Reflectivity vs Object Size

[gn.times]

Guys, can yu please help me in explaining the relation between object size and wavelength of EM wave with respect to reflective property of the wave when it strikes the object.

Is it like if the wavelength is smaller than the object size then the wave will be reflected and vice versa?

 

[Terminator3]

Also i learned that metal grid with small holes will reflect the wave, maybe this gives you an idea. And waveguide can stop frequencies it not designed for.

 

[jiripolivka]

Also i learned that metal grid with small holes will reflect the wave, maybe this gives you an idea. And waveguide can stop frequencies it not designed for.

The effect of wave hitting an object is named DISPERSION. The wave itself has no "reflective properties".

Electromagnetic wave travels through open space until it hits a material object. Object properties like permittivity and conductivity affect how charges in it respond to the wave. Charge movements then generate another wave that can interact with the original wave. Typically, standing waves are created in the object vicinity, object material can get warm, and around it a new wave field is generated.

Object dimensions measured in wave length do affect things that would happen.
Dispersion study represents an entire scientific field. Go find a good textbook on it to understand it.

 

[Terminator3]

The effect of wave hitting an object is named DISPERSION. The wave itself has no "reflective properties".

Then why corner reflector called so? Or a satellite dish made of metal grid.

 

[davenn]

Then why corner reflector called so? Or a satellite dish made of metal grid.

its called a corner reflector cuz it has a corner angle bend in the reflector, commonly 60 or 90 degrees

the vertical height of the reflector is usually a bit larger than the length of the radiating element.
The length of the sides are usually 1 wavelength minimum and can be out to 2 wavelengths
The reflector can be a solid sheet of metal as in the photo. Or they can be made of spaced rods (or mesh) as in
the drawings. The wavelength of the freq used determines the maximum distance between the rods/mesh
From memory ~ 1/10 of a wavelength is the common max distance


Dave

 

[jiripolivka]

Then why corner reflector called so? Or a satellite dish made of metal grid.

Corner reflector and satellite dish utilize reflected waves so they can combine in a "focus". In this way, like in optics, the wave front is dispersed and the larger area of the aperture combines the power. The effect is named directivity or gain and all antennas take advantage of it.

 

[gn.times]

For example, take the two below situations:

1) A radar wave of wavelength 5mm is striking a perfect metal sphere of diameter 3mm

2) the same radar wave hits a perfect metal sphere of diameter 8mm

In which case/cases the radar wave will get reflected and why?

 

[jiripolivka]

For example, take the two below situations:

1) A radar wave of wavelength 5mm is striking a perfect metal sphere of diameter 3mm

2) the same radar wave hits a perfect metal sphere of diameter 8mm

In which case/cases the radar wave will get reflected and why?

In both cases the wave will be reflected. Intensity of the reflection will be higher by the larger sphere (or any other object).

Why was answered earlier.Any inhomogeneity in the free space caused the wave to disperse. Reflection is the part of dispersion directed to a definite direction (most often, back to the transmitting antenna).

 

[gn.times]

In both cases the wave will be reflected. Intensity of the reflection will be higher by the larger sphere (or any other object).

But in many places I have read that the wave will not be reflected since its wavelength is larger than the object and wave will be reflected since its wavelength is smaller than than the size of object. But I don't know the technical background.
Ex: Millimeter wave penetration through rain droplets but microwave won't in some cases