Tuesday, November 2, 2010

Microwave antenna alignment

Simplifying the Path Alignment of Microwave Communication Systems:
Wherever microwave links exist, the path between antennas has always required accurate antenna alignment. This process requires highly trained tower crews to physically align the antennas as well as ground technicians and sophisticated, expensive, and complex test equipment to monitor the results. The process of optimizing the transmission path of microwave communication systems is about to undergo a significant development in process, simplification and cost benefit without compromising performance or accuracy. The process can now be accomplished with the use of the Path Align-R™ , Models 2200–2241, ยต wave Antenna Path Alignment Test Sets from Pendulum Instruments, formally XL Microwave. Tower installation crews can now perform the entire alignment process themselves, up the tower, at the antenna,
without the need of additional ground technicians, equipment, or indeed, even the waveguides or radios installed.

The Traditional Process:

The traditional process, more typically described as “microwave path alignment,” requires the use of a transmitter and a receiver located at each end of the microwave link. The transmitter generates the signal that passes through the transmission line to the antenna, which radiates the signal over the free space link. The signal propagates towards the other end of the path and is received by the antenna, forwarding the signal through the transmission line to the receiver, connecting the two sites. The receiver processes the signal,producing information on its value relative to the amount that was originally generated at the transmitting end. If the antennas are optimally pointed to each other (aligned), the largest concentration of signal (main beam) is emitted and received, reducing the free space attenuation of the signal. Provided the transmission lines do not lose too much of the signal between the antennas and the radios, maximum signal transfer is achieved. If the antennas are not optimally aligned, then signal transfer is degraded and received dynamic range is lost.

There are several steps involved in the traditional preparation and process of aligning the antennas of a microwave communication system. These steps may include making sure the cable or waveguide transmission line was properly installed, with minimal RF reflection of the microwave signal; that each antenna polarization is properly setup; and that the transmitter output power is calibrated. A voice communication link between the personnel inside the radio room of each site and the tower technicians,located at each antenna, needs to be established using two way mobile radios or cellular phones. Some spread spectrum radios have an order wire over which to communicate, however, communication to each of the tower technicians will still need to be completed. The engineering profile is reviewed to determine the expected RSL (receive signal level) for the path under test and any adjustments for output power are applied.Once this setup is complete, the tower technicians are instructed to commence the adjustment of the azimuth alignment (bearing) of the antennas, one at a time. The antennas are panned over their azimuth profile and readings of the receiver signal output power are taken. Careful observation of the output power reading is necessary to distinguish the antenna side-lobe to main-lobe response. Once the maximum signal is achieved,the antennas are aligned for elevation optimization. It is evident that the communication between site to site and tower technician to receiver technician needs to be continuous and clear to ensure that the antennas optimum alignment setting is achieved.