Antenna challenges


Whether it be VHF, HF SSB or satcom, radio communications is impossible without an antenna. Marine electronics installers and ham radio enthusiasts know this and will pay particular attention to getting antenna installations correct. Why are these elements so important and will the increasing use of satellite-based communications make the antenna issue a thing of the past?

Any electrical conductor can act as an antenna — you just need electrons flowing through it. Depending on its size and the material used and some other factors, this antenna may be well suited or a complete bust at the task of efficiently radiating a particular radio frequency. The length of an efficient antenna is tied to the frequency you’re trying to radiate; the accepted rule is that a quarter-wave antenna will be in the sweet spot of efficiency and manageable size.

In the case of marine very high frequency (VHF) radio, the frequencies used are short enough so that the small antenna you place at your boat’s masthead is good enough to fulfill the quarter-wave requirement. For example, for 156.8 MHz, which is marine VHF channel 16, the full length of a single wave is 6.27 feet and the quarter-wave is 1.56 feet.

When we move on to the marine high-frequency (HF) band, which is roughly 4 MHz to 26 MHz, we’re talking about lower frequencies than VHF and thus longer wavelengths. A 1.56-foot antenna clearly won’t be suitable for our quarter-wave antenna. Take the lower end of the band: 4 MHz. The full wavelength at that frequency is 246 feet, and a quarter-wave is 61 feet. A 61-foot long whip antenna might prove unwieldy to mount on a voyaging boat. Not only that, but we will probably want to broadcast and receive at more than just that one frequency. We can get close to the proper length by using a nice long wire. One of the longest wires on a voyaging boat is the backstay — few boats are big enough, however, to have a 61-foot backstay. The way around this problem is to use an antenna tuner. This is a way to change the electrical length of the backstay antenna and allow us to transmit on a variety of frequencies.

When we transmit from a boat, whether at VHF or HF frequencies, we are sending radio energy out in all directions. This omnidirectional approach makes sense because as our boat moves and changes course, the direction to other boats and receiving stations on shore will change.

What if you’re a small HF coast station like one of the members of the new Trans-Atlantic Cruisers’ Net sponsored by Seven Seas Cruising Association (see this issue’s special section on communications for more on this) and you want to send your radio energy out over the ocean and not toward land areas? The solution for these intrepid radio operators is to use a directional antenna. An insulated backstay element is officially considered a dipole antenna. Coast station operators like Glenn Tuttle of KPK in Punta Gorda, Fla., often use a type of antenna called a Yagi. This antenna type looks like an old-fashioned TV antenna on steroids. It has a long central beam crossed at 90 degrees by several shorter elements. One of those cross elements is fed by the hot antenna lead from the radio. This driven element is usually offset from the center so that there are, for example, two elements on one side of the feed line and one element on the other. The driven element radiates the signal; the other elements radiate it as well but slightly later so they are out of phase with the driven element. The two elements are set up such that when their signals combine they are in phase. The signal going to the other end of the antenna with the single cross element ends up out of phase. The result is that a very strong signal goes in one direction and a weak signal goes in the other direction (check out the excellent animation at–Uda_antenna). This is the way that stations like KPK can use their Yagi antennas to send their signals out over the ocean and better serve their marine users.

What about antennas used for satellite communications? The sight of fiberglass domes on ships and large yachts has become standard. Inside those domes are stabilized dish antennas, a different animal entirely from the long wire HF antenna or even the multi-element Yagi.

Since most satcom services — such as Inmarsat — make use of geostationary satellites, they are attempting to pick up signals from 23,000 miles away. That means a simple dipole isn’t enough for sending and receiving a high bit rate; they must use a dish to concentrate all the available signals to the receiving element, and stabilizing motors are needed to keep the dish aimed at the satellite.

What about satellite phones? Inmarsat, the Iridium satellite system and Globalstar system all offer satellite phones with simple antennas. One reason is that these services use UHF signals, which allow for a shorter antenna. In Inmarsat’s case, the signals do go out to geostationary satellites and back again. But Inmarsat is able to keep the antenna small by keeping the data rate low compared to its dish-stabilized services.

Iridium and Globalstar also use small antennas on their phones. But in the case of these two services, they make use of low-Earth orbit (LEO) satellites that are only in 500-mile-high orbits.

There are a several companies, such as SpaceX, that have announced global Internet satellite service. These services will make use of thousands of LEO satellites to provide high data rates to users worldwide, which will include voyagers at sea. It will be interesting to see how these companies will solve the antenna challenge.

By Ocean Navigator