With 1,600 television stations simulcasting both NTSC and DTV until their final conversion to digital, there is a great demand for premium vertical real estate to optimize the placement of their antennas.
For broadcasters and tower manufacturers, a major setback is the need to double tower aperture space in most locations by maximizing vertical real estate, antenna density, and antenna height. Broadcasters need assurance that antenna performance is maximized, and tower operators will want to realize an economic benefit by making more real estate available.
Four methods can be used to at least double vertical real estate. Three proven methods include the in-line stack, offset stack and horizontal stack, in either a T-bar or candelabra configuration.
Another new system that can accommodate more than two antennas in the same aperture is a two-level stacked system. Each method produces different performances and challenges. What system best suits a particular RF application can be determined by examining the advantages and disadvantages of each method.
In-Line Stack
An in-line stack mounts one antenna on top of another. There are two forms of an in-line stack – the feed is either internal or external. In both cases, the lower antenna serves as a structural support for the top antenna. With an internal feed, the bottom antenna also serves as a conduit for the transmission line to the top antenna. An antenna with an external feed routes the transmission line externally along the side of the bottom antenna.
Advantages, Internal Feed: This technique creates unobstructed vertical real estate for two antennas when the top antenna’s transmission line feeds through the bottom antenna. Both antennas have unobstructed azimuth patterns, which is very efficient for omnidirectional patterns, and offer minimum wind loading.
Disadvantages, Internal Feed:. The bottom antenna may require specialized overmoded design because of a large inner conductor. If either antenna requires repair, both the top and bottom antennas may need to be taken off the air. Off-air time results in a higher total cost of ownership.
Advantages, External Feed: If only the top antenna of an in-line stack with external feed requires maintenance, the bottom antenna may still be operable. In addition, both antennas are manufactured from standard components, reducing the cost of ownership.
Disadvantages, External Feed: When the upper antennas’ transmission line is routed on the outside, reflections impact the radiating characteristics of the bottom antenna. Scalloping of the pattern can be caused by re-radiating energy off the transmission line. The exposed transmission line also results in slightly higher wind loading.
The bottom antenna is typically more mechanically specialized because it supports the top antenna. The larger pipe affects pattern selection; unique cardioid-shaped azimuth patterns may be more difficult to achieve. This also depends on the frequency range required. Height may nullify the in-line stack as a solution if it exceeds FAA restrictions.
Horizontal Stack
Also known as the candelabra or T-bar arrangement, this structure holds two antennas about 40 to 50 feet apart horizontally to minimize pattern interference.
Advantages: The candelabra uses standard antenna components and is a particularly good choice for directional antennas. This is the most ideal configuration for placing two antennas in a common aperture. Antennas operate independently, so if one needs repair, the other is unaffected.
Disadvantages: The antenna structure can be very large – approximately 70 feet high atop a 1,500- to 2,000-foot tower. These towers are the most costly of any of the solutions because of the extreme wind loading.
The horizontal spacing of each antenna is critical. As opposed to the in-line stack, both antennas impact each other’s aperture, so the azimuth pattern of both is affected, although the overall effect is minimal.
Two antennas placed in close proximity to each other may require filtering, which could be costly. If two antennas are relatively close in frequency range, the possibility of re-radiation between the two antennas may occur.
Offset Stack
In this configuration, a standard top antenna is supported by a steel structure with a side-mounted antenna attached to it. Transmission line for the upper feeds through the steel support structure; a separate transmission line feeds the side-mounted antenna.
Advantages: This is a proven design that uses standard antennas on both top and side. It also requires only a standard mount because one mount does not support both antennas. The offset stack has the lowest total cost of ownership of any of the systems discussed.
Each antenna operates independently, so either antenna can be removed for repair or replacement without affecting the other. In addition, the bottom antenna is re-deployable, which becomes relevant when a station wants to change antennas because of channel swapping once the simulcast period is finished or if it has been allocated a DTV station above Channel 51.
Disadvantages: The steel support structure affects the pattern of a side-mounted antenna. The pattern has more scalloping than that of an antenna in a candelabra arrangement. The configuration requires two structures: the support mast and the bottom antenna, which produces higher wind loads than an in-line stack support structure. Additionally, the cost of the support needs to be factored when deciding which approach best suits the needs of the broadcaster.
Stacked Structure
The stacked structure, which includes Andrew’s Stacker structure, offers high antenna density without the tradeoffs and expense of a candelabra or T-bar. The structure has three legs that support a platform on top. Up to two antennas can be installed on the bottom level and one or two on top. This solution doubles the amount of antennas that can be installed in the top aperture.
Advantages: An antenna on the Stacker structure produces a pattern comparable to an antenna on a candelabra. Standard antenna components can be used on both top and bottom levels, though those used as bottom legs are hybridized with slight differences in the top and bottom flanges. This solution provides the highest density of antennas on the tower top. It is a lower-cost solution than a candelabra because the antennas are integrated into the tower legs. The overall wind load of the Stacker structure is greater than the other stacked solutions such as in an in-line or offset stack but carries the lowest wind load per antenna.
Each antenna operates independently, so tower operators can remove, re-deploy or change antennas at any time without affecting the operation of the others. The Stacker unit is manufactured and shipped with support legs to use while replacing any of the radiating elements. The structure can start with only one of the three tower legs as an antenna. One additional antenna can be added at any time. Two antennas on the bottom can be positioned in any of the legs; the third leg is used for transmission line to the top-mounted antennas. Antenna spacing, typically around 90 to 95 inches on both levels is based on frequency and pattern, so the effect on the pattern is minimized.
Another configuration places one Stacker unit on top of another. In this situation, the lower Stacker unit typically has leg spacing of 120 inches with two of the three legs as radiating elements. The top Stacker unit spaces the legs at 90 inches. One other option could place three antennas in the Stacker unit with not top-mounted antennas.
Disadvantages:. Because antennas are mounted within each other’s aperture and can radiate into each other’s pattern, additional filtering may be required.
With careful selection of what channels are placed where, the cost of filtering can be minimized. For example, if four antennas – Channels 14, 19, 35 and 43 – were installed on a Stacker structure, antennas would be positioned with Channels 14 and 35 on the top and Channels 19 and 43 on the bottom. Again, with antennas radiating plus or minus 4 channels of each other in the same aperture, some filtering may be required, depending on the azimuth patterns of the antennas.
Broadcasters and tower owners can use many configurations to increase vertical real estate. In-line horizontal, and offset stacks are established solutions. The Stacker structure that can be expanded to allow for up to four antennas in real estate commonly populated by two. Broadcasters need to review the trade-offs of all situation and determine which best suits their needs.
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