DTV antennas

In the November 2006 column about the use of antenna ranges to measure patterns, I stated that Jampro had the only full-size range in use. I have since found out that ERI also has a range. That range has existed for several years, and many FM antennas have been tested there. Following the acquisition of the TV transmitting antenna and rigid transmission line departments from Andrew, the company expanded the range to permit television measurements.

The inner-workings of an antenna range

For engineers under the age of 50, an explanation of antenna ranges might be in order. For that purpose, I'll refer to the range shown in the photo at right. While this particular range belongs to ERI, the general functions of the various on-site facilities are similar to most ranges.

The purpose of an antenna range is to measure the antenna's pattern on the structure where it will ultimately be mounted. To that end, ERI uses several tower sections for measurements. When an actual section of the proposed mounting structure cannot be delivered to a site, it is necessary to fabricate an equivalent tower. The buildings near the top of the photo are used for that purpose. Those and other manufactured tower sections can be seen stacked near the access road.

Two test sites with towers make up the heart of the range to test the antennas. The towers are erected on large turntables so they can be rotated. The antennas are mounted on the tower sections in what will be the final configuration along with any other conduit, transmission lines, ladders and other hardware that might be in the aperture.

Again, the antennas receive the signals from the source. The source antennas are located off the photo on the left side. The buildings at the test locations contain the necessary instrumentation to measure the received signal and to plot that signal versus azimuth as the antennas are rotated.

Some engineers prefer to see the measured patterns on a range using full-scale components. On older ranges, the antennas could also be rotated on a large stand to measure the vertical pattern. This method has mostly disappeared, with the vertical patterns either simply calculated or measured on a model.

The photo at left shows an anechoic chamber with an antenna being tested. In this system, a model of the antenna is constructed and placed in the chamber. The chamber has one open side where the signals from the RF source are received. The material on the chamber walls, ceiling and floor absorbs all the reflected signals as well as the unused energy from the source. The construction of that material can be seen on the left side of the photo.

The purpose of the chamber is to ensure that the signal measured by the antenna comes directly from the source rather than from any reflections. Again, the tests are performed using the antenna as a receiver because it is significantly easier to construct the system in that manner.

Types of DTV antennas

DTV antennas vary in size, configuration and operation. For low-power, low-band VHF use, many operators use Yagi or log-periodic antennas. Such antennas make more sense for ERP values up to a couple of kilowatts rather than erecting a five- or six-bay batwing antenna. When the power starts increasing, the most common antenna to use is a batwing with a few bays. Thanks to the commission, most low-band VHF DTV stations will not have a high ERP, and the simpler antennas will work fine.

For high-band VHF, the ERP values are somewhat higher. In this situation, analog stations commonly use the standard antenna types. There are some low-cost alternatives but the majority of DTV antennas in that band are batwings and dipole arrays. Those antennas have stood the test of time and are of moderate weight. Size will continue to be based on frequency and gain until someone finds a way to bypass the laws of physics.

At UHF frequencies, antennas range from the simplest dipoles to the familiar slot and panel configurations. Several manufacturers offer DTV products with questionable claims of performance. The mechanical characteristics of such antennas vary considerably as well.

Some antennas use flexible feed lines from one or more power dividers to connect to the elements. For many DTV stations, especially those of lower ERP, these antennas offer a workable solution. When properly mounted, they should provide successful operation for a long time. The antennas themselves work well, but users should plan on more maintenance than for a slot antenna, for example, because the line will need periodic mechanical service or replacement. These antennas are highly flexible for directional use, and electrical performance is really not a consideration. The power-handling capability and life expectancy are the real criteria to consider.

For high-power UHF DTV, a good choice is a slot antenna protected by radomes. It is a proven antenna design that will produce excellent performance with a long life expectancy. There are still slots in service that are more than 30 years old, although the newer antennas offer improved performance.

Going cheap may cost you

The big issue in buying an antenna is that it could be totally transparent for the station operation. That is, other than checking the gas pressure in the line, the transmission line and antenna systems should provide excellent performance without more than the occasional mechanical mount checkup.

When buying an antenna system, spending the least amount of money may not be wise. The antenna and line are not located where repair work is convenient or inexpensive. Therefore, stations should select a system that is built solidly enough to stand the harsh environment of its location and electrically sound enough to perform for years without trouble.

Granted, any antenna system may develop a problem. Such problems are usually caused by exterior forces such as lightning, frustrated hunters or the failure of other components on the tower. That type of problem, while irritating, cannot be anticipated and is simply a risk of operating a system. Poorly built antennas or lines with cheap hardware and materials cause unacceptable problems.

When an antenna can't handle power and causes the station to go off the air, the managers or ownership of a station won't care how much money you saved them by choosing a cheaper antenna. Then, the only consideration is that the antenna or line failed. This isn't to say that there aren't savings that can be realized by checking with different vendors before making a purchase. Sacrificing quality to save a few bucks can be a real disaster when trying to keep the station operating.

Don Markley is president of D.L. Markley and Associates.

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