Part 1 of this story discussed the mechanical and phased-array design features of the TracVision A7 hybrid antenna.
Part 2 discussed the challenges of acquiring and locking onto satellites while in motion, multi-satellite capability, and the enhancements provided by GPS.
In addition to the key technological advances cited in the first two installments of this series (noted above) that enable the TracVision A7 mobile satellite TV system from KVH Industries there are also supporting elements vital to operation of the system.
The TracVision radome
The antenna housing had to meet several goals: It had to be rugged enough to survive life on the top of a car; it needed to be "invisible" to the satellite TV signals in order to maximize reception; and it should look stylish in order to blend with vehicle looks. The resulting patented enclosure is made out of a rugged automotive-grade plastic that has a structure designed to "match" the RF signals broadcast by the TV satellite so there is virtually no loss of signal as it passes through to the antenna. Thus the maximum signal reaches the antenna and permits reception even in foul weather. In addition, the material is tough enough to withstand road hazards like flying debris, inclement weather, and even a car wash.
Custom stacked low noise block (LNB)
Because of the restrictive height requirements of the TracVision A7, not only must the antenna be small in cross-section, but the low noise block (LNB), or amplifier and intermediate frequency (IF) converter, must also be thin to fit into the antenna housing. The TracVision A7 LNB unit is a small package mounted to the underside of the antenna where it receives the in-phase signal after it passes through the waveguides and combiner.
Compared to a conventional LNB used with parabolic antennas (left), the TracVision A7 LNB (right) offers a significantly thinner profile.
The LNB takes the 12.45 GHz signals from the antenna, cleans them of noise, and amplifies and then converts them using band pass filters, amplifier circuits, notch filters, and mixers to an IF range of 950 to 2,025 MHz. The IF range, which is easier for the receiver to use than the higher frequency satellite signal, is fed into the receiver and there decoded into programming and relayed to the TV screen.
Slip-ring rotary joint
A big challenge for mobile satellite antenna systems is passing the satellite TV signal from a continuously rotating antenna above to the receiver secured below. Traditionally this is done via a cable wrap using a ribbon cable connecting the antenna's base to the rotating antenna. But given the limited length of the cable, the antenna could only rotate up to 720° before the cable would reach its limit.
As a result, if the vehicle made two full turns (navigating a parking area, for example), the antenna would reach a point at which it could rotate no farther. The antenna would then have to rotate 360° in the opposite direction to unwrap the cable but doing so would cause a brief interruption in TV reception.
KVH engineers designed a slip-ring rotary joint that solves this problem with a two-piece component that passes electricity across several points of contact. The joint is configured so that the top moves while the bottom stays stationary. This arrangement allows the antenna to rotate freely and continuously to provide a constant, uninterrupted signal no matter what maneuver the vehicle is performing. The designers improved the technology for low path loss and a steady signal with stronger points of contact than ordinary, high-loss slip rings.
Given that the TracVision A7 antenna unit is operating only a few inches above the passengers heads, the motors that point the antenna needed to be as quiet as possible because whirring motors could detract from the viewing experience. KVH engineers utilized ultra-quiet stepper motors on the A7. These units are strong enough to reliably handle the antenna-pointing requirements, yet are basically silent.