7. ) SUBJECT : BEST POLARIZATION OVER THE WATER
Question :
Regarding with the best polarization in microwave links over sea water; I’ve learn is Horizontal don’t remember though. Can somebody send me theory documentation about that?.
Anwer:
This is news to me. In link design over water the major issue is Reflections since water is a plane reflector. For Frequencies below 10Ghz Polarization is considered only for frequency planning purposes. Polarization is usually an issue in links at frequencies above 10Ghz. Horizontal stretcxhing of rain drops usually make the horizontal polarizated signal get slightly more attenuated than vertical
The underlying principle is that a wave travelling over a surface has currents absorbed by the surface.
A horizontally polarised wave has more ground absorbtion than a vertically polarised wave.
A vertically polarised wave over water has more absobtion than a horizontally polarised wave
The issue here is conductivity of the surface. The more conductive the ground is, the stronger the reflection will be (and therefore cause more multipath problems).
Reflections from sea are thus more critical than reflections from terrain with vegetation.By selecting vertical polarization for over-the-water links, you can achieve 2-17 dB reduction in surface relection in comparison with horizontal polarization.
"conductive - having the quality or power of conducting heat or electricity or sound; exhibiting conductivity"
A vertically polarised wave over water has more absobtion (less reflections) than a horizontally polarised wave. A conductive surface will absorb a wave, not reflect it. Hence a vertically polarised wave will have less reflections.
Bottom line - verticle over water
your statement: “A conductive surface will absorb a wave, not reflect it” is contradictory to any known electromagnetic wave propagation theory. I will try to illustrate my point without getting into beautiful Maxwell’s equations, Faraday’s equations and other fun things.
Generally speaking, for a plane electromagnetic wavefront (EI)incident on a plane boundary between two different dielectric media having different refractive indices (n1 and n2), part of the energy (ET) will penetrate the second dielectric (refraction) and part of it (ER) will be reflected back (reflection).
ER = EI x (n1-n2)/(n1+n2) ET = 2EI/(n1+n2)
Consider a plane wave falling normally on a flat metallic surface and hence, the case of refection from a (perfect) conducting surface:
ER = - EI ET = 0
For good conductors n2 is a large number, tending towards infinity in the case of a perfect conductor. Therefore, no field is transmitted into the metallic medium, and the field is fully reflected back with a 180º change in the phase of the wave (due to the minus sign).
This is exactly the principle waveguides use for electromagnetic wave propagation. They must have perfectly reflective and conductive coating on the inside to function properly.
Conclusion is that more conductive the surface, more electromagnetic waves will be reflected from it.
The extreme case is a totally metallic surface in which absolutely ALL of the energy gets reflected.
TEST PROCEDURE FOR DRY TESTING ODU OF SRAL RADIO
The correct way to test the SRAL units is as follows.
1. You will require two -48VDC power points for the two SRAL IDU units.
2. You will require two IF cables to connect the IDU’s to the two ODU’s
3. There needs to be a RF connection between the two ODU’s to complete the link. This is also known as a virtual link. If the test is done without this link you stand a good change in damaging the ODU because of high VSWR levels.
The virtual link for a 7GHz SRAL test scenario consists out of the following.
2 x Waveguide to Coax Transitions (Flange = UDR84 / Waveguide = WR112 / Coax type = Type Female(FM).) (Order number from Andrew = C122LNSG)
2 x RF attenuators (0 to 12GHz) 30dB N-Type Male to Female (To obtain a nominal receive level of -40dBm (+/- 5dB)) (Order number from Huber+Suhner = 22550187)
1 x RF test cable 1meter (Sucotest Straight plug N-type male to male (DC up to 13GHz)) (Order number from Huber+Suhner = 23005046)
The virtual link will be arranged in the following way.
Starting from left to right.
1. #1 ODU (UDR84) to #1 Waveguide to Coax transition (UDR84)
2. #1 Waveguide to Coax transition (FM) to #1 RF attenuator (M)
3. #1 FR attenuator (FM) to #1 RF test cable (M)
4. #1 RF test cable (M) to #2 FR attenuator (FM)
5. #2 FR attenuator (M) to #2 Waveguide to Coax transition (FM)
6. #2 Waveguide to Coax transition (UDR84) to #2 ODU (UDR84)
8. ) SUBJECT : EFFECT OF TOWER GUY TO MICROWAVE LINK
Question:
Can we really have a problem in the field when a microwave antenna is installed on a tower, just below the attachment point of the guys, and that the azimuth of the antenna is about the same as the azimuth of one guy ?
If yes, what is the minimum angle to respect between the antenna azimuth and the guy azimuth compared to the antenna beamwidth for example ?
If yes, is there a minimum distance to respect between the antenna and the guy in order the negative effect disappears ?
What about the possible accumulation of ice on the guy ? The ice can surely becomes a significant obstruction to the antenna!
Is there any practical or theoretical standard about this phenomenon ?
Answer:
When you use microwave parabolic reflector or horn antennas, you should be aware of the 100% path clearance in the near-field area (also called the Rayleigh zone) where the radiated waves are travelling inside a cylinder that continue the dish circle (the dish is then the radiating surface of a planar wave that is a zero-loss propagation mode)
The distance of the Rayleigh zone is given by a simple equation:
dR= D²/2*Lambda where:
dR: distance of the Rayleigh zone
D: antenna diameter
Lambda: wavelength
in the Rayleigh zone the Radiated Power Density is constant along the distance (zero-loss propagation)
the next zone is the Fresnel-Kirchoff zone that extend between dR and dFK with
dFK= 2D²/Lambda
in this zone the Radiated Power Density is fluctuant
the last zone is the Fraunhoffer zone where the wave is now spherical and the Radiated Power Density decay according the 1/d law; yhis is the far-field zone
Conclusion:
Free space loss is computed with assumption of the far-field propagation.
You should avoid to place any obstacle inside the Rayleigh zone. Otherwise big impact in term of:
RPE (radiated pattern enveloppe)distorsion
Cross-Polarisation degradations
Another thought....
With the guy wire returning to ground, it would act as a parasitic element and absorb the radiated energy. The net result would be a field strength distortion / reduction in the desired direction.
It looks like this thread was done some time ago, but I thought I would add one other interesting point.
We once had a guy going through a u-wave path that seemed to either reflect or retransmit the signal causing interference not in the adjacent hop, but in the next hop around the loop (we were using the standard 2-frequency plan). It made no sense at the time, but when we replaced the guy with a non-metallic material, the interference went away.
My rule has always been a minimum of 10 degrees for highly directional microwave, just to make sure you are nowhere near the guy.
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