Taming the RF system

The tuning of combiner or diplexer assemblies is a job for the factory representatives.

As is well-known and often discussed in laborious detail, most stations are either installing new RF systems to accommodate DTV or are still cleaning up the new DTV system. Part of that work obviously includes new RF plumbing inside the building as well as, in most cases, one or more new transmission lines and antennas.

The good news is that the combiner or diplexer assemblies to be installed usually don't require further tuning. Those units are tuned at the factory and are normally installed in exactly the same configuration. If not dented or otherwise damaged, they will not usually require field adjustment. If they are subsequently determined to not function properly, their tuning is a job for the factory representatives. They can be field adjusted with a network analyzer but they are a bit like a directional AM array in that if you start making adjustments without knowing what you are doing, they can be taken from needing a minor adjustment to a total mess.

The rest of the equipment inside the building, consisting primarily of the transmission lines between the transmitters and passive components, can be tuned for proper impedance match. If the passive hardware is new, the input and output impedance values are usually right on. In that case, the short runs of transmission line or waveguide will not cause problems. It is more difficult to incorporate older existing equipment that has either drifted a bit over the years or was never tuned well in the first place.

In the worst of systems, there may be a need to add an additional fine matcher or two into the system. Those can then be adjusted with a network analyzer to bring the system impedances back to normal. If only small problems are observed, two different approaches can be taken. First, just forget about it. Most transmitters will tolerate a minor impedance mismatch without causing trouble. For this purpose, a minor impedance mismatch might be considered something on the order of a VSWR of 1.1 to 1.15:1. The short runs involved won't cause reflections that would appear as ghosts in the picture.

The other response is to tune the interconnecting lines to eliminate the mismatch problems. This is normally done by adding slugs to the waveguide or rings on the center conductor of coaxial lines. The slugs and rings are sized and placed through a process of trial and error — mostly error. A network analyzer is almost a necessity for this work. It is attached to the system using a tuned adapter and the system response is measured. A tuning element is then inserted in the system and a second measurement taken. The process is repeated until the right size of tuning element has been correctly placed and the element is fixed in place. It doesn't take as long as it sounds and does result in a total system with flat response. It is often surprising to see the improvement in the transmitter tuning when the little irregularities in the system are corrected. One major note of warning is important at this point. The adapters for connecting the network analyzer must be known to be good, usually not simply an off-the-shelf component. Otherwise, the tuning simply matches the line to the impedance of the adapters. That can actually cause more of a mismatch than if you simply left the lines alone.

The above references to a network analyzer are meaningful. The instrument offers abilities that simply weren't available 20 years ago. The ability to measure the input VSWR to the system is only a start. With a modern vector analyzer, it is possible to look at only the antenna or only the transmission line system without moving the test equipment from the input. This facilitates the final tuning of the input match between the transmission line and the antenna as well as any necessary tuning of the transmission line components. This is particularly important for waveguide installations where tuning sections are involved. Using the time domain functions of the analyzer, it is possible to observe the reaction to the tuning in the area where the tuning is done, not simply at the input to the waveguide. That eliminates the need to repeatedly break the waveguide and insert a dummy load, greatly shortening the time needed to optimize the waveguide system.

With a good network analyzer, an experienced engineer not only can determine a problem at a bad flange, but can define the problem down to a particular insulator in a section of line. The only bad point is that a novice with the network analyzer will be quite limited in doing such an analysis.

Accurate power measurement has always been a problem. With traditional calorimeters, temperature and flow monitoring must be accurate and the proper correction factors known for the coolant in use. That problem has been greatly eased by the use of the current series of power meters using various sensors and probes. The main problem with such units is the initial calibration to accurately determine the degree of coupling between the probes and the transmission lines. This usually requires a network analyzer or similar device.

One new item that works well for both DTV and NTSC and does not require field calibration is the new series of power metering equipment from Bird Electronics. That equipment measures the power levels for complex waveforms accurately using a thru line approach. The equipment is calibrated at the factory and is installed in a line section in the same way as the sampling sections used for years. The difference is that the older systems do not measure the more complex signals, including DTV, accurately. In addition, the new system presents digital outputs that can be used in sophisticated monitoring systems to keep track of the system performance.

Also, Dielectric now offers the VSWR Vision for monitoring the antenna and transmission line system. The equipment samples forward and reverse power and maintains a file of the system performance. Periodically, the information is downloaded by the manufacturer and analyzed by their software. First, the equipment will place the necessary phone calls to alert both the station staff and Dielectric that a VSWR problem has occurred in the antenna system. Perhaps more importantly, the ongoing analysis contains trend-spotting capabilities that will alert the station of a worsening condition that may cause future trouble.

As an example, a station monitoring by Dielectric was alerted by a small, gradual increase in VSWR to the onset of the problem that would inevitably have led to a burnout. The monitoring system allowed the problem to be corrected without any loss of airtime for the station other than for the maintenance.

Modern equipment is so stable and dependable that stations are now accustomed to allowing the transmitter to sit quietly in the country and run without constant supervision. However, failures are going to occur in the best of systems. Equipment such as the systems mentioned above will monitor the power and the antenna system performance continuously and provide the station's technical staff with information that will help to avoid catastrophic failures. We all know what catastrophic failures are. They are the ones that cause the building lights to dim and the transmitter room to become suddenly so quiet that the screams from the front office as the ratings suddenly drop are very easy to hear. It is highly advisable to do everything possible to maximize the dependability of the transmitting plant. Finding the money to install a modern monitoring system will be much easier for management to accept than being off the air for a couple of days while repairs are made to the antenna system.

Don Markley is president of D.L. Markley and Associates, Peoria, IL.