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Transmission lines revisited

Elbows are like people. They don't mind working together, just not too close.

Don't even think of applying power to a new system without having it carefully checked out with a network analyzer.

As we get deeper and deeper into the DTV world and transmission line systems are added or replaced, more errors are appearing. UHF isn't as forgiving as VHF, and tolerances that were acceptable in VHF result in bad systems at the higher frequencies.

The author and his staff have been experiencing a common problem in new installations. The standard way of getting around obstacles like tower legs or cross-members has been to simply add more elbows. That method still works, but has to be applied with caution. It is not enough to simply call your vendor of choice and order the required number of elbows. If your vendor tells you that is all that is needed, seek a new vendor immediately.

Elbows are like people. They don't mind working together, just not too close. When elbows are simply connected back to back, they have a tendency to introduce unwanted reflections in the transmission line.

Elbow complexes

Placing elbows back to back seems to create mismatch problems that don't exist when they are separated by a length of transmission line. However, such systems can still be used. When it is necessary to use a complex of elbows, have the elbows optimized on the desired channel by the manufacturer. If possible, this should include all short sections of line that have been cut to fit your specific project. If the elbows have been properly optimized, adding a cut length of line in the final project probably won't be a significant problem unless the length is short and simply connects to another elbow.

An ideal approach is to first make sure that the elbows purchased are optimized on channel. Where two elbows are to be connected back to back, have them tuned and match marked in that configuration. If it is necessary to add a short length of line between them or from them to an additional elbow, any problems that result will be immediately apparent when the system is checked out after installation. By the way, don't even think of applying power to a new system without having it carefully checked out with a network analyzer. The author recently checked out a system that had been carefully assembled by a good tower crew. It was found that a piece of hardware had been dropped into the line and was resting on a Teflon insulator. If the line had been energized before that material was removed, a significant cleaning project would have been required and at least one piece of inner conductor probably would have had to be replaced.

If adding a piece of line into the system in the elbow complex appears to cause a problem, it is best to remove the complex, including the elbows and cut sections, and return the whole assembly to the factory. The parts should be match marked to show the factory the order in which they will be used. The assembly can be put back together at the factory and optimized as a unit. This will result in the best possible performance on site.

The biggest elbow complex is normally at the top of the tower where the line is connected to the antenna. It is common to see three or more elbows and some small cut sections at that location. Often, some of the small sections of line will have tuning slugs on the inner conductors to obtain the best match between the antenna, elbow complex and main transmission line. Their location is critical. The components should arrive match marked, with the locations of the tuning slugs clearly identified. If this isn't the case, don't bother to install the system. Just send it back.

It is possible to do some tuning in the field if necessary. One way in which this is done is by adding slugs to the inner conductor of the coaxial line. This adds a reflection that can be used to cancel out an existing reflection. The size of the slug will largely determine the magnitude of the reflection and its location will determine the phase. The problem with using only rings is that they can cancel out a mismatch at one point in the channel and leave a problem at another frequency.

A solution is to place a sleeve on the center conductor that may have a lesser outer diameter than the ring but will exist along a greater portion of the line. The greatest problem here is that a box full of these little patches is necessary to solve the problem by experimentation. That is why the preferred method is to send the offending complex back to the factory. First, they have the necessary range of tuning pieces. Second, they can lay this all out on the bench to work on it rather than trying to add the tuning sections while hanging on the tower. Third, they do this all the time and can get the whole thing done while you are working up the courage to try it in the field.

Another problem is created when an elbow of unusual angle is desired. Normal shelves only contain 90-degree components. If a special angle is required, it will usually require special order and manufacture. Those parts, if needed, should always be optimized at the factory on the desired channel. Another solution to this problem is the use of semi-flexible transmission line. This has been found to be desirable in wideband systems where numerous elbows are necessary. Rather than use all those elbows, the overall performance of the system may be improved by using a hybrid system where the more complex paths are accomplished with semi-flexible lines and rigid line is used for the straighter runs up the tower.

All in all, obtaining excellent performance from a new transmission line system is a strong possibility. It requires some planning and careful coordination with the manufacturer and the installer. Or you can take your chances that totally off-the-shelf products won't cause the IOTs to come flying out of the cabinet when you turn on the transmitter. Let's see now — just how much does an IOT cost these days and just how does one explain that to the front office?

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