Exploring Corner Reflector Antenna Design

Key Takeaways

• The corner reflector antenna, operating mainly in the 70 cm band (420-450 MHz), features a dipole element before two flat, converging rectangular screens, offering strong directional capabilities.

• The polarization is primarily linear, with circular polarization achievable. However, its gain doesn’t surpass that of a parabolic dish.

• The antenna allows for adjustments in dipole placement, reflector size, and angle to modify resonant frequency, impedance, and gain.

Diagram of corner reflector antenna. Each parameter is discussed in detail below.

A corner reflector antenna operates mostly in the VHF and UHF frequency bands and is known for its directional capabilities. It is built of a dipole driven element, positioned before two flat, rectangular reflecting screens that converge at an angle, typically 90 degrees. In scenarios where a broadband directional antenna is required, around one to one and a half wavelengths in size, corner reflector antennas are a good choice. Compared to a parabolic dish of similar size, the corner reflector does not confer any gain advantage. However, its relative simplicity in design and construction makes it a more attractive option in such applications. 

Corner reflectors mostly operate in the 70 cm band (420-450 MHz), which comprises the television and amateur radio bands. They are also popular among amateur radio enthusiasts, particularly for use in the 144, 420, and 1296 MHz bands. However, although the corner reflector antenna was highly prevalent, its usage has significantly declined. Regardless, read on as we explore corner reflector antenna design.

Corner Reflector Antenna Design Basic Guidelines

Basic Corner Reflector Antenna Design Introduction

Typically, the antenna employs a dipole as the radiating element, but a folded dipole variant or half-wave dipole is also feasible. Enhancing the directivity of an antenna can often be achieved through a simple yet effective method: incorporating a reflector, typically set at a 90-degree angle between its plates, as illustrated in the figure. The will be mounted ahead of two flat, rectangular metal sheets, which are joined or bent at an angle, (ϴ) ranging from 80° to 120°. 

Designing the Reflecting Surface 

1. Step 1: The reflecting surfaces, typically flat, are commonly constructed from wire screen or rod elements aligned parallel to the driven element. This choice is made to lessen both the weight and wind load on the antenna. The spacing of these rods, denoted as D in the diagram, should ideally not exceed 0.06 (6%) of the wavelength. The angle (θ) formed between the sides of the reflector is most frequently set at 90°.  

2. Step 2: To ensure the desired characteristics, the length (H) of the reflector's sides should exceed twice the wavelength. Additionally, the reflector width (W) must be greater than one wavelength for a half-wave radiator. The reflector can be constructed using various materials, such as wire netting, sheet metal, or fabricated metal spines arranged in a V-formation. These spines should be parallel to the radiator, with their spacing being less than 0.1 wavelength of the operating frequency.

Driven Element Position

3. Step 3: The positioning of the driven element, S, in relation to the reflector’s convergence point is roughly 0.5λ. This placement is not exceedingly critical; for antennas with a 90° angle, the gain does not fluctuate more than 1.5 dB when S ranges between 0.25λ and 0.75λ.

 Notably, the radiation resistance of the dipole increases with this spacing, enabling the adjustment of the spacing to align the driven element with the feed line effectively. In applications requiring wide bandwidth, such as television antennas, bowtie-driven elements are often employed. Additionally, the polarization of the antenna can be altered to elliptical or even circular forms by rotating the dipole relative to the corner reflector.

The impedance of these antennas varies depending on the operating frequency, but typically ranges between 50 ohm and 75 ohm. It's common to encounter a slightly higher Standing Wave Ratio (SWR) of around 1.7:1 at the lower end of the band. 

How to Increase Bandwidth

4. Step 4: The bandwidth of the corner reflector can be increased by thickening the dipole antenna. However, it's crucial to remember that a dipole is inherently a balanced structure, necessitating the use of a balun when feeding the dipole. 

How to Modify Gain

5. Step 5: Adjusting the angle of the reflector allows for gain modification: a sharper, more acute angle results in greater directionality, whereas a wider, more obtuse angle leads to less directionality. Although the gain of the antenna does increase as this angle is reduced, the gain enhancement becomes minimal below 90° and necessitates the use of longer reflector screens. Nevertheless, angles as acute as 45° have been successfully implemented. 

Corner Reflector Antenna Features 

Antenna Design With Cadence AWR Software

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