Traditional Pulsed Energy vs. Sweeping Frequencies
Revised by BoatUS editors in June 2012
Radar and sonar both transmit a short, powerful, single-frequency burst of energy. In radar, units since World War II utilize a microwave-emitting device called a klystron or magnetron; sonar transducers vibrate a hunk of ceramic material at high frequencies. Both blast out a “main bang” or “tone burst” and wait for a return echo, then the software inside the display translates this reflected energy into a picture.
CHIRP (Compressed High Intensity Radar Pulse). Instead of transmitting only 200 or 50kHz, for example, CHIRPing devices transmit a signal that sweeps linearly upward (from 40 to 75kHz, 130 to 210kHz, or other frequency ranges).
Traditional “tone burst” fishfinders have always demanded a trade-off between pulse length and target resolution, and no targets can be resolved unless they’re larger than the physical wavelength of the pulse.
Broadband Radar sends a continuous transmission wave with linear increasing frequency (hence the term Broadband). The wave retains its frequency as it travels out and reflects back from any objects. Meanwhile, the transmitter continues to output an increasing frequency. The difference between the currently transmitted and received frequencies, coupled with the known rate of frequency increase, is how the radar precisely calculates a “time of flight” and target distance. Since FMCW constantly builds up radar return energy (vs. transmitting a single pulse), this system provides target detection superior to pulse radars while transmitting at far lower energy levels.
Broadband Radar Plusses
Navico’s radars offer the following advantages over traditional radar:
Lowest RF transmission for safe, flexible installation: Broadband Radar 3G transmits at 1/10,000 the power of typical pulse radars (emitting about 1/5 the energy of an average cell phone), so the radome is safe to mount in locations never before possible. In addition, the lowest DC power draw of any X-band marine radar makes this system well suited for sailboats and other vessels with limited power.
Improved short-range target discrimination: Broadband Radar provides better target resolution, even at an amazingly close 1/32 nm range. Docks, channel markers, moored vessels and other critical targets are displayed with clarity and separation, for added confidence in close quarters. Broadband technology also eliminates the “main bang” of a pulse radar—the obscured “dead zone” immediately around the vessel—which interferes with close target detection.
No warm-up time: no more waiting 2-3 minutes for a magnetron to warm up. When darkness falls or the fog rolls in, you are always ready. Ideal for sailboats wishing to save power or boats at anchor not wishing to run the radar continuously.
Broadband Radar Minuses
Now for the bad news: much lower maximum range than conventional radar. Although they excel in close-range navigation, broadband radar is inferior to the old-school high-powered technology outside the three-mile range or so, but they’re closing the gap. Navico has mitigated this problem with their 3G radar, doubling the transmitting power of the BR-24, and increasing the range by 30 percent. Their 4G device narrows the gap even more, with maximum range of 36 nautical miles.
If you have ever been suddenly engulfed by fog, making passage at night or disoriented by an array of confusing navigation lights against the backdrop of a light cluttered shoreline, then you will appreciate how radar's electronic vision will enhance your ability to navigate safely. Radar has advanced significantly in recent years and your radar screen today (if you want to pay the money) can not only tell you what the transmitter above “sees” but, if interfaced with other electronics such as, for example, a GPS chart plotter and AIS, it can also tell you names of other ships, their direction of travel, make collision risk avoidance solutions and even supply a three dimensional image of the bottom or of the relevant chart.
How Radar Works
Radar "sees" objects by reflected radio waves instead of light waves, but the principle is the same. For example, a basic radar transmitter sends out a constant stream of short pulse radio waves (at 162,000 nautical miles per second) through a revolving antenna (or scanner), and a computer inside the radar receiver measures the time delay and calculates the distance. The rotating scanner simultaneously determines the bearing from which the echo came. The results are then converted and displayed on the radar screen.
But recent developments have gone far beyond that old basic technology. Now, with developments such as “CHIRP” (Compressed High Intensity Radar Pulse) technology, the unit, instead of sending out single bursts of energy, will send transmit signals over a frequency range. New technology has the units sending out a signal in increasing frequency over a set range, hence the term “broadband” is often used to describe these units. This can result in a fare more accurate and easier interpreted depiction and clearer resolution. This also results in other improvements such as lower and thus more efficient and safer energy levels, less of a “dead zone” problem (the densely cluttered zone close in to the boat), better short range resolution and less warm up time. This later may seem unimportant, but when you suddenly need radar and it’s off (to save power or save aging of the unit) a 2 or 3 minute wait can be a problem. But shorter range may be an issue on some sets. Like most electronic technology, this is changing daily and it’s best to seek out the very latest info before you buy. Don’t just go in and “pick one up off the shelf.” This technology and others in the pipes was developed originally for military use. Eventually the public benefits and you can usually expect more improvements. Like anything on a boat electronic in nature, stay tuned, it’ll continue to change and usually for the better.
There are different types of displays available. For example, there are LCD displays and larger, heavier, CRT displays. LCDs are most common in cockpits and on flybridges because they show up well in sunlight, are easily disconnected and stored below, and some are waterproof. Color displays are more expensive than monochrome displays, but can be much easier to read. A critical consideration will be ease of reading the screen in sunny daylight. Even if your display is to be mounted inside in a wheel house, its readability in high light is important. Most sets allow you to adjust brightness. This is also important because at night the same brightness as needed during daylight will impair your night vision. Ask for a demonstration of the display before you buy.
Transmit power determines the strength of the signal sent by the antenna and the corresponding echo back--the stronger the transmitter, the more likely soft or distant targets will show up on the screen. It has been reported that most users operate their radar within a 4-mile range 95% of the time, where high power is not critical. Antenna height is also a factor because radar is a line-of-sight system.
Open or closed array antenna?
As a rule, the larger the antenna, the better its chances of distinguishing separate objects that are close together in congested areas. Open arrays are generally larger and more likely to be found on powerboats. Closed arrays, called radomes, are usually reserved for sailboats where protection from sails and rigging is a concern.
Magnetron: a common cause of radar demise is the failure of the magnetron, which is the wave emitting device. This happens eventually with age. Many think that the magnetron is not “aging” when the radar is on standby. This is a setting available on many units. (Commonly there is no sweep on the screen when the unit is on standby.) But in many sets the magnetron is still being used when the set is in this mode and if you want to “go easy” on the magnetron you need to turn the unit off. However technology is developing other components to do the job of the magnetron. As always with electronics, check for the latest developments before you buy.
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