High-power antennas

High-power antennas come in a variety of flavors. Which is right for you?

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Last month, we discussed peanut-whistle power for DTV. Specifically, we talked about facilities that have been operating with greatly reduced power under a special temporary authority to serve their community of license until they can get permission to operate at higher power. We covered the use of low-power antennas, and the fact that many facilities that use them are able to cover a greater area than anticipated.

Radiation patterns

This month, it's time to move on to big antennas and higher transmitter-power output. To start, there is good news and bad news. The bad news is that there is absolutely no such thing as an omnidirectional antenna. That is, there will always be some variation in an antenna's radiation pattern as a function of azimuth. That variation increases when an obstacle is placed in the antenna's aperture. When that obstacle is a tower, as in the case of a side-mounted antenna, the variation becomes significant. It is possible to calculate roughly the effect of the tower on the antenna's pattern, but you have to accept the fact that the pattern will never be as uniform as that of a top-mounted antenna. The good news is that this isn't usually a problem. While the side-mount antenna's pattern will vary, the effect of that variation is much less than you might expect.

Size matters

Big, high-priced antennas have several advantages, including robust structural characteristics, highly predictable pattern and low maintenance requirements. You can't top-mount lightweight antennas or use them to support other antennas. They require a tower leg or other support structure which, in turn, affects the pattern. But the big antennas can stand alone and support other antennas. Facilities often mount a UHF antenna on top of either a VHF antenna or another UHF antenna. Andrew even has a slot antenna that can be incorporated into the tower as a leg. Obviously, this requires a much more massive and robust antenna.

The inside story

Large slot or traveling-wave antennas are fed internally with rigid transmission line. Being inside of the cylinder, the line is well protected from the elements. Many antennas of this type serve for over 30 years with no service to internal components. The most common failure mode is at the input, where the feed system is subjected to vibration and movement. Still, there are no flexible lines to be replaced and essentially zero external hardware to be serviced.

The big panel antennas, while using a pot full of lines, splitters, etc., offer the advantage of wide bandwidth to accommodate multiple stations. The components can sometimes be serviced by having the technician climb up inside the antenna. In particular, RFS and Shiveley are marketing antennas that are serviceable from the inside. Panel antennas also offer the unusual advantage of allowing a different amount of beam tilt at different azimuth values. But, remember, electrically the big antennas don't offer a great improvement over their lighter-weight cousins. The patterns available are similar, their gains are essentially the same and they will all provide good service within their mechanical or structural limits.

Service contours

The question becomes one of determining just what you accomplish by going to a higher-power operation. As we discussed last month, DTV service is determined by use of Technote 101, the Longley-Rice propagation model. That method is not normally considered to generate a contour. Rather, it determines areas that receive the desired level of signal or more. Figure 1 is a map that shows the service area for WTVP-DT in Peoria, IL, based on both Longley-Rice and the FCC curves. The clear area enclosed by the irregular, light-blue ring is the Longley-Rice service of 41.86dBu or more. For comparison, the FCC F(50,90) contour is shown in black. The significant variation is caused by the even nature of the terrain once the signal gets well clear of the river. On the other hand, the map shows areas along the river, especially to the north, where the signal drops due to terrain blockage at the river bluffs.

The interesting thing here is that there isn't a lot of difference when changing from 100kW to 400kW. That is especially true where the terrain is the most even, as in the areas northeast of the site. Where the terrain is rougher, the difference becomes greater. This tends to agree with what is observed in actual signal reception. The higher power doesn't have as much effect in the distance to the edge of service as it does in filling in the holes in the service. That is something that doesn't show up much in the propagation calculations. But, higher power provides a more “solid” signal.

Remember what happened when Class A FM stations were all upgraded to 6kW (separations permitting)? Many expected it to make a huge difference in the coverage area of the stations. It did make some difference, but many feel that the small increase in the distance to contours was much less significant than the overall improvement in signal quality within the service area. The increase in power helped penetrate buildings and greatly reduced the size of “holes” in the existing service. That is something that doesn't show up much in the propagation calculations. But, higher power provides a more “solid” signal, for lack of a better word. Such is the case with analog television, and we can expect it to be so with DTV. The big factor isn't how far out the station's service goes, but how much better the service quality is within the coverage area.

Reality bites

The problem with any method of prediction is that the real world isn't as nice and uniform as we calculate it to be. The real world has buildings, big trees and ridges that don't show up well, even on 3-inch databases. Granted, it is possible to include a “fudge factor” based on the type of terrain, the average extent of foliage or forestry, or the existence of buildings. While such factors do improve the overall accuracy of signal-strength predictions, it still isn't possible to predict the field-strength value at a given point. We must admit that prediction methods are only viable over an area and are so considered by the commission. That is, they clearly state the field strength at 50 percent of the locations in an area.

If you want the real field strength at a given area, it is probably most accurate to attempt field-strength measurements. But, everything can affect the value of readings, especially in UHF. That includes the receiving antenna, the exact location of the measurements, the accuracy of the meter, etc. The commission says that you should take measurements at 30 feet above ground level (AGL) over a distance of 100 feet, recording the measured values. But then, taking measurements is a good subject for another column.

High and low

Lightweight antennas can provide good service when their mechanical limitations are acceptable and low power is necessary. The higher-power service does extend the distance to service contours but, more importantly, it provides a better-quality signal in the service area.

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

Send qusetions and comments to:don_markly@primediabusiness.com