Why VHF is not considered as a desirable frequency choice for a long range air surveillance radar

VHF (Very High Frequency) is generally not considered a desirable frequency choice for long-range air surveillance radar for several reasons:

1. Wavelength and Antenna Size:

• Long Wavelengths: VHF wavelengths range from about 1 to 10 meters. Longer wavelengths require larger antennas to achieve the same beamwidth and directivity as shorter wavelengths. Large antennas are impractical for many radar applications due to size, weight, and mechanical complexity.
• Antenna Size: Effective radar systems require directional antennas with high gain. At VHF frequencies, achieving the necessary gain and directivity would require extremely large and cumbersome antennas.

2. Resolution and Detection:

• Poor Range Resolution: VHF radars have lower range resolution compared to radars operating at higher frequencies (like UHF or microwave). Lower resolution can make it difficult to distinguish between closely spaced targets.
• Doppler Resolution: Doppler resolution, crucial for detecting moving targets and filtering out stationary objects, is lower at VHF frequencies due to the longer wavelengths. This can reduce the radar's ability to detect and track fast-moving objects accurately.

3. Atmospheric and Environmental Factors:

• Ionospheric Reflection: VHF waves can be reflected or refracted by the ionosphere, causing propagation beyond the horizon (skywave propagation). While this can extend the range, it can also introduce multipath interference and make it challenging to determine the actual position of targets accurately.
• Environmental Noise: VHF frequencies are more susceptible to environmental noise from natural sources (e.g., lightning) and man-made sources (e.g., industrial equipment, communication signals). This noise can degrade the radar's signal-to-noise ratio and reduce detection performance.

4. Clutter and Interference:

• Ground Clutter: VHF radars can pick up significant ground clutter (reflections from the ground, buildings, and other static objects) due to their longer wavelengths. This clutter can obscure the detection of airborne targets, especially at lower altitudes.
• Civilian and Military Interference: VHF frequencies are heavily used for communication (e.g., FM radio, TV broadcast) and other applications, leading to potential interference. Military radars need to avoid frequencies heavily used by civilian systems to reduce interference and ensure reliable operation.

5. Propagation Characteristics:

• Diffraction and Refraction: VHF signals are more prone to diffraction and refraction by obstacles (e.g., terrain, buildings) compared to higher frequency signals. This can create shadow zones and reduce coverage in certain areas.
• Multipath Effects: The longer wavelength of VHF signals increases susceptibility to multipath effects, where signals reflect off various surfaces and create multiple, slightly delayed copies of the signal. This can complicate signal processing and target detection.

Summary:

While VHF frequencies have certain advantages, such as better penetration through foliage and buildings, their limitations in terms of antenna size, resolution, environmental noise, and clutter make them less suitable for long-range air surveillance radar. Higher frequencies (UHF, L-band, S-band, etc.) are preferred for these applications due to their better resolution, smaller antennas, and more favorable propagation characteristics.