Chris Watson, KVH Industries - January 10, 2007
With a growing number of video passenger entertainment systems being sold both in the aftermarket and as a factory-installed component, the question then becomes "What will people watch on all these video screens?" Just like at home, where people watch more live TV than pre-recorded content, that same trend occurs anywhere TV screens appear, including cars. In addition, pre-recorded media like DVDs provide only short-term entertainment value and must be stored in the car. Clearly, the time has come for expanded content and live programming. In fact, Frost & Sullivan projects that more than three million automobiles in the U.S. will be equipped with live mobile satellite systems by 2011.
In 2003 KVH Industries introduced the TracVision A5, which allowed passengers to watch DIRECTV satellite television programs and enjoy XM Satellite Radio in their vehicles while on back roads or rolling down the highway.
In August 2006, KVH advanced the technology again with the introduction of the TracVision A7 (see above), a system that builds on the original technology of the A5 and significantly advances the state-of-the-art in mobile satellite TV. The A7 offers faster acquisition, increased versatility, greater reliability, and quieter operation, due to patented advances in satellite TV technology developed by KVH engineers.
Several key innovations have made the TracVision A7 possible:
At the heart of the TracVision A7 is the hybrid phased-array antenna. Until the introduction of the A5 and now the A7, the longstanding form of a satellite antenna was the familiar round or elliptical parabolic dish. However, these dish-style antennas have obvious drawbacks when used on the roof of an SUV or automobile. A dish antenna's overall size, height, and aerodynamic drageven when covered by a domemake the dish-approach unusable as a mobile satellite TV antenna.
To fit the low profile and small size requirements of a car or SUV mobile TV unit, a new design was needed. In response to these daunting constraintsfor example, the antenna package could not exceed a 5.3 inches in heightKVH engineers took a hybrid approach. They combined the best capabilities of electronic phased-arrays and mechanically pointed antenna systems to produce the TracVision A7 antenna hybrid. The resulting antenna has impressive operational features and excellent signal capture capability engineered into a thin package.
Electronic phased-array antenna
Antenna designers have long known the benefits of "aiming" radio energy using multiple antenna elements. In the field of terrestrial broadcast radio, AM radio stations in crowded urban markets have used multiple antenna towers in precise groupings to produce a directed lobe of energy that provides service to specific FCC-defined areas. Based on the radio frequency used and the geometry of the antennas, the radio signals are subtractive in one direction and additive in another.
Later, in the field of radar, electronic antennas were developed that used many small antenna elements. When these elements are fed with signals in the proper phase relationship, the resulting RF energy can be focused, aimed and then rapidly re-aimed electronically, all without physically moving the antenna. This technology is used by the U.S. Navy, for example, in the phased-array radars on Aegis guided missile cruisers.
KVH engineers adopted this multi-element, solid-state approach for receiving the Ku band signals from DIRECTV satellites. Each TracVision A7 antenna is fitted with a flat array of 280 separate receiver elements.
Each elliptical element is designed to receive the 12.45 GHz DIRECTV signal. The signal strength from a single element, however, would not be high enough to reproduce a DIRECTV program. It is the combined signals from multiple elements across the array that produce the needed signal strength to reconstitute the TV picture and sound.
Because DIRECTV signals are broadcast using circularly polarized signals, the radio energy arrives at the antenna as either a right-hand, circularly polarized (RHCP) or left-hand, circularly polarized (LHCP) signal. The design of the A7 antenna elements allows the array to pick up signals of both polarizations.
Before the signal ever reaches the array, however, it must first pass through three layers of additional antenna elements or "parasites" that "bend" and refocus the satellite signal before it reaches the antenna array itself. This design eliminates the need for the antenna to be perpendicular to the satellite signal as required by traditional parabolic and "flat panel" stationary satellite TV antennas. It also permits the TracVision A7 antenna array to lie virtually flat while having reception capabilities comparable to or better than an 18-inch diameter parabolic satellite TV antenna. In addition, this "beam bending" enables the antenna to "see" satellite signals that are at lower elevations.
While the antenna has performed two key functions, receiving the satellite signals and segregating them by either right- or left-hand polarization, one more crucial task is necessary to produce a useable DIRECTV signal. Unlike a dish antenna, which focuses the received signals to a single point in space and time, a single-plane array of antenna sensors, like that used on the A7, can experience a slight difference in the time of signal arrival for each sensor (skew). Given the speed of light, some sensors are slightly closer to the arriving signal and thus receive it ahead of other sensors. If this time difference is not accounted for, the result will be out-of-phase signals and scrambled data.
To solve that problem, KVH invented a new waveguide structure as well as new low-loss combiner technology that compensates for the difference in phase among the separate signals relayed by each antenna element. Each separate signal arrives at the antenna's low noise block (LNB) at exactly the same time, creating a single, unified signal to form the DIRECTV picture.
Mechanical antenna tracking
Even with the added capability added by the beam bending effect, a purely electronic fixed flat array is far less effective than a unit that can move and tilt to detect satellite signals. The TracVision A7 can rotate through 360 degrees in the horizontal plane while also tilting its antenna array ±15 degrees in the vertical, allowing it to pick up the signals from the DIRECTV satellites. The combination of beam bending and the ±15 degrees physical tilt eliminates the need for the antenna to be perpendicular to the satellite signals. The A7 antenna array is able to lie nearly flat and still have reception capabilities better than an 18-inch diameter parabolic satellite TV antenna. One major result of this capability is the thin cross section of the A7 antenna caseno more than 5.3 inches thick.
Part 3 of this feature goes into additional advances made possible by the technology incorporated into the TracVision A7. These include the physical radome, the low noise block amplifier/IF converter, motors, and rotary joints.
Chris Watson is an engineer and marketing manager at KVH Industries. He can be reached at firstname.lastname@example.org.