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Marine Electronics Frequently Asked Questions

by Dave Alston

VHF Radio

Q: I’ve been looking at a new VHF radio and noticed that the transmitter specifications say 25/1 watt power. Why are there two power ratings, and are all radios the same?
A: All fixed mount (non-portable) marine VHF radios have high and low power modes of 25 and 1 watt. FCC regulations limit the maximum transmit power to 25 watts and restrict transmit power to 1 watt on certain channels.

Q: My VHF radio receives well, but when I transmit other boats say there is a buzzing or humming noise and the range is less than it used to be. What do you think the problem is?
A: It sounds like you have a voltage issue. When your radio transmits it draws more current, and if your battery is low or you have a bad connection anywhere on the power cable, connector, or even fuse panel, the voltage will drop below 12 volts. Check the 12 volts DC as close to the radio as possible while you key the microphone. If you don’t have a voltmeter, one quick test is to watch the panel lights on the radio and see if they go dim when you key the mic. Find more VHF radio technical tips here.

Q: Someone told me that if I bought an extension for my VHF antenna that I could increase my range. How much further will my radio reach if I add an 8’ extension to my antenna?
A: A VHF radios range is limited to line-of-sight. While increasing antenna height is the best way to gain more range, raising your antenna by 8’ will produce only a small increase in the distance you can effectively transmit or receive. Using a high gain antenna (9dB) will “focus” the radiation pattern of your radio and provide increased range. As a general rule of thumb, every 3dB in antenna gain “effectively” doubles the radios transmit power.

Depth Sounder / Fish Finder

Q: I mainly fish in less than 100’ of water. My fish finder has both 50 and 200 khz. Which frequency is the best?
A: 200 khz provides higher resolution than 50 khz, and is best suited for operating in depths of 200 – 300 feet and less. Your ability to define bottom structure, shape and size, as well as “mark” individual fish in shallow water is much better at 200 khz. Most transducers send out a narrow beam at 200 khz and a wider beam at 50 khz. Using the 50 khz signal to search a wider area then switching to 200 khz to “zero-in” on your target is a common method used by fishermen. For more information on depth sounders and transducers check out the article, “Behind Every Successful Depth Sounder is a Great Transducer”.

Q: I have a new boat and plan on installing a fish finder. I really don’t like the idea of putting a hole in the bottom of my boat to install a transducer. I’m thinking about going with one that shoots through the hull. Do you lose a lot of signal with this kind of transducer?
A: In-hull, or shoot-through-the-hull transducers offer an attractive alternative in some installations. However, there is a substantial loss of signal when the transducer face is not in direct contact with the water. The exact amount of attenuation depends upon the thickness of the fiberglass. You should never attempt using in-hull transducers with metal, wood, or composite hulls. Always following the manufacturer installation instructions carefully.

Q: How can I tell how much of the ocean bottom I’m looking at on my depth sounder?
A: Different frequencies and different transducer designs affect the beam width of the signal your fish finder has. As a general rule of thumb, higher frequencies (200 khz) have a narrower beam width than low frequencies (50 khz). Look up the specifications to your particular transducer in your operations manual and plug in your numbers to the Beam Angle Calculator on our site.


Q: I’ve heard that open radar antennas are better than dome units, and why are the open ones so much more expensive?
A: As far as the cost goes, the primary reason an exposed radar array (antenna) is more expensive than a dome is the manufacturing cost. It’s just cheaper to put everything in a waterproof dome than it is to build a unit with exposed moving parts that can withstand the elements. Generally speaking, exposed antenna units outperform domes but the dome doesn’t have anything to do with it. Longer arrays have a more focused or narrow beam width and therefore function more efficiently. Think of it like comparing a flood light to a spot light.

Q: My radar just doesn’t pick up as well as it used to, and I’ve noticed that some targets look like they have a shadow behind them. Could re-tuning and adjusting other settings correct this, or is it time to call a technician?
A: The symptoms you’ve described are classic signs of an aging magnetron, which is the main transmit component in the radar. The “shadow” effect you noticed is commonly called target doubling. This isn’t a task for the average boat owner. It’s time to call in the professional. Find other useful radar tips on our Radar Technical Tips article.


Q: I bought a new GPS for my boat and the instructions say to mount the antenna clear of all obstructions. Unless I put the antenna on an extension mast there’s no way to be totally clear of everything on my boat. How critical is this?
A: The ideal situation is to position the antenna where it has an unobstructed view of the sky from 20 degrees above the horizon in a complete circle. In reality, that can seldom be achieved and your unit will work fine even with a few things in the way. At any given time there may be from eight to twelve satellites in view of your GPS antenna. It only takes a minimum of three to obtain a good fix, so if a few are temporarily blocked, it won’t adversely effect the operation of your GPS. Just use common sense and do your best to keep as much separation as possible between the GPS antenna and any potential obstruction.

Q: I’ve noticed that the GPS readings seem to be wrong in and around my marina. My own boat position on the plotter isn’t correct either. The amount of error seems to vary also. What could be causing the problem?
A: The fact that you noticed an inconsistency with the error, and that it happens in and around the marina suggests that the problem may be from electrical interference, or a condition know as multi-path error. Interference can come from a number of sources. One of the most common are TV antenna amplifiers. The other possibility is that multi-path error is caused when the GPS signal from the satellite reflects off local objects and provides two different signal sources of adequate strength for your GPS receiver to process. When you’re away from the dock and your GPS is operating normally, check the signal strength and SNR (Signal to Noise Ratio) of your unit. Most marine GPS receivers have this function and can be found in your operation manual. Look for a significant change in your readings when the unit is working well and when you’re experiencing problems. It’s then a process of elimination to find the source of your problem. Read more about marine GPS receivers here.


Q: I hear a lot of talk about AIS these days. What exactly is AIS?
A: AIS, Automatic Indentification System, is similar to an air traffic control system, but for boats. Although busy ports do often have Vessel Traffic Control centers, AIS units work independently and automatically communicate vital position, navigation, and identification data between themselves. AIS units can interface with most modern electronic chart plotters and provide a “picture” of other AIS equipped vessels in your area. Most commercial vessels are required to have AIS but the typical pleasure craft is not. AIS is one of the most significant navigational aid developments in recent history, and vessels that use AIS have a distinct advantage when it comes to safety and convenience.

Q: What is the difference between and AIS “A” and “B”?
A: There are currently two classes of AIS. The primary difference is that Class A units report their position and other data every 2 – 20 seconds where as Class B units report (transmit) every 30 seconds. Transmit power on a Class A unit is 12 watts in order to maximize range but only 2 watts with Class B. There is also a significant price difference between the two units. Class A transponders will cost between $2000 and $4000, where as a Class B unit will be from $800 to $1400. For pleasure craft thinking of adding AIS, a Class B unit is well suited for the application.