Antennas
VHF, SSB Marine radio, Satcom, satellite radio, GPS, Loran, cellular phones and satellite TV, what do all of these electronics have in common? Marine radio navigation and communication devices are the most sophisticated and complex in the world. But all depend entirely upon clear reception of radiowave frequencies, without reception they won't function. Therefore, whenever installing high grade radio navigation or radio communication electronics, the foremost priority should be a quality marine antenna optimized for receiving signals in that frequency range.
What difference will a good antenna make? No matter how much you spend on a marine radio, performance relies ultimately on the quality and performance capabilities of it's antenna. Here are some basic considerations for commonly used marine electronics.
Antenna Height
For line of sight transmission such as VHF radios, antenna height is paramount. Mounting an antenna of adequate length at the best possible height assures the furthest range along the curvature of Earth. (Information based on Geographic Range Table calculations in Coast Pilot. i.e. 5 ft above sea level has line of sight of 2.6nm, 10 ft height has a range of 3.7nm.) As a general rule, sailboats typically mount a 3-5ft antenna on top of the mast. This mounting elevation offers superior range of communication. Racing sailboats sometimes opt to mount an 8 ft antenna on the stern as a fail safe against dismasting at sea. Powerboats up to 25 feet in length typically install a standard 8 ft antenna, while bigger vessels may require a larger antenna with higher gain. The other limiting range factor is power of transmission, but standard radios are most often limited by geographic range before power becomes a consideration. In US waters, transmission power is also regulated by the FCC.
Gain
Gain, measured in decibels (dB), rates communicating range. In general the higher the gain, the further the transmission will carry. The compromise of a higher gain is a narrower signal beam. This compressed beam can cause weak or intermittent signals in a rolling sea. For smaller boats prone to excessive roll, the gain used does not exceed 6dB. Larger, more stable vessels can effectively use a higher gain. Shakespeare antennas offer a low angle radiation which maintains a normal beam width consistent with gain, but has a lower trajectory for more power across the horizon. Low angle radiation doesn't suffer as much weak signal strength in heavy seas.
Mounts
Mount style is dependent upon the needs of the boat. Many boats require a fold down antenna mount to pass under bridges or for trailering. In such a case, be sure the installation site allows enough room for the antenna to lay down. If using two-piece antenna systems, an upper support clamp is necessary. Be sure to check with the antenna manufacturer for the appropriate mount setup to allow flex without damage to the antenna.
Cable
Cable selection depends upon the length of the cable run. For VHF, HF/SSB, and cellular communications, the optimum is the shortest possible cable run. For cable runs feeding marine radios of up to 20 ft, RG-58 cable is sufficient. For cable runs in excess of 20 feet, the recommended cable size is the larger and better, low loss RG-8/X, RG-8A/U or RG-213. Some antenna manufacturers offer a low loss cable engineered specifically for the needs of dual band cellular communications. These cables have the higher conductive characteristics of RG-8A/U and RG-213 cables without the bulk.
Radio Navigation - GPS, Loran C, RADAR
The Federal Navigation Plan
"The ideal navigation system should provide three things to the user. First, it should be as accurate as necessary for the job it is expected to do. Second, it should be available 100% of the time, in all weather, at any time of day or night. Third, it should have 1005 integrity, warning the user and shutting itself down when not operating properly. The mix of navigation systems in the US is carefully chosen to provide maximum accuracy, availability, and integrity to all users, marine, aeronautical, and terrestrial, within the constraints of budget and practicality.
"The Federal Radionavigation Plan (FRP) is produced by the U.S. Departments of Defense and Transportation. It establishes government policy on the mix of electronic navigation systems, ensuring consideration of national interests and efficient use of resources..."
American Practical Navigator, Bowditch.
As electronic navigation has advanced over the years, many systems have phased out in favor of modern improvements. Many systems have become active or defunct through the policy of the Federal government and the FRP. Radiobeacon signals were once the primary navigation aids during the first half of the 20th century. They are no longer in use today. Loran C, which is accurate to as far as 2000 miles offshore, is slated to be discontinued in the indefinite future, but still remains current because of a large number of users. Today, GPS has become the tool of choice for electronic navigation.
There are two principal types of GPS devices available on the consumer market, the Wide Area Augmentation System (WAAS) and Differential GPS (DGPS). The difference is algorithms within the device to correct for accuracy. DGPS is the type used in commercial and military marine navigation, WAAS is more prevalent in terrestrial use. Antennas for GPS are required for units installed in wheel houses and used in cars. The GPS signal is filtered out by modern windshields and enclosures. This necessitates installation of short antenna run to open skies.
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