5.5 GHz Dual-Polarized Parabolic Reflector Antenna Using a Planar Yagi Feed

1. Introduction

Parabolic reflector antennas are among the most important high-gain antenna structures used in microwave engineering. They are widely used in point-to-point wireless backhaul, satellite terminals, telemetry links, radar sensors, microwave test ranges, and long-distance broadband communication systems. The key reason for their popularity is simple: a parabolic reflector can convert the energy radiated by a small feed antenna into a narrow, highly directive beam.

In the 5 GHz band, long-range wireless links require strong link margin, good polarization purity, and controlled sidelobe radiation. A single low-gain antenna may not provide sufficient received power over long distances. A reflector antenna solves this problem by increasing effective aperture area, thereby improving gain without requiring active amplification. In practical systems, this improves received signal strength, reduces multipath pickup from undesired directions, and increases interference rejection.

The design discussed here is a compact 5.5 GHz parabolic reflector antenna with a dual-polarized planar Yagi feed. The target is approximately 25 dBi gain with stable performance across the 5.15–5.85 GHz band. The use of dual polarization allows horizontal and vertical polarization channels to be used for polarization diversity or capacity enhancement, provided that the port isolation and cross-polarization levels are sufficiently controlled.

Design Snapshot

2. Theory and Principle of a Dish Antenna

A parabolic dish antenna is based on a geometric property of the parabola: rays originating from the focal point are reflected from the parabolic surface and emerge parallel to the main axis. When a feed antenna is placed at the focus, the reflector transforms the feed’s spherical wavefront into an approximately planar wavefront. In receiving mode, the same process works in reverse; incoming plane waves are concentrated at the focal point.

This focusing property makes the parabolic reflector a high-aperture antenna. Unlike a simple dipole or patch antenna, the gain is primarily determined by the physical aperture area of the reflector and the efficiency with which the feed illuminates that aperture. A larger aperture captures or radiates more electromagnetic energy in the desired direction, resulting in higher directivity and narrower beamwidth.

Here, f is the focal length. For a circular dish of diameter D and depth d, the focal length can also be estimated as:

The ratio F/D is one of the most important reflector design parameters. A very small F/D ratio creates a deep dish and may increase blockage or feed spillover sensitivity. A very large F/D ratio creates a shallow dish that requires a narrower feed pattern to illuminate the aperture properly. For compact microwave dishes, an F/D value around 0.3–0.5 is commonly used depending on feed pattern and mechanical constraints.

Aperture Gain

The ideal gain of a reflector antenna depends on its aperture size relative to wavelength. Practical gain is lower than ideal directivity because of aperture efficiency losses such as spillover, taper loss, phase error, surface error, polarization mismatch, blockage, and feed mismatch.

At 5.5 GHz, the free-space wavelength is approximately 54.5 mm. Therefore, a 470 mm dish has an electrical diameter of about 8.6 wavelengths. This is large enough to produce a narrow directional beam and a gain around the 25 dBi class when aperture efficiency is well optimized.

Beamwidth

The half-power beamwidth is inversely related to dish diameter. As dish diameter increases, the main lobe becomes narrower. This improves long-distance link performance but also makes mechanical alignment more critical.

For a 470 mm aperture at 5.5 GHz, this approximation gives a beamwidth close to 8 degrees, which matches the simulated beamwidth trend. Such a beamwidth is suitable for directional point-to-point links, but antenna alignment must be performed carefully because even a few degrees of pointing error can reduce received power.

3. Reflector Geometry and Antenna Setup

The antenna geometry consists of a parabolic reflector and a compact feed assembly positioned near the focal region. The reflector diameter is approximately 470 mm and the focal distance is approximately 185 mm. This gives an F/D ratio close to 0.39, which is a practical value for compact high-gain microwave reflectors.

Correct feed placement is critical. If the feed is moved away from the focal point, aperture phase error increases and the main beam becomes distorted. This may reduce gain, increase sidelobes, and degrade cross-polarization performance. Similarly, feed tilt and polarization alignment directly affect port isolation and radiation symmetry.

4. Planar Yagi Feed and Dual-Polarization Concept

The feed antenna determines how efficiently the reflector is illuminated. A good feed should illuminate the dish aperture with the correct amplitude taper, maintain phase symmetry, reduce spillover beyond the reflector edge, and preserve polarization purity. In this design, a planar Yagi-type feed is used because it offers compactness, directional radiation, and easier integration of orthogonal polarizations.

A Yagi antenna typically uses a driven element, reflector element, and one or more director elements to shape radiation in the forward direction. In planar form, these elements can be implemented on a dielectric substrate or metallic support structure. The feed pattern should not be too broad, because excessive energy misses the dish and contributes to spillover. It should also not be too narrow, because under-illumination reduces aperture efficiency and gain. The ideal condition is a balanced illumination taper, often with reduced field strength near the dish edge to suppress sidelobes.

Dual polarization is obtained by using two orthogonal feed excitations, typically corresponding to vertical and horizontal polarization. In a practical dual-polarized reflector antenna, the two ports must remain isolated so that energy from one polarization does not strongly couple into the other. Good isolation improves MIMO performance and reduces cross-channel interference.

5. Design Methodology

1. Select operating band: define the target frequency range around 5.15–5.85 GHz with centre operation near 5.5 GHz.
2. Estimate wavelength: calculate λ = c/f to understand aperture size in wavelengths.
3. Choose dish diameter: select aperture diameter based on target gain and beamwidth.
4. Set focal length: choose F/D ratio based on feed pattern and mechanical feasibility.
5. Design feed: optimize the planar Yagi feed for impedance match, polarization purity, and suitable illumination pattern.
6. Place feed at focus: tune feed position and orientation for maximum gain and minimum pattern distortion.
7. Evaluate S-parameters: verify return loss and isolation for both polarization ports.
8. Optimize radiation pattern: improve gain, beamwidth, sidelobe level, front-to-back ratio, and XPD.

6. Practical Thumb Rules

• For high-gain 5 GHz P2P antennas, a dish diameter of 8–10λ can provide gain in the mid-20 dBi range.
• Maintain accurate feed placement; small displacement at 5 GHz can noticeably affect pattern symmetry.
• Use edge taper to suppress sidelobes; over-illumination increases spillover and back radiation.
• Surface accuracy should typically be better than λ/16 for efficient reflector operation.
• For dual-polarized operation, optimize both co-polar gain and cross-polar discrimination, not just S11.
• High port isolation is essential when using polarization diversity or dual-channel radio links.
• Mechanical alignment is as important as EM design because a narrow beam is sensitive to pointing error.

7. Simulated Electrical Performance

The simulated response shows that the antenna maintains stable performance across the 5 GHz band. The return-loss plots indicate impedance behavior of the vertical and horizontal polarization feed ports. In a dual-polarized reflector system, both ports should be well matched over the intended operating band, because mismatch loss directly reduces realized gain and system link margin.

Vertical Polarization Port Response

Horizontal Polarization Port Response

Port Isolation

Port isolation describes how much energy from one polarization port couples into the other port. Good isolation is required for clean dual-polarized communication. Poor isolation can reduce diversity gain, increase correlation between channels, and degrade cross-polar performance.

Radiation Pattern and Gain Stability

The elevation-plane radiation pattern shows a narrow main beam, low sidelobe behavior, and strong forward radiation. This is the expected signature of a properly illuminated parabolic reflector. The sidelobe reduction near the centre of the band indicates good feed taper and reflector-feed alignment.

8. Result Summary Table

Discussion

The antenna achieves approximately 25.3–25.4 dBi gain across the simulated band. This confirms that the 470 mm aperture is effectively utilized at 5 GHz. The beamwidth of around 8 degrees is consistent with theoretical aperture-based estimates, confirming that the reflector geometry and feed placement are well matched.

The sidelobe level is especially strong near 5.55 GHz, where it is below -26 dB. This suggests that the feed illumination is well controlled near the centre of the band. At the band edges, sidelobes remain below -20 dB, which is still suitable for many point-to-point wireless applications where off-axis interference control is important.

Cross-polar discrimination remains above 20 dB near the centre frequency, indicating that the dual-polarized feed maintains acceptable polarization purity. The reduction at the upper band edge is common in compact dual-polarized feed systems and can be improved through further feed symmetry optimization, substrate tuning, or reflector-feed spacing adjustment.

9. Built-in Antenna Calculators

The following calculators provide quick first-order estimates for reflector gain, wavelength, beamwidth, focal ratio, and far-field distance. These values are useful during early design and sanity checking, but final validation should always be done using full-wave simulation and measurement.

10. Key Takeaways

• A 470 mm parabolic reflector at 5.5 GHz can practically achieve around 25 dBi gain when feed illumination and aperture efficiency are optimized.
• The planar Yagi feed provides a compact and directional feed solution suitable for reflector illumination.
• Dual-polarized operation requires both good impedance matching and strong port isolation.
• Sidelobe level is strongly influenced by feed pattern, edge taper, and feed location.
• The simulated results show stable gain, narrow beamwidth, good front-to-back ratio, and useful cross-polar performance across the 5 GHz band.

References

1. C. A. Balanis, Antenna Theory: Analysis and Design, 4th ed. Hoboken, NJ, USA: Wiley, 2016.
2. W. L. Stutzman and G. A. Thiele, Antenna Theory and Design, 3rd ed. Hoboken, NJ, USA: Wiley, 2012.
3. J. D. Kraus and R. J. Marhefka, Antennas for All Applications, 3rd ed. New York, NY, USA: McGraw-Hill, 2002.
4. S. Silver, Ed., Microwave Antenna Theory and Design. London, U.K.: IET, 1984.
5. R. C. Johnson, Ed., Antenna Engineering Handbook, 3rd ed. New York, NY, USA: McGraw-Hill, 1993.
6. IEEE Standard for Definitions of Terms for Antennas, IEEE Std 145-2013, 2014.
7. P.-S. Kildal, “Factorization of the feed efficiency of paraboloids and Cassegrain antennas,” IEEE Transactions on Antennas and Propagation, vol. 33, no. 8, pp. 903–908, Aug. 1985.
8. A. W. Rudge, K. Milne, A. D. Olver, and P. Knight, Eds., The Handbook of Antenna Design, vol. 1. London, U.K.: Peter Peregrinus, 1982.
9. Y. T. Lo and S. W. Lee, Eds., Antenna Handbook: Theory, Applications, and Design. New York, NY, USA: Van Nostrand Reinhold, 1988.
10. T. A. Milligan, Modern Antenna Design, 2nd ed. Hoboken, NJ, USA: Wiley-IEEE Press, 2005.