A Microwave Antenna Path-alignment Test Set

A Microwave Antenna Path-alignment Test Set

XL Microwave Inc.
Oakland, CA

Microwave links require accurate path alignment to ensure proper operation. This process has traditionally required highly trained tower crews to physically align the antennas as well as ground technicians and complex and expensive test equipment to monitor the results. A new test set has been developed that simplifies this task at a low cost without compromising performance or accuracy. Tower installation crews now can perform the entire alignment process on the tower without the need for additional ground technicians or equipment. The associated system's radio equipment and waveguide feeds do not even need to be installed.

The Traditional Process

The traditional antenna alignment process requires the use of a transmitter and 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 free space to the receive antenna. If the antennas are optimally aligned, the largest concentration of signal (main beam) is emitted and received and maximum signal transfer is achieved. If the antennas are not aligned properly, the signal transfer is degraded and dynamic range is lost.

Several steps are involved in the process of antenna alignment in a microwave communications system. A voice communication link between the personnel inside the radio room of each site and the tower technicians located at each antenna is required using two-way radios or cellular phones. Some radio systems contain an order wire link for these communications; however, communications to each of the tower technicians still must be established.

Once this setup is completed the tower technicians may begin the adjustment of the azimuth (bearing) of the antennas (one at a time). Careful observation of the output power is necessary to distinguish each antenna's side-lobe to main-lobe response, as shown in Figure 1 . Once the maximum signal is achieved, the antennas are aligned for optimum elevation. Throughout this process communications between the two sites and between the tower and receiver technicians must be continuous to ensure optimum antenna alignment.

A New and Improved Method

A new method utilizing a recently developed high performance test set permits antenna alignment to be accomplished without the use or presence of the system's radio equipment. There are several good reasons for not utilizing the actual system radios to accomplish the antenna alignment process. First, the system radios may not be available at the time the antenna test has been scheduled or their reliability may be questionable. Also, Federal Communications Commission permits for the radios may not have been granted. In addition, if the anticipated path is questionable, a quick, cost-effective method is required to test the link prior to the significant investment of constructing permanent towers and purchasing radios and other equipment.

The Path Align-R™ model 2200 microwave antenna path alignment test set has been specifically designed to quickly and accurately optimize the transmission path between two microwave antenna sites. The process can be completed without the need for the individual site's system radios or other equipment. The battery-powered Path Align-R test set directly drives the site's antenna, providing an indispensable tool for antenna site installation and maintenance personnel. Figure 2 shows a simplified block diagram of the test set. Table 1 lists the model 2200 test set's key specifications.

The Path Align-R test set consists of two identical portable units in weather-resistant, instrument backpack measuring 3.5" x 8.375" x 13.1" and weighing only seven pounds each. Four tunable operating frequency bands are available, thus allowing the test technician to choose the appropriate frequency to match the antenna's required operating frequency. The antenna can be adjusted for minimum path loss by utilizing a test unit at each antenna site. The path loss is displayed on the test set's front-panel meter and/or an external instrument, such as a pocket digital voltmeter. The Path Align-R units enable test technicians to talk to each other during alignment over the antenna link in full-duplex FM using the included headset.

The Path Align-R test set provides 100 dB of effective dynamic range, the result of the -100 dBm sensitivity of the individual receivers and the 0 dBm output of the transmitter section. The four frequency bands provided in the standard test set are 1.8 to 2.5 GHz, 5.8 to 6.6 GHz, 11 to 12 GHz and 18.1 to 19.4 GHz. The operator can tune within these bands by means of thumbwheel switches to within 1 MHz of the required frequency. An antenna system with an operating frequency outside of the unit's frequency band edge (for example, 6.8 GHz) may still have its alignment correctly adjusted as long as the antenna system can operate at both the link frequency and a nearby frequency within the test set's operating bands (for example, 6.6 GHz).

A liquid-crystal display indication of direct path loss, within 0.1 dB resolution, is automatically updated every 300 ms and can quickly detect subtle changes to the antenna response, thus permitting small adjustments to the antenna's azimuth or elevation for optimum signal transfer. Communications between sites and from the radio room to the tower top are significantly improved with a full-duplex FM voice channel. One tower technician can speak to the other using the included headset without having to key a radio. Also, voice communication is enabled immediately after setup and activation - the antennas do not need to be fully aligned for the voice channel to function.

The test set's output is from a synthesized internal signal source that provides an accurate and stable test signal. Comparisons to tests run using an HP 8360 synthesized source and a full-featured HP 8594E spectrum analyzer produced path loss agreement to within 1 dB. However, the Path Align-R test set is priced far below that of a separate synthesized signal source and spectrum analyzer combination. In addition, the cost of communicating between sites can become considerably expensive when using cellular phones or other means. Additional savings are achieved because the required test personnel can be reduced to just two tower technicians and the cost of shipping bulky, expensive test equipment to the sites is eliminated.

Conclusion

The difficulties of scheduling an antenna alignment with the system's radios can cause significant delays. Furthermore, traditional test methods can result in significant test costs. The Path Align-R model 2200 test set offers an accurate and cost-effective method of microwave antenna alignment that is easily performed by two technicians without the use of system radios. Additional information may be obtained from the company's Web site at www.xlmicrowave.com.

XL Microwave Inc.,
Oakland, CA
(510) 428-9488.