A narrow view helps
  |  First Published: June 2004

MOST sonar users are well aware that the transducer portion represents the ‘eyes’ of the sonar.

For the uninitiated, however, the transducer is the part which plugs into the sonar’s head unit and is mounted to the transom of the boat, or set in a wet box or fitted through the boat’s hull. The transducer is the integral piece of equipment that emits and receives sound waves to establish the picture that appears on the sonar screen.

The sonar signal that the transducer emits shoots an ever-increasing sound beam, similar to that of a torch beam. The strongest signal is in the centre of the beam and the power of the signal decreases the farther you move away from the centre.

Transducers differ in producing various strengths and various degrees of angle in their beams. The degree or angle that the beam is emitted at is referred to as the ‘cone angle’.

The industry standard for establishing cone angle is measured by locating the peak power point directly under the centre of the transducer and then locating the half-power point (a measurement of -3dB) on either side of the peak power point. See the diagram for further explanation.

Over the years, certain sonar manufacturers have made claims that transducers with wide cone angles are far superior to those with narrower cone angles. Their argument was that the larger the area that your transducer beam covers, the more chance you have of finding fish.

While this may be correct, the big problem (particularly for offshore anglers and deep-water scenarios) is that a fish echo return from a wide cone-angle transducer may well be displaying fish tens of metres away from the boat.

My long-time association with marine electronics company Lowrance Australia has allowed me access to a wide range of products for testing and I have established my own ideas relating to cone angles in transducers.

From my experiences very early in the piece, I have found that narrow-beam (8° to 12°) transducers are far superior for offshore reef fishing. When targeting reef fish and many pelagic species I find it essential to know exactly where I am on a reef structure.

When targeting jewfish on offshore reefs there are times when I want my boat positioned directly over a gutter in a reef system. For kingfish, I might want to be on top of the reef right where the baitfish are holding. And many of my best snapper are taken on the edges of the reef. It is imperative that my sonar tells me what is directly under my boat – not what might be 10 metres off to my left, right, front or rear.

Consider the diagram hereabouts that shows a vessel searching for a wreck– it could well be a reef structure or similar. Yes, the structure would have displayed on a sonar screen if a 20° transducer was being used, but not if using an 8° transducer in the boat’s current position. To find it using the 8° would have required a little more searching.

The most important point is that the boat may well be anchored in its current position and, if using the 20° transducer, the occupants may believe themselves to be fishing on top of the structure because it falls within the transducer’s beam and appears on their sonar screen.

The prevailing conditions would play a part in whether they were or weren’t actually effectively fishing the structure, but there is simply no doubting that if the structure appeared on their sonar screen using an 8° degree transducer, they would be guaranteed of being that much closer.

I know I’d much rather be guaranteed of what is virtually directly under my boat than what might be within a 20-metre radius or so of the boat. I’ve also found that narrow-beam transducers appear to offer better target separation and a clearer, crisper picture at normal sounding speed and at higher boat speeds. Interesting food for thought I think. Lowrance currently offer the choice of 20° or 12° transducers for the majority of their Lowrance and Eagle range. If you’re interested in further information on narrow-beam transducers for these products, call Lowrance Australia on 02 9905 9700.

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