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Selecting a marine antenna | Size of the vessel and hardware budget | Operating Area and Footprint Coverage | Rain Fade | Expected Sea Conditions | DirecTV HD in North America |
There are few hard and fast rules on what antenna would work on any particular vessel but here are some guidelines when making a selection.
It is not uncommon to see tiny antennas on large yachts and ships, or large antennas on smaller vessels, but on average, vessels over 30 meters (100 feet) would have a 1 meter or greater TV antenna. This is largely determined by the weight and windage of the antenna and the budget of the vessel.

The larger the antenna, the better the reception in adverse conditions, so if the budget allows, you will want to get the largest antenna that will comfortably fit on the vessel.

Marine TV antennas range in price from about $3000 for a 14 inch Ku-Band antenna to about $18 000 for a standard 1meter Ku antenna to  $80K for a large, 3.6m C-Band antenna.
 
If the vessel is going to operate well within the center of a strong satellite footprint, a small antenna can work just as well as a larger antenna. You only need a certain amount of signal to satisfy the receiver and in normal conditions the picture quality will be just as good with a smaller antenna.

If the vessel plans to stay within a good coverage area there is no advantage in cluttering up the decks with huge antennas. If the vessel plans to operate over a larger area, the larger antenna will provide coverage further out toward the edge of the footprint.   

You should also consider how long you expect to spend in a fringe area of the footprint. Vessels on passage from, say  the US to the Caribbean may only gain a day, or a few extra hours by having a larger antenna, as they quickly pass through the edge of coverage. However, if they plan to spend some time in the eastern Bahamas, the larger antenna would provide a big advantage.
One other important exception is rain fade. Ku and Ka Band signals are subject to attenuation by water droplets in rain and heavy cloud between the antenna and the satellite. A larger antenna with a much stronger signal will be far less susceptable to loss of signal when the skies open up with rain.

If you can live with the occasional loss of picture during a heavy downpour, you could opt for a smaller and less expensive antenna. You may regret this though, when you lose the picture during a critical scene in your favorite program.
 If the vessel is only expected to operate in calmer waters, and if one only expects to watch TV in fair conditions, one could go for a two axis antenna that is much less sophisticated and quite a bit less expensive. After all, most people tend to watch TV while relaxing in a secure anchorage or marina. 

If the vessel expects to spend much time at sea in all weather conditions, and watch TV while underway, then a three axis antenna will be required to keep the antenna on satellite while the ship rocks and rolls in the sea below.
The High Definition TV coverage in the Caribbean and Europe is presented on normal Ku-Band satellites that can be acccessed using a standard Ku-Band LNB on almost every brand and model of antenna.

DirecTV in North America is presented on two Ka-Band HD satellites at 99W and 103W and three Ku-Band Satellites at 101W, 110W and 119W. There are some special marine TV antennas that allow one to access HD TV simultaneously on 3 or even all 5 satellites. 

This does add complexity and price to the antenna, but if high definition TV is important to you, then this could be the answer.

It is important to note that you must have a HD TV or display, either connected directly to the decoder via HDMI or DVI cables, or have a TV distribution system that is capable of distributing HD signals to the HD TVs onboard. If this is not the case, having a HD capable antenna will be of little use, as the channels will still be displayed in standard definition.

All of the HD channels on the Ka-Band satellites are also available on the standard definition on 101W so if you do not have HD display capability with your TV system there is not a great advantage in having an HD antenna. You can see a DTV channel listing here.

The Intellian S6HD, S80HD and the KVH HD7 and HD11 are examples of antennas that can receive the three primary DIRECTV satellites 99W, 101W 1nd 103W. SeaTel produced the DTV04 antenna using a domestic style DirecTV dish with a five LNB feed that could simultaneously receive all 5 DirecTV US satellites. I am not sure if the DTV04 is still in production. There are rumpors that Cobham Satcom will be coming out with a 1 meter HD antenna in Septrember 2014.