WIRELESS COMMUNICATIONS
If you have wireless broadband communications aboard your vessel you are able to access a wide variety of information that is very useful. You can use email, access websites and make low cost phone calls to mention just a few applications.
As a minimum you need a computer and a wireless modem for your chosen service provider, however by enhancing your system you can do quite a bit more.
COVERAGE
Unless you can access a wireless network cell of your chosen service provider, the game is over. Each service provider uses one of the carriers (eg Optus, Telstra and others) who provide and manage the cells. You need to find out which carrier your service provider uses and have a look at their coverage maps. The maps usually show coverage with both a normal mobile setup as well as coverage with an external antenna attached. Coverage with external antennas is obviously more extensive.
Unfortunately coverage is often optimised for land based use because seaward coverage is less of a priority for most users. Nevertheless, quite useful offshore coverage is available in many areas. Adding an external antenna is likely to improve matters.
ANTENNAS
Coverage of mobile cells is basically line of sight which means that if there is a visual path between the cell and the user’s antenna, then communication is likely to be OK. In numbers of offshore situations, the line of sight will be limited by the curvature of the Earth (and sea).
This can be calculated by multiplying the square root of the antenna height (in feet) by 1.2 which will give the mobile horizon distance in nautical miles. It is important to recognise that both the cell antenna and the user’s antenna have to be considered. If for example the user’s antenna is 9 feet above the sea level, then the user’s horizon will be 3.6 nautical miles. If the cell antenna height is 900 feet above sea level, its horizon will be 36 nautical miles. Adding the two together gives a line of sight communication distance of 39.6 nautical miles. If the user’s antenna is located on the mast at a height above sea level of 49 feet, the user’s horizon will be 8.4 nautical miles and the line of sight communication distance will now be 36 + 8.4 = 44.4 nautical miles. Antenna height is probably the most critical factor in optimising communication distance.
Arguably, the next most important factor is the antenna cable. All antenna cables have loss and as the frequency of the service increases so does the loss. For marine VHF (160 MHz with masthead installations), cable losses often halve the signal strength of the received signal as well as reducing the effective transmitter power by the same amount. Better quality cable can reduce these losses to an acceptable level. The situation is far worse with wireless mobile systems which often operate at more than 2000 Mhz. Extremely high quality cable is necessary in this application.
Lastly, the gain of wireless broadband antennas bears consideration. This is a case where more is not necessarily better. Antenna gain is not achieved by some miraculous multiplying process. It is achieved by refocusing the available radiated power in much the same way that a lens is able to focus the power of the Sun to the point where fires can be lit. For an omnidirectional antenna, this is achieved by compressing the radiation lobes of the antenna by design changes. This means that power radiated at right angles to the vertical antenna is increased while radiated power above and below that is reduced. As gain increases, this effect becomes more pronounced. The antenna may perform well when the antenna is vertical but as the boat heels or rocks in a seaway, the power radiated from the vessel to and received from the cell may be significantly reduced. The message here is that modest gain figures are likely to produce the best results.
Antenna gain is normally expressed in decibels (dB) which is just a (logarithmic) ratio which must be referenced to something else in order to make sense. If the ACME washing powder company claimed that its washing powder was 10 times better, it would be reasonable to ask “better than what?” The same applies to antennas that advertise a gain of 10 dB, and the logical question remains “compared to what?”. Just to confuse matters further, there are a variety of standards used as references for evaluating antenna gain, for example dB(I) references the performance of the antenna to an entirely theoretical isotropic radiator (in concept rather like a point source of light). Another way of expressing antenna gain is dB(D) which compares the antenna gain to that of a basic (and common) dipole antenna which to my way of thinking is a bit more practical.
WIRELESS ROUTERS
These provide another option for overcoming antenna cable loss with masthead antennas. The router is mounted near the masthead in a waterproof non metallic housing so that the standard length antenna cable can reach the wireless modem attached to the router.
This arrangement has the other benefits of enabling numbers of computers, tablets and the like to share the same wireless broadband access.
CONCLUSION
Viewing the latest and predicted weather reports with maps, radar and satellite imagery, catching up with friends and family through voice (and images) over internet services like Skype, managing your finances or just buying bits and pieces for the ship are at your fingertips with a wireless broadband connection.
The preceding information is not meant as a complete do it yourself guide but may provide a starting point to enable you to have some idea of what you need to consider when engaging professional help. The article contains some approximations designed to explain concepts without becoming too onerous.