New technology and the FCC broadcast spectrum repack channel re-assignment process is driving many US TV stations closer to replacing their broadcast antennas.
A TV broadcasting antenna is a highly-engineered, passive mechanical device that accepts the full transmitter power output (TPO), typically following some attenuation through filters and the transmission line. A TV broadcast antenna directs a RF signal toward a targeted area or population with minimal reflected power at its operating frequency. The antenna gain multiplies the TPO to achieve the station’s effective radiated power (ERP) as licensed by the FCC.
Sooner or later
The unofficial countdown is on for ATSC 3.0 and it's rapidly gaining momentum. Whether your facility needs an RF upgrade generally depends on its designed headroom in ATSC 1.0. Some stations may only need to change their exciter software. Others may need to change-out nearly everything from the antenna down.
Meanwhile, the official FCC 39-month Repack deadline countdown is about to start, and some big users are already placing some major RF hardware orders.
If your station avoided Repack, ATSC 3.0 isn't far behind and both require attention to RF.
Once you’ve learned your new channel, antenna selection begins with its location. Where will it be mounted, side or top? How high? Horizontal, circular, or elliptical polarization? Repurpose or replace feed line? Will the actual moment of channel change-over take place at your station instantly by flipping a couple of switches at 2 a.m., or will it be vastly more complex?
With these questions answered, the many antenna RF design options become more easily identifiable. The right combination of TPO and a massive number of mechanical and electrical antenna design characteristics will custom-tailor the licensed RF field-strength contour and desired coverage at a specific antenna height. Variables include gain, beam tilt, null fill, elevation and azimuth patterns, and polarization.
Length, weight, shape, and wind load of the antenna are essential factors. Mechanical upgrades to antennas and/or feedline on existing towers will require a structural analysis to verify the tower structure’s capability to handle the proposed load within applicable building codes and with adequate safety factors.
High Band VHF and UHF DTV broadcast transmission has evolved into two basic types of antennas; panel and coaxial slot. Each has unique characteristics and different construction.
The slot antenna is the most common antenna used in TV broadcasting today. It’s based on a cylindrical conducting material with slots cut into it to couple the RF energy to the atmosphere.
The location and number of slots generally determine the antenna pattern and gain. Slot antennas are specific to one (sometimes two) RF channels, because the antenna length and the location of the slots are based on the operating frequency.
Lower channels are longer wavelengths that usually require longer antennas. If your station is moving to a lower channel, a new slot antenna will require additional length to maintain the same gain. If the additional length is not available due to antenna aperture restrictions or overall tower height constraints, the gain will be reduced on the lower channel.
Slot antennas can generate a radiation pattern to a customer’s specification. This pattern should be verified in an appropriate anechoic chamber to ensure compliance with the desired spec, as well as to verify the arrangement of any pattern shaping elements used to achieve the pattern.
A slot antenna completely enclosed in a radome can be pressurized to further protect the radiating elements from the effects of atmospheric moisture. Pressurized slot antennas have been known to operate for decades without any sign of corrosion to the radiating elements or the mechanical structure of the antenna.
Compared to panel antennas, slot antennas provide several advantages. Typically, they have a wider variety of custom patterns in both horizontal and elliptical polarization. Slot antenna construction uses far fewer cables and connectors than a panel antenna, which means a more dependable design with few potential points of failure. A slot antenna creates less wind load on a tower than a panel antenna of similar performance.
Pattern performance and null fill
With slot antennas, elevation patterns are critical. As the elevation pattern becomes more “focused,” or higher in gain, the nulls (low points) in the pattern become more pronounced. These nulls can mean that population near the tower may not receive sufficient signal to achieve reception.
Because of the physics of the antenna architecture, some center-fed slot designs exhibit substantial, deep nulls. Reducing the gain of a center-fed slot antenna to achieve “null fill” and reduce the deep notches in the elevation pattern requires increased TPO and antenna length to achieve the same gain performance as an end-fed slot.
Many slot antennas use an end-fed architecture for smoother elevation patterns and naturally occurring heavy null fill, especially at higher gains. End-fed designs are also capable of greater electrical beam tilt without a gain reduction. This results in higher signal levels inside the close-in coverage area when combined with the increased null fill. End-fed slots typically provide a smoother elevation pattern and more consistent coverage, particularly at higher gains.
Panel antennas are an array of paddle-type radiators enclosed in radomes and arranged in a specific pattern to achieve the desired pattern and gain performance. The main advantage of the panel array is that it is broadband. The pattern of a panel antenna changes with frequency, so different channels will not have exactly the same patterns, especially for complex pattern and high gain designs.
Panel antennas are available for both top and side-mount, and are usually a much higher wind load than a slot antenna. A unique benefit of a panel is that multiple stations with the same (or very similar) patterns can use the same aperture on the tower. Panel antennas use a complex feed system to supply each panel in the array. A separate system is required to feed V-pol radiating elements within each radome, and its addition greatly increases antenna complexity.
A full-power TV antenna is a large, heavy, static load to place on a tower. It also has wind load coefficient dynamics that must be factored into the tower’s load safety evaluation. A panel antenna is particularly important because of the high wind load associated with it.
The structure that holds the individual panels in the array needs sufficient strength to support the panels, withstand the wind (based on tower location), and provide appropriate mounting points for attachment to the tower. Local building code will specify the standard to which your tower needs to be evaluated. TIA 222 G is the current version of the tower construction standard.
Slot antennas can be more than 50 feet long, weigh more than 5 tons, and are often raised and positioned with a gin pole, a device used in construction of tall towers for stacking tower sections. The tower structural analysis is critical to ensure it can not only support the load of the antenna, but also any other rigging and fixtures required to complete the antenna installation work.
This story is based on a portion of the Rohde & Schwarz Whitepaper Go 2 Repack: An Expert Guide to Technology and Logistics. To download the full Whitepaper, click here, scroll to Repack and click the red “Download our Whitepaper” bar.
The author also thanks Dave Benco, Director of Television Sales at Elecronics Research, Inc. for the valuable information he provided.
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