More on getting good TV pictures on board
A recent issue contained a communications column on receiving television on a boat ("Television on board," Issue No. 93, Nov./Dec. 1998). The author, Jay Robbins, did a good job of surveying the various actively stabilized DBS antennas. However, a few points might have been a bit clearer. I work as a broadcast engineer at a network affiliate and have considerable experience in KU-band video transmission.
Robbins states, "forget about watching Weather Channel updates while sailing through a class-five hurricane: no system can compensate for that much movement." From practical experience in trying to send and receive KU-band satellite transmissions, I can safely say that movement is only one of the problems in watching satellite TV in heavy weather. Long before the ship motion became a problem, the video would go away due to rain-fade; in the case of an 18- to 24-inch direct satellite dish, a heavy rain will reduce the signal to an unusable level and cause the receiver to lose lock on the signal, which causes the antenna stabilizer system's electronic feedback loop to be broken and send the dish into a search mode to reacquire the signal. The loop would only be restored after the receiver regains a signal strong enough to reestablish lock. At the time that the receiver lost lock, the best thing to do would be to shut the system down and save the boat's batteries.
In terms of weather degradation, small-dish systems suffer most commonly in heavy rain. Dense fog will decrease the signal, but it usually isn't enough to completely block it. Snow usually causes problems when the viewer allows the dish to become snow packed. In the air, snow only causes difficulties when the density of snowfall is roughly equal to a heavy rain.
One other weather-related problem Robbins didn't mention is electrostatic discharge. A nearby (one and a half miles) lightning bolt can be enough to damage a small-dish system's circuitseven if the system is turned off and properly grounded. I have had to replace modules on my shore-based system twice after nearby lightning strikes.
One large variable to good reception is physical obstruction. Few people realize it, but a KU-band signal can actually be completely blocked by tree limbs, leaves, and pine needlesnever mind steel shrouds, metal in mast fixtures, or even large amounts of nylon or canvas. A good rule to remember is that a satellite signal is a microwave transmission, and microwavesas well as their close cousins radar wavestravel line-of-sight paths. Ideally, a small satellite dish should be placed in a location where it always has a stable and unobstructed view of the satellite. On a sailing craft, the only time this can be absolutely guaranteed is when the dish is mounted to a piling dockside. Failing that (who wants a boat that never sails?), perhaps a mount on either beam amidships for a stabilized system would work.
Robbins is correct in his urging to get the largest antenna you can get with a given system. Digital KU transmissions, by their very nature, allow smaller antenna sizeswitness the increasing use of smaller and smaller satellite news-gathering vehicles, some of which are the size of a sport-utility truck. Unfortunately, the marketing departments of the various direct-to-home satellite services used the "smaller is better" approach as a major selling point for their systems and pushed the designers to make the reflectors as small as practically feasible. In terms of coverage, the DBS 2 & 3 satellites are located directly above the equator at approximately 101° west longitude, placing the strongest signal for DirectTV and PrimeStar roughly in the center of the U.S. However, the footprint of the satellites allows for good fringe coverage to almost 300 miles offshore in placesperhaps farther with a very good system in calm water. A larger dish will increase the chances of a usable signal near the fringes of a satellite's coverage area.