Radar installation

Radar installation

It would be difficult to overestimate the value of radar as a navigational tool. Given the choice, as a navigator, if I could choose only one electronic navigational aid for my nav station, it would be radar. Is this heresy? Perhaps. While it may not be able to tell you where you are to within a few yards (although the radar “fix” is an excellent and relatively accurate navigational tool), it will tell you when there’s a 600-foot containership traveling at 22 knots bearing down on you, or when you’ve made a fatigue-induced plotting error that has you, your vessel and your crew standing into danger. It will help you find aids to navigation, both those that are shown on your chart and those that are not, and it will allow you to penetrate fog, rain and darkness to see other vessels, floating debris and the shoreline.

Scanner units

Scanner installation locations, as mentioned above, are for the most part limited to one of three location; the mainmast, a dedicated pole or the backstay. Any of these locations will offer acceptable performance from your radar scanner; however, there are several pros and cons to consider. Mast mounts are among the most popular; this places the scanner well above the deck (we will discuss elevation issues later) and out of the way of nearly everything, except perhaps the jib, chafe being an ever present problem with this location. Off-the-shelf and custom-made scanner guards mitigate the effects of chafe, and are a necessity for larger, open-array scanners.

The cabling for a mainmast- or mizzenmast-mounted scanner must be run within a conduit within the mast. If a conduit is not present, or if it’s not large enough to accommodate the typically large cross section of a scanner cable, then this may present a problem. If the cable is run outside of a conduit, its life will almost certainly be shortened as a result of chafe, not to mention the incessant slapping that invariably occurs with this type of nonapproved installation. In some cases, mast-mounting a scanner will require unstepping the mast to pull the cable and mount the scanner, or to install a supplemental conduit.

Dedicated pole mounts (the term pole is used here to differentiate it from a main- or mizzenmast; however, these often are referred to as radar masts), usually located aft near the transom, also offer an out-of-the-way location for scanner installation. The elevation is invariably lower, but this usually isn’t enough of a factor to discourage its use, and cable runs are considerably easier and less chafe-prone than mast mounts. The pole often can be used to mount other antennas, although care must be taken to keep them out of the scanner’s beam, to prevent interference or damage to other electronics.

Finally, the backstay, using one of the available dedicated backstay-mounting systems, also offers a suitable location for reasonably sized closed-array scanners. All backstay mounts include a self-leveling or gimballing device, which keeps the scanner’s horizontal beam parallel to the waterline. Backstay mounts also place the scanner lower than if it were mounted on the mast. Once again, this isn’t a serious enough issue to disqualify it as a suitable location. Stepping and rigging a mast with a backstay-mounted antenna is more laborious and time consuming, but once it’s rigged, this is a nonissue.

Whichever location you choose, take into account two factors. Primarily, the scanner must have a clear view of the horizon, and it should not be installed where it may irradiate crewmembers. It is often thought the main- or mizzenmast presents an unavoidable obstacle or shadow for any scanner installation. However, if the antenna element, the portion of the scanner that rotates, is wider than the mast (most voyaging-vessel masts are no more than 12 inches wide), then the beam actually envelopes the mast section, which makes it nearly transparent to the signal. Even if this were not the case, unless you are capable of holding your vessel on a rock-steady course that deviates not even a single degree to port or starboard, the normal yawing motion allows the signal to sweep and move enough to overcome this shadow.

Exceptionally small scanners, the dinner-plate variety, mounted directly ahead of a mast, may have a semipermanent blind spot astern. The blind spot would be a possible safety hazard for vessels overtaking this type of scanner installation, and the operator should be aware of this possibility. For pole and backstay mounts, this is much less of an issue. I have sailed aboard vessels with a noticeable blind wedge caused by the mast (it’s often evident by a lack of sea return – waves reflecting the radar signals – in a clearly defined wedge on the mast side opposite the scanner). This does occur even on pole- or backstay-mounted scanners, particularly for large mast sections. However, disconcerting though it may be, unless the vessel is moving on rails, most returns will be “painted�VbCrLf or bombarded by the radar’s signal as a result of normal yawing motion. If your radar installation suffers from this anomaly, it may be worth intentionally yawing your course if you are operating in placid conditions while in close proximity to other targets, such as entering a calm anchorage at night or in fog.

Two final notes on mast mounts: First, never install a scanner in the same plane as the spreaders, boom or a radar reflector. These can create sizable blind spots or spurious echoes that will render your radar returns unreliable. Second, if you are installing a radar that offers ARPA (automatic radar plotting aid), it is important that the scanner be kept as stable as possible. This system requires stable, repeatable targets to function properly. This means a high mast mount may be less than ideal, as the vessel’s motion will be accentuated by the elevation. Because of this, the ARPA feature also will benefit from a self-leveling mount.

Gimballing or self-leveling mounts are worthy of elaboration. The vertical beam angle, or VBA, of most small marine radars is between 20� and 25�. This means the signal is vertically wedge-shaped at this angle. It paints some of the sea and some of the sky nearly all the time, as long as the vessel remains on an even keel. While heeling, however, some of the wedge points down into the sea while some points more toward the sky (it’s possible to see low-flying aircraft with marine radar).

Beyond a heeling angle of about 15�, this can become problem. A disproportionate section of the wedge is painting the sky and the sea directly adjacent to the vessel, rather than looking out toward the horizon. Thus, if you intend to use your radar under sail and your vessel routinely heels beyond 15�, you may wish to consider a self-leveling mount. These are available for all three of the previously mentioned installations: mast, pole and the backstay.

As mentioned, one consideration when selecting a scanner location is elevation. Because radar signals, much like your VHF radio, travel in a line of sight, it would seem that the higher the antenna, the better. A higher scanner can extend the radar’s range. To an extent, this is true; however, a higher perch for your scanner also presents a potentially less stable platform.

The math also deflates some of the argument of maximum elevation. A scanner’s range, as a function of its height, is derived by adding the square roots of the antenna and target heights and then multiplying by 1.23. If, for instance the antenna is 20 feet above the waterline and the target, a mountainous coast in this case, is 485 feet high, the radar should be able to see the peak when the vessel is 32 miles away. As a comparison, a scanner mounted 15 feet above the waterline has a range of approximately 4 nm (to the sea surface rather than an object above the water, such as a vessel or land), while a 30-foot elevation increases this to about 6.5 miles, not much of a gain for doubling the elevation. Thus, attaining maximum elevation for a scanner’s installation is not necessarily a prerequisite.

Additionally, greater elevation also means the radar’s minimum detection range is increased. Since radar is most often used on the shorter ranges, this could be considered a serious handicap. For example, a scanner mounted 9 feet above the waterline will have a minimum detection range of approximately 27 feet and a maximum range of 3 nm, while a scanner mounted 32 feet above the sea surface increases those numbers to roughly 145 feet and 7 nm, respectively (these figures are approximate and may vary from manufacturer to manufacturer). The user will have to make this decision. However, my preference is for enhanced short-range rather than long-range detection. When you are nudging up to a navigational mark or clearing an entrance in a jetty, close-range performance will pay dividends.

Display issues

The display installation options make for a relatively short list when compared with those of the scanner. They can be placed in the cockpit, either at the helm or under the dodger or at the nav station (or both, most modern marine radars can be easily set up with repeaters). Nearly all LCD radar monitors are weather resistant and work well in sunlight, so helm mounts work nicely, although it’s almost certain that these monitors won’t last as long as those tucked away under the dodger or at the nav station. Still, it’s hard to deny the advantage of the helmsman being able to see the image up close and manipulate the radar’s functions, particularly for short-handed crews. If you opt for this installation, be certain the mounting does not obstruct any drain holes that are located on the monitor’s casing, and follow the manufacturer’s instructions concerning the minimum safe compass distance. Ideally, the compass should be swung once the installation is complete.

Primary radar installation connections are straightforward. They call for a reliable power supply and the interconnection of the scanner and monitor. The DC supply for the radar should be routed through the appropriate circuit-breaker and wire combination, a minimum of 16-gauge wire, perhaps larger depending on the distance from the monitor to the main electrical panel (this is an issue for helm- or cockpit-mounted monitors). The American Boat & Yacht Council’s 3 percent voltage-drop formula should be used for the radar and all navigation electronics installations. Most radars are equipped with their own case-mounted fuse or fuses, which sometimes are proprietary (obtain spares and make sure they fit). However, this does not negate the need for a panel-mounted circuit breaker.

When making cable runs, provide appropriate drip and service loops. These will prevent water from running directly onto the connections, and it will allow for future repairs or replacement of plugs or other terminations, should damage or corrosion occur.

Scanner cable care

Scanner cable runs create a much greater challenge for the installer. This run often is circuitous, and it must pass from below to above the deck. Because the run often is long and difficult, these cables frequently are damaged during installation. Even the smallest hole in this cable’s jacket can wreak havoc months or years down the road. Once water enters the jacket, corrosion will set in quickly, compromising the cable’s shield and possibly the other gossamer conductors found within this cigar-sized bundle. Avoid running scanner cables parallel to electrical supply cables or adjacent to other electrical gear, such as motors, fans or pumps.

Chafe also is a problem for this cable, particularly where it exits the mast and/or enters the scanner housing. Further exacerbating this problem are self-leveling mounts. While scanner cables often are rugged, they are not designed to accommodate constant movement, such as that which may be found at the interface between a self-leveling mount and the vessel. Most self-leveling mounts advertise their dampened, non-oscillating feature, which means they won’t swing like a pendulum. That’s more for the benefit of the radar image than the cable. However, it reduces but doesn’t eliminate the possibility of chafe. The bottom line for this scenario calls for careful installation and regular inspection of the cable in this region. Augmenting the cable’s jacket with UV-resistant electrical tape or heat-shrink tubing, and a proper grommet where the cable passes through the mast or base may reduce damage.

Some radar manufacturers call for an unbroken run of scanner cable; i.e., no connections are allowed. While this makes for the most reliable type of electrical installation, it’s impractical for mast-mounted scanners. When it comes time for the mast to be unstepped, unthreading the cable from the mast or the vessel could be exceptionally time consuming and may actually stress or damage the cable. In the event a connection needs to be installed (it should be made below deck), a proper, barrel-type locking, shielded plug should be used (unless you are exceptionally electrically inclined, this plug should be installed by an experienced marine electronics technician). Avoid using exposed terminal strips, as these are unshielded and may create problems with signal loss or interference.

Finally, don’t forget to bond or ground your monitor. Most radar manufacturers call for a dedicated chassis or case ground on the monitor and some require this in the scanner base as well. Bonding will reduce the likelihood of interference and may offer some measure of protection in the event of a lightning strike.

Power needs

It’s difficult to discuss radar-connection cabling and power supplies without mentioning power consumption. The amount of power a radar uses is related to its power output (2-kw vs. 4-kw scanners, for instance) and the display type (LCD vs. cathode ray tube). For comparison’s sake, let’s look at two average radar units, one small and one large, to determine their comparative power consumption and what this means for the voyaging vessel’s battery bank.

A 15-inch (radome or closed array), 2-kw radar will draw approximately 2.5 amps while it’s transmitting, while a 3.5-inch (open array), 4-kw unit may draw roughly 5.5 amps in the transmit mode. This means, over the course of an eight-hour watch, if the radar was used in the transmit mode for the entire time, it would consume between 20 and 44 amp-hours. If your vessel’s house battery bank is moderately sized, say 400 amp-hours (two 4D sized AGM batteries), about 200 of which are usable (it’s preferable to not use more than 50 percent of your battery bank’s capacity between charging cycles), then you will have consumed between 10 percent and 22 percent of your usable battery capacity during this watch period. Most radars draw about half power while on standby, so if you leave the radar on standby for some portion of this time, the consumption will be reduced.


Modern radars are capable of displaying a plethora of additional information on their screens, including position, speed, satellite reception info, true and magnetic heading, course, depth, temperature, time and barometric pressure. Connections to the monitor chassis, in addition to the power supply and scanner cable, include a heading sensor (this is mandatory for ARPA operation), NMEA interface cable, GPS input and an external alarm. In some cases, if your vessel is equipped with a heading sensor (this is simply a fluxgate compass, such as those used for autopilots), you may be able to direct that heading information to the radar via the NMEA interface. The options and possibilities are nearly endless. However, a careful review of your owner’s and installation manual will yield the required connection and port setting information.

Today’s voyager has a host of options and features where radar is concerned. Proper installation, however, shouldn’t be considered an option. If you want a reliable radar to guide you and your vessel safely, one that works well and works when you need it, then make sure it’s installed properly from the start.

Contributing Editor Steve C. D’Antonio is vice president of operations at Zimmerman Marine in Cardinal, Va.

By Ocean Navigator