Broadband coax

Television and radio broad- casters face an expensive problem if they want to multiplex additional channels into an existing single-channel transmission system. Among the equipment that will need to be added or modified to facilitate such a change is the coaxial transmission line. The bolted flange joints used to connect the individual sections of rigid line cause small reflections that add constructively whenever the length of each section is a multiple of one-half of the electrical wavelength at the frequency of operation. This constructive addition of small reflections may result in unacceptably high VSWR on the transmission line if the run is sufficiently long. The result is that for a transmission line run comprising multiple bolted sections of single length there are channels that are unusable because of a high VSWR.

With this in mind, the transmission line manufacturers are careful to select the proper length for the desired operating channel. When multiple channel performance is required it is often necessary for the manufacturers to employ a scheme whereby the length of the individual sections vary throughout the transmission line run, thereby avoiding the constructive addition of small reflections. In the past the only option available when broadband performance was required from an existing run of transmission line was complete replacement.

EXH has developed a patent-pending method to eliminate the constructive addition of the flange reflections in existing rigid transmission line. This method consists of opening the flange joints at regular intervals and inserting custom-designed transmission line sections throughout the length of the run. These custom sections are typically inserted between every three of the existing sections and they are nominally two-feet long. The material cost is a small fraction of the cost to replace the entire run. In addition, because of the relatively small size of the custom sections and the fact that the original run does not have to be removed from the tower, the rigging charges are also reduced to a fraction of the cost to replace the entire run. The ease of installation also facilitates a staged project completed at night if a station must be kept on the air during the modifications. Finally, the electrical performance, once the method has been applied, rivals that of some of the best broadband lines available.

The first application of the EXH solution occurred at the WCSC transmitter site in Charleston, SC. The team from Jefferson Pilot Communications had installed a premium run of 7 3/16”, 75V line. The design channel was 52, and so it was cut for a single channel at 19.75 feet per section. There were 85 sections of this line reaching to the centerline of the new panel antenna at 1700 feet. Two years later the requirements had changed. The new requirement was an operating channel of 47 with additional channels multiplexed into the system on channels 34 and 49. Fortunately, the panel antenna and the elbow complexes in the system had been designed to accommodate this wider bandwidth. The coaxial line, however, was narrow-banded and channels 34 and 47 presented a VSWR of approximately 1.6:1.

Figure 1. The final measured data for the WCSC transmission line at Channel 47

The first step in applying the EXH method to broadband transmission line is to collect benchmark data and create a mathematical model of the existing system. D. W. Sargent Broadcast Service provided both broadband VSWR and time domain reflectometry (TDR) measurements of the WCSC transmission line. This data was then used to determine the magnitude and location of the small discontinuities that created the VSWR characteristics of the existing line. These values were then plugged into a wave transmission matrix model. Once the theoretical data was adjusted to match actual measured data, the model was modified to simulate the insertion of the custom lengths that would break up the addition of the flange reflections and result in a line with broadband VSWR performance. In the case of WCSC, the model showed that a VSWR of 1.2:1 could be achieved using custom sections inserted every three line-lengths along the vertical run. The total number of new line sections was 27 and their length ranged from 13 to 22 inches.

The actual installation of the line sections was done over the course of two weeks. The station supervised the installation, while D.W Sargent and EXH provided on-site technical support. Tower & Communication Services handled the rigging. They removed the fourth and fifth vertical section of the coaxial run, inserted the first custom section, lowered the next three vertical sections, and then repeated the process for the entire length of the vertical run. During this process the line was monitored with a network analyzer to ensure that all of the bolted flanges were seated properly. While the vertical run was being modified, the elbow complex at the base of the tower was returned to the manufacturer for re-optimization over the new frequency band. Finally, a new cut section was made at the tower top to compensate for the difference in overall line length.

Figure 2. The measured data for the transmission line over the band from channels 34 through 49

Discounting weather days, a five-man rigging crew completed this modification in only five days. The short lengths of the new line sections allowed the riggers to make the modifications using only the tower elevator and eliminated the time and expense of having to rig the tower with a winch. The final measured data for the transmission line at Channel 47 is shown in Figure 1. The measured data for the transmission line over the band from channels 34 through 49 is shown in Figure 2. The measurements include the effects of six elbow miters that help make up the horizontal run of transmission line, and the transitions used in the measurement. The inclusion of the transitions in a measurement of this type is significant because it is impossible to perfectly match them over wide bandwidths. They add a margin of error of 1.02:1 VSWR. Even so, only one small spike reaches a 1.23:1 VSWR in the entire band.

An understanding of what the EXH solution does not do is important when making a decision to re-task the transmission system. It does not correct for old transmission line that should be replaced for mechanical reasons. It does not affect the power handling capacity of the transmission line in either a positive or negative manner. It also does not correct the poor VSWR of any narrowband devices in the system. These devices may include the antenna, elbow miters, gas barriers and filters. The solution does, however, provide an alternative for television and radio broadcasters who for any reason need to change the operating frequency of their coaxial transmission line.

Brett Grandchamp is the vice president of engineering for EXH.

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