Directional antennas

New stations need to know the criteria for these antennas
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Prior to the digital television evolution, directional antennas at full-power television stations were primarily used to transmit a signal over a desired area as opposed to provide protection to other stations. For example, a station with a transmitting antenna located within visual distance of the coast might use a directional antenna to place the signal over the land area and avoid wasting the signal over the water. The FCC permitted this if the antenna conformed to the guidelines in the rules and regulations.

In some instances, directional antennas provided protection to close-spaced stations or to radio-quiet areas. For grandfathered stations using directional antennas, this wasn't a major problem, but new stations were required to provide a detailed argument to get a construction permit. The situation improved when the FCC adopted the Longley-Rice propagation model for determining interference to television stations. But the precedent had to be pressed for variances to be granted.

Criteria

The commission created a strange set of criteria for directional antennas. The criteria focused more on the proposed service than physics.

The maximum-to-minimum ratio of VHF TV antennas was set at 10dB. For UHF stations, the ratio was set at 15dB. If the station had a maximum effective radio power (ERP) of 1kW, all limits were off, and the station could do virtually anything.

With regard to low-power stations having no max/min limit, there is some obvious common sense involved. At that low-power level, variation in the antenna pattern won't cause a significant problem.

The 10dB to 15dB change between UHF and VHF is a bit more difficult to understand. Variations in the environment, such as ice or wind movement, cause more instability in the VHF pattern than the UHF. A buildup of 1in of ice on the UHF antenna would have a greater effect on the pattern than the same buildup on a VHF antenna. The clue here is to look at the ice buildup in terms of wavelengths rather than absolute size.

It is interesting to note that directional antennas in the FM service were permitted to have a max/min ratio of 15dB while the television antennas were only allowed 10dB. Perhaps ratios were added without reviewing or changing the older ones.

Another anomaly is FM antennas have a requirement that the change rate on the pattern cannot exceed 2dB per decade of azimuth. Television antennas do not have that limitation. However, there should be some degree of commonality in those regulations.

Low-power stations

When low-power television service came along, directional antennas were used to provide protection to other stations. Previously, they were used in the translator service, primarily to obtain good service over isolated communities from low-power transmitters. The directional antennas for translators and low-power television stations had no criteria. The stations chose their own rate of pattern change and the max/min ratio. No measurements had to be performed on the antenna to confirm the actual pattern with strictly theoretical values acceptable. Obviously, the 1kW ERP limits, which existed in the full-power service, were exceeded.

Remember, the first low-power stations were limited to 1kW transmitter power, which could reach 75kW or more ERP. Subsequent rule changes for low-power television and Class A stations permitted up to 150kW ERP. In other words, enough power was involved to cause interference if stations were not properly designed.

The lack of any criteria for low-power stations has caused some obvious abuse. I know a manufacturer that claims its system has a max/min ratio of 40dB and more than 180 degrees of azimuth. This is pure nonsense. Such a ratio cannot even be reached over a span from dish antennas — let alone a slot with reflectors.

Still, the rules permit such claims to be made without proof, so the commission's staff is helpless in these matters. Applicants are not required to prove that the antenna meets the criteria. They only need to show that the theoretical pattern would provide such protection.

The pattern and mounting of the antenna

The next issue involves the pattern to be presented and the manner of mounting the antenna. When the station's consulting engineer specifies an antenna, the calculations concern limiting the signal to other stations in an effort to prevent interference or limit it to an amount acceptable under the rules.

The engineer will normally attempt to use off-the-shelf patterns published by manufacturers. In some cases, none of those antennas do, so the engineer will contact the manufacturer and explain what is needed. The manufacturer will then modify a published pattern and provide that information to the engineer in both plotted and tabular form for the horizontal and vertical planes. The FCC application includes those patterns, along with a statement that the vertical plane pattern is identical for all values of azimuth (even though it often is not).

The directional characteristics of a television antenna are commonly obtained in one of two ways. Either the slots or the dipoles are arranged around a mast to obtain the pattern, or reflectors are used on a simple radiating element. This usually involves slots in a row, down one side of a mast, with passive reflectors or parasitic elements attached to the radiators (usually dipoles). The power and phase relationships to the radiating elements are fixed by the factory and are not normally field-adjustable.

The antennas are simply installed, the voltage standing wave ratio is trimmed, and all is considered to be in order. The license application must only state that the antenna is pointed in the right direction.

If the desired antenna is a new design, the manufacturer will probably model it in an anechoic chamber. This means that the size and frequency will be scaled to permit the antenna to be a more workable size. The antenna will then be placed in a chamber that has no reflections from the sides, permitting the radiation to be measured without outside influence.

In the past, the full-scale antenna was placed on a range and the horizontal and vertical patterns were measured. The antenna was always used to receive the transmitted signal coming from a dipole or yagi at some distance.

My next statement might upset some amateur radio readers, but the truth is that antennas work exactly the same whether transmitting or receiving. Full-size test ranges are primarily gone. Many manufacturers believe the patterns can be satisfactorily measured with probes in the lab. That may be the case, but there is something positive about seeing the whole antenna move on big turntables with a plotter drawing the patterns.

This process is still used by ERI and Jampro for FM antennas. Anyone who has witnessed such measurements will confirm that even minor changes in antenna characteristics can largely affect patterns, especially, the size and placement of parasitic elements.

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

Send questions and comments to:don.markley@penton.com