Good antennas make good neighbors

Written by Joseph Darlington, P.E.
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When building any radio system, the scarcest and most precious system component is not the radios, the antennas nor the towers, but available “clean” radio channels on which to transmit. That’s why specialized narrow-beam antennas can ease frequency planning and reduce interference for railroad communications.

That scarcity of radio spectrum became clear when Trinity Metro was putting together the voice radio network for its new 27-mile-long TEXRail commuter rail line linking downtown Fort Worth to Dallas/Fort Worth International Airport Terminal B. In the planning stages of the TEXRail line, the 160 MHz frequency coordinator for the Association of American Railroads (AAR) was able to assign two voice radio channels to TEXRail for use in train dispatch operations. However, those two channels were also being used on other railroad towers located south and west of Fort Worth. Some degree of co-channel interference with the TEXRail radio network was inevitable … or was it?

An Evolving Design

The potential for radio interference became apparent in the computer simulation of radio coverage for the original system design, which consisted of two 100-foot towers topped with omnidirectional, folded dipole antennas. As seen in Figure 1, the radio signal provided by the new TEXRail radio towers would cover most of the right-of-way, but would over-propagate several miles on each side of the track.

Propagating radio signal much past the railroad right-of-way is wasteful, making about as much sense as setting up a lawn sprinkler system to irrigate one’s driveway and sidewalk. The real problem with such sloppy antenna patterns is that they will pick up stray, interfering signals from neighboring transmitters, such as locomotive radios and other radio base stations. Clearly, an alternative design approach was needed.

The computer simulation in Figure 2 shows TEXRail’s alternative design using three 60-foot towers and highly directional narrow-beam antennas. These narrow-beam antennas squeeze the available radio signal to confine it to the right-of-way. The base radios’ total transmitted power in watts has not increased; instead, the radiated power is better-focused onto the track.

Specialized Antennas

The improved radiation patterns could only be achieved with the use of specialized, narrow-beam Yagi antenna arrays. How the Yagi antenna uses its multiple elements to morph a basic omnidirectional radiation pattern to a narrow directional pattern is a subject for antenna designers. Fortunately, for the system integrator, 160 MHz Yagi antennas are available from several manufacturers as a finished, off-the-shelf product. Figure 3 shows how the four Yagi antennas were precisely arranged and cabled together for TEXRail.

The trickiest part of assembling the Yagi antenna array was building the phasing harness. The interconnecting cables had to be cut to specific lengths depending on the wavelength at 160 MHz, the physical antenna spacing and the cable type. When built properly, the antenna array will deliver the full effective radiated power to the intended coverage area. Also, interference-causing radiation pattern sidelobes will be minimized.

Big Antennas

Assembling the antenna arrays in the field and installing them on the towers at TEXRail was a Texas-sized adventure. Yagi antennas tuned to operate at 160 MHz are large—more than six feet in length—so they had to be assembled on site. Once the antennas were assembled and clamped to the tower, a power divider was used to join the west-facing and east-facing antennas. All coaxial cable connections were then meticulously weatherproofed using butyl and nylon tape.

Before the tower could be cranked up to its vertical position, antenna alignment was checked using a magnetic compass and inclinometer. Because the radiation pattern of the antenna array is a narrow 28 degrees, an error in antenna azimuth of as little as 14 degrees (28 ÷ 2) will result in a half-power, or 3 dB, loss in signal delivered to the locomotive radios. Beyond the 14-degree error point, signal strength drops off rapidly. It’s wise then, to hire an installer who knows how to read a compass, correct for magnetic declination, and set antennas to the design azimuths indicated by the computer simulations.

Once the tower was raised, VSWR (voltage standing wave ratio) tests confirmed that the coaxial cabling work was flawless. Two-way audio checks between TEXRail’s dispatch center and radio users in the field at first uncovered some radio configuration errors that had to be corrected. Once the configuration and network connectivity bugs were worked out, the delivered voice radio quality of the radio system proved to be nothing short of excellent.

Looking Back … and Forward

Judging from the quality of the radio communications between the dispatcher and train engineers, the time invested to install the narrow-beam antenna arrays appears to have paid off. Simpler omnidirectional antennas would have been an easier hardware option, but a price would have been paid in noise reception and audible radio traffic from neighboring railroads. For a commuter rail line like TEXRail, the safety afforded by eliminating such distractions to the dispatcher is priceless.

Should this specialized antenna approach be used on future rail projects? Future system integrators may have little choice. Jim Reimer, the 160 MHz frequency coordinator for the AAR, reminds us that the radio bands for voice communications and Positive Train Control are only getting more crowded. Smart antenna design will prevent new radio licensees from stepping on incumbent radio operators, and will obviate expensive region-wide radio system retunes. Certainly, in the case of TEXRail, good antennas make good neighbors.

Joseph Darlington is a Senior Communications Engineer at Xorail, a Wabtec Company.

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