wavenumber and d is spacing).
  • Elements are typically equally spaced.

The direction of the main lobe of a linear array with uniform spacing 'd' and progressive phase shift '$\alpha$' is given by the angle $\theta_m$ (relative to the array axis) where the Array Factor is maximum. This occurs when $\psi = kd\cos\theta_m + \alpha = 0$ or integer multiples of $2\pi$. For the principal maximum, we set $\psi = 0$.

$$kd\cos\theta_m + \alpha = 0$$

$$\cos\theta_m = -\frac{\alpha}{kd}$$

For a broadside array, the main lobe is at $\theta_m = 90^{\circ}$, so $\cos\theta_m = \cos 90^{\circ} = 0$. This requires $\alpha = 0$, meaning the elements are in phase.

For a conventional endfire array, the main lobe is at $\theta_m = 0^{\circ}$ (along the axis) or $\theta_m = 180^{\circ}$ (opposite to the axis). For $\theta_m = 0^{\circ}$, $\cos 0^{\circ} = 1$, requiring $\alpha = -kd$. For $\theta_m = 180^{\circ}$, $\cos 180^{\circ} = -1$, requiring $\alpha = kd$.

This confirms that the in-phase condition ($\alpha=0$) is characteristic of a broadside array.