In the past few months, this column has discussed many issues involved with propagation. But it might be helpful to look at a broad overview of television transmitting antennas in general. This might benefit those who have recently entered the broadcast field or those who are being driven to move from VHF to UHF by DTV.
No magic stick
The first thing that the newcomer to the television RF business should realize is that there is no magic radiating stick that will cure all the station's ills, including its advertising rates. Second, there are usually several antennas available that will do a decent job of serving a market, especially when you are considering simple, nondirectional systems. In fact, there are only a few antenna combinations that you should avoid.
VHF batwing antennas
For the VHF band, the old tried-and-true standby is the batwing, or superturnstile, antenna. It uses broadband radiators mounted around a pole. At each level on the pole, the batwing radiators are arranged in two pairs. Each radiator in a pair works with its counterpart on the opposite side of the pole. Each pair of radiators is fed a signal that is 90° out of phase with the other pair at that level, generating a fairly omnidirectional, horizontally polarized signal. The four radiators at each level constitute what is usually called a bay. In other words, if there are four levels of batwing-radiating elements, the system is called a four-bay batwing or superturnstile. The advantages of this type of antenna are good bandwidth, simple design, good long-term stability and relatively low cost. The disadvantage is that there are numerous pieces involved. Power dividers send cables to each radiator. A six-bay batwing needs 24 radiators, 24 transmission lines cut to exact length, at least two power dividers (probably four) and at least two fine matchers. In time, the owner of this type of antenna will need to replace the feedlines and maybe the power dividers.
The batwing is primarily a nondirectional antenna. But broadcasters can achieve some directional patterns by varying the power-division scheme and/or the phase of the power to the elements. These methods are commonly used, although obtaining varying amounts of beam tilt and null fill is common and easily attainable. Remember, think of the batwing as an antenna with a parallel feed — that is, the power is divided down until each radiator gets its own feedline. A big advantage to antennas of that type is that they allow broadcasters to manipulate the power and phase of individual elements — almost like an AM array — to get the desired pattern and performance.
VHF panel antennas
VHF TV stations also use panels of one type or another. These are usually wrapped around the tower and may have three, four or five panels at each level. Individual radiators are mounted in front of the panel. The radiators can be simple dipoles, crossed dipoles for circular polarization, cavity-backed radiators, or other configurations for horizontal, elliptical or circular polarization. As with the batwing, the power is divided down until the radiator on each panel receives its signal from its own transmission line. These usually employ only one radiator per panel because of the relatively large wavelengths involved. At UHF frequencies, the smaller wavelengths allow antennas to have one or several smaller radiators mounted on each panel.
VHF dipole antennas
Finally, some VHF stations use large dipole arrays mounted around a mast. Usually, they do this to achieve directionality, circular polarization or both. The dipoles may be fed either in parallel or in series like an FM antenna. Some VHF stations use large, traveling-wave antennas that have excellent bandwidth and stability. Such antennas can have good life expectancy with only minimal maintenance. The only drawback is that they are a bit heavy. But, if the tower can support the weight, they provide the ability to be directionalized and offer great performance.
UHF slot antennas
Now let's consider UHF antennas. Here, the odds-on favorite configuration is a slot. These antennas use either a cylinder or panels with an opening cut in them that will radiate well at the desired frequency. The signals feed the slots either in an end-feed or center-feed configuration. Convention attendees often hear arguments that favor one configuration or the other. Suffice it to say that both configurations work — period.
Slot antennas offer some big advantages. They have good bandwidth over a single channel, although some are usable over several channels. Generally speaking, their power-handling capacity is limited only by what the feed system will handle. They are normally stable and require little maintenance. They are available in a broad range of power-handling capacities, from small types such as the Scala SL-8 up to models that will handle over 300kW input power. Much to the favor of the front-office suits, both models — nondirectional or directional — are relatively inexpensive.
Some antennas use slot radiators mounted in sections, which are then fed RF power individually. The transmission lines run from power dividers that are either internal to the cylinder or mounted below the tower top with multiple feedlines coming out of the antenna. This configuration makes it a little easier to get broad bandwidth. Several versions of this antenna are available, from LPTV to high power. Some of the LPTV configurations use a few slots mounted in a bay fed by a single transmission line. This enables the broadcaster to stack several bays for added gain.
Caveat engineer
Now let's look at what to avoid. First, avoid any antenna whose maker claims it has enormous gain coupled with small physical size. Manufacturers of such antennas are conjuring the gain from either snake oil or magic because they sure won't find it in physics. An antenna's gain is always going to be a function of its size in wavelength. Lest the author be accused of heresy, the reader is advised to check any text on antennas and/or antenna design. The size of the aperture in wavelength is always involved in gain. For a given pattern, the antenna has to be longer to offer more gain. It is possible to increase gain in one or more directions by reducing the gain in others, but the RMS value of that pattern remains a function of aperture length.
Next, retreat post haste from anyone who claims that one or more components on the antenna aren't critical or sensitive to antenna performance. In the process of adjusting an antenna for a return loss of 30dB or more all the way across a 6MHz channel, everything becomes critical and touchy. To keep VSWR down around 1.05, every adjustment has to be within that last little RCH (referring to the Red Chaffrich, an often loud and rather abrasive South American bird). If the manufacturer isn't touchy about the antenna's handled, tuned and installed, it probably isn't worth the copper he wasted making it.
In a nutshell
So, for VHF, the most common antennas use batwing or dipole radiators, either mounted around a mast or in front of panels that are around a pole or tower. For UHF, the most common antenna is a slot-type radiator. The slots may be cut in a cylinder and fed by a coaxial scheme, cut into small panels fed by multiple lines, constructed two or more to a small antenna with several such antennas stacked, or simply cut into the sides of a circular waveguide. Some UHF antennas use other radiator types, but their use is often specialized for a unique set of requirements. Before buying any antenna, check carefully with the consulting engineer. Usually, he will be experienced in selecting transmitting antennas to meet the station's general or specialized needs.
Don Markley is president of D.L. Markley and Associates, Peoria, IL.
Send questions and comments to:don_markley@primediabusiness.com
