RF Systems
By Walter Mamak
If you’ve recently upgraded your station’s equipment, you’ll need to optimize your RF system before the station goes on the air. Of course, system performance has many facets. This article focuses on the physical layout of a typical passive RF system and the technical concerns of optimizing VSWR performance.
Figure 1. A typical plot produced by scattering analysis.
Having a detailed plan right from the start will move your system performance testing quickly and keep costs down. But even the best system optimization procedure will not compensate for non-optimized components, poor system design or faulty installation processes. Always insist on trained, knowledgeable engineering technicians to optimize and monitor installation of all components. Keep in mind that these services are available to broadcasters without in-house expertise.
A few words of caution: It is essential to coordinate the activities of station engineering and management, equipment suppliers, consulting engineers, tower vendor, installation crew, and the system optimization service, to make sure that everyone is on board.
The upgraded facility
The facility discussed here is a high-power facility typical for a U.S. broadcaster, upgrading either within NTSC analog or to DTV, and providing coverage on a UHF channel.
A high-power broadcast facility requires an effective radiated power (ERP) of 1500 kW to cover the market. The transmitter in the upgrade discussed here has a total power output (TPO) of 60 kW. This facility also has special radiation-pattern requirements. Therefore, it will require a high-power, top-mounted, 25-gain directional antenna with a 6-1/8-inch, 75 V rigid transmission line.
The new antenna system will be added to an existing tower already loaded with broadcast, cellular and microwave equipment.
Antenna and transmission line
After selecting the antenna, you must consider the physical details of the entire passive RF system and its installation at the site. The ultimate performance of the system depends on proper planning in this initial phase.
There are several factors that determine the position of the antenna on the tower, including availability of space, antenna pattern and gain, the desired signal coverage, and wind and weight load. The other equipment on the tower may also affect the placement of the antenna and transmission line.
At this Mt. Wilson, CA, site, the center tower has a side-mount antenna with a rectangular waveguide transmission line. Because of the complexity and routing of the line, special attention had to be paid to the optimization process. Photo courtesy Andrew.
The proper mechanical and structural interface between the antenna and tower varies, of course, with the characteristics of the selected antenna and the existing tower. Generally, this interface entails fastening the antenna to the tower structure, providing adequate space for the antenna input and transmission line connections, routing the transmission line to the antenna, and positioning the antenna to attain the desired coverage. These procedures require detailed information about the antenna and the tower. Information on structural loads and mounting interface details for the antenna are available from the manufacturer. But, for an existing facility, the only way to be sure that the tower information is correct is to inspect the site and obtain a structural analysis, which you should do before designing the system.
Next is antenna orientation, which involves determining the azimuth position of the antenna, tower and tower mounting bolts for a top-mount antenna.
Other steps in the installation of a top-mount antenna include confirmation that the transmission line will be correctly routed to the antenna.
In the case of a side-mount antenna, perform a “scattering analysis” (see Figure 1) to obtain the best RF signal coverage. Antenna manufacturers typically have developed proprietary software for this purpose. But, to obtain accurate results, this software requires accurate, detailed information on the tower’s physical details. Make sure the antenna is correctly located to minimize pattern degradation by the surrounding tower structure, transmission lines and other tower appurtenances. Side-mount antennas can be leg-mounted or face-mounted. This selection is based on the required azimuth pointing direction, the available space on the tower to clear other obstructions, and the structural requirements of the tower. Broadcasters should also ensure tower bracing and/or other tower appurtenances do not obstruct the correct routing and support of the transmission line for weight, ice and wind load.
Transmission line considerations
When choosing transmission line, you must balance cost of the line with the mechanical, structural, thermal and practical considerations of routing the line from the transmitter to and through the tower and to the antenna input.
Three installers placing a side-mount, low-power, circularly polarized antenna on an existing tower holding existing antennas. Photo courtesy Andrew.
A detailed plan for the transmission line system includes the following items: a complete bill of material; design drawings, with all components positioned in the system; a plan for routing the transmission line from the gas barrier in the equipment building through the ice bridge and up the tower to the antenna-input flange; the location of the flange, hangers and guides; and, finally, a plan for installing the line on the station’s tower.
For the application discussed here, a single-channel, rigid transmission line was chosen. Other choices are available, including broadband rigid line, circular or rectangular waveguide, or flexible coaxial cable. The choice depends on the parameters of a particular installation, such as the need for broadband service, power-handling capacity, tolerable level of line attenuation, tower wind-load limitations, structural-mounting details and overall cost effectiveness.
Optimizing the aforementioned parameters is no trivial undertaking. Even at the most basic level, there are trade-offs:
1. Physical size and RF system loss vs. tower structural loading
2. Ease of installation vs. optimization of power output and attenuation
3. Structural support type and location vs. flexibility for thermal expansion/contraction
4. Mechanical routing simplicity vs. reduction of system components
5. Location of structural supports coordinated to the existing tower’s structural steel
6. Strategic location of fine matcher sections (tuners) to optimize system performance
System testing and VSWR optimization
Once you’ve selected all the major components for the tower, you must be sure that they are up to the task of integrating into a top-performing, long-serving system. After all, tower installation is a lengthy, labor-intensive, costly process.
When the antenna is delivered to the site, ask the following questions: Were all components delivered in good working order? Have the RF tuners located throughout the system been optimized? Were the electrical and mechanical ground checks of the antenna performed? An engineering technician should supervise unloading of the antenna and perform detailed visual, pressurization and VSWR sweep tests to assure that it is in factory-quality condition.
After the transmission line system is completely installed, best current practice requires system sweep and optimization services. To obtain the best results, inspect the transmission system after installation to ensure structural and mechanical integrity prior to the final system sweep. Inspections should be done from the input flange of the transmission line system to the top of the tower.
Follow-up inspections should be conducted at least once per year — more frequently in a high-humidity or corrosive environment, or in high-wind areas.
The following steps detail the inspection points for all transmission line systems:
1. Check the complete transmission line system for compliance to design drawings.
2. Verify that the line is plumb to 1/8 inch per 100 feet.
a. Verify free movement of spring hangers and transmission line.
b. Verify proper extension of spring hangers.
4. Check all mounting hardware for compliance to torque requirements.
5. Check field-cut sections for proper fit by looking for binding or stress between adjacent sections of transmission line.
6. Check for proper clearances between transmission line sections and the tower’s structural steel or other obstructions that may damage the transmission line under extreme conditions of thermal expansion/contraction.
7. Verify proper seating of fine-matcher hardware.
8. Verify pressure integrity of the system by performing a timed pressurization drop test.
If you chose a waveguide for your installation, follow these additional inspection points:
1. Check constant-force springs for proper extension and tightness of hardware.
2. Inspect the bearing flange; verify bolt torque.
3. Verify that one vertical hanger is installed for every two sections of waveguide above the specified level in the manual.
4. Verify that the guideline pins are aligned in the same plane. Note: To avoid additional personnel costs, you should pressurize the systems and hold pressure for at least 24 hours before beginning the system sweep optimization services.
After visually and mechanically verifying the transmission line installation, perform RF sweep measurements and record VSWR according to the manufacturer’s procedure. Several independent-contractor engineering services offer system optimization. Additionally, you can hire site technical advisory service field engineers with experience in new equipment installations to manage the installation process. Typically, RF sweep measurement is an interactive process to optimize the system’s return-loss performance. Use a network analyzer to determine the characteristics of the transmission line and adjust the system’s fine matchers to achieve optimum performance. This is accomplished by attaching transitions (adapters) to the transmission line near the gas barrier. This adapter is tuned to better than -50 dB (return loss). Initial data is then taken in both the frequency domain (return-loss sweeps) and the time domain (time-domain reflectometry — TDR).
By using modern TDR techniques over broad bandwidths, you can identify the most severe mismatches because the tuning process is usually initiated at these points with the fine matchers designed into the system layout.
Broad-bandwidth, high-resolution TDR measurements check for anomalies in the installation such as pinched gaskets, split bullets, loose hardware and elbow complexes that exhibit high VSWR. You might have to replace components or correct faulty installation procedures before finalizing system sweep measurements.
Proceeding in this manner, optimize each of the fine-matcher locations to yield the best system VSWR. When complete, re-inspect the system for pressure integrity. Usually, you can expect long transmission line systems to maintain a 0.5 psi pressure drop over a 24-hour period.
The consultant or service provider will perform an array of measurements, which can include the following:
1. VSWR and return-loss plots with each 6, 25 and 100 MHz span.
2. TDR (VSWR) format with each 6, 25 and 100 MHz span.
Final thoughts
A truly optimized system includes good planning, a detailed design, high-quality products, certified installation crews, and qualified system optimization services. Pre-test inspections are essential. Skimping here may result in costly on-site expenses for crews and RF technicians, and ultimately have a negative impact on station performance.
Walter Mamak, PE, is product engineering manager for Andrew, broadcast products division.
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