Cross polarization Antenna

Hi All

could you please to supprt me for the following quastion

Why the rain is affect on antenna discrimination value ( decrease with rain fading increase)

If you are asking why your received signal power decreases when it rains, then the answer is simply that you no longer have a pure medium (air) that the signal is traveling through. Things like water droplets in the air will absorb some the signal, or refract (scatter) it. This causes less signal to reach the antenna, compared to the air on a normal day (minimal disturbances).

Mostly important in satellite-communications but also experienced in terrestrial microwave links, antenna depolarization by precipitations has been studied since ~1950s.
A good antenna can have its polarization ratio ( direct/orthogonal) better than 30 dB. This is, however, only valid in a clear space.
Electromagnetic wave propagating in space, when it passes through a dense medium like water droplets or snow flakes, suffers a dispersion. The conductive/dielectric particles absorb some of wave energy causing signal loss. Another effect is reflection to other directions, and refraction. Those effects take the energy from the original wave and turn its vector, changing the wave polarization.

Mostly in satellite communications, power budget on the downlink is adjusted so the receiver system signal/noise ratio is higher than ~10-16 dB but lower than 30 dB. This way the orthogonal polarized waves are suppressed enough to be separated. The required system feature is frequency reuse- two independent streams of information can be transmitted at the same frequency but with mutually orthogonal polarizations.
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During precipitations, the desired signal suffers a loss while the undesired (transmitted with the orthogonal polarization) signal, due to dispersion in the same precipitation cell, becomes strong enough to be received as an interference.

For more actual data, find good satellite-communication textbooks and CCIR microwave-propagation data.

Hi

thanks for you reply but my quastion was totaly difffernt:

if i have antenna with dual polarization the manfacture provide cross polarization discrimination value betwee Vertical and Horizantal for example 35dB and in raining condition this value is decrease to like 30 dB and this increase the interference between Vertical and Horizantal signal if they use same channel frequency fo same link

Rain does not consist of round droplets but have more elliptical size and are tilted (due to wind).

So when you excite a rainy area with a vertical polarized wave, the induced current in the rain droplets may have a (small) horizontal current component. This reradiates and creates a horizontal polarized EM wave component. This component couples with your horizontal polarized antenna.

Dear Vessamm77:

I still do not see your question. I tried to explain how a wave with a defined polarization, upon interacting with a lossy/dielectric material like precipitation, can be split in a part having an orthogonal polarization.

If you want a physical explanation, find a new textbook where you can find it. Dispersion is a quite complex phenomenon and cannot be described in one page.

In real nature, depolarization effects differ in various rain intensities, temperature and wind patterns; also location on Earth is important. This is why CCIR gradually presents Recommendations for Communication system design where updated data can be used.