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: Obstruction of a microwave antenna by a tower guy
01-15-2004, 03:42 PM
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 ?
Thank you all in advance for sharing your practical experience with me.
01-30-2004, 03:31 PM
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= D?/2*Lambda where:
dR: distance of the Rayleigh zone
D: antenna diameter
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
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
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
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.
07-05-2005, 03:34 AM
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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