“The amount of seabed displayed on the
fishfinder screen depends on the frequency used.”
The amount of seabed displayed on the fishfinder
screen also depends on the frequency used. Generally,
the ‘bottom band’ displayed will increase when
using low frequency and decrease when using high-
frequency ultrasonic waves.
When you want to get a good view of the seabed and
judge its properties, it is recommended to switch to
the 50 kHz transducer.
In deeper water you may need to reduce the
scrolling speed that the image moves across your
screen in order to allow the processor time to display
the high volume of information.CHIRP
All of the major brands of depth sounders featured
in this article offer CHIRP (compressed high-intensity
radar pulse) technology and/or a rebranded version
of similar intelligence. The CHIRP ideology is quite
simple: instead of sending just one frequency from the
transducer, it sends a continuous sweep of frequencies,
ranging from low to high. CHIRP sonar technology
then interprets the multiple frequencies individually
when the echoes return. This continuous sweep of
frequencies provides CHIRP with a much wider range
of information, which is why this sonar is able to create
a much clearer, higher-resolution image.According to Raymarine’s website, CHIRP
transmissions allow more sonar power to penetrate
the water and are in fact 10 times more powerful
when compared to conventional sonar modules.
Traditional echo sounders rely on a
distinct frequency pulse to rebound off fish, an object
or the bottom. The amount of energy required for the
signal to reach the bottom in deep water when using
a low-frequency signal such as 50 kHz requires a long
sonic pulse (possibly 40m in length). Any objects that
are closer together than the length of this pulse will not
show up as separate objects on the screen.
In contrast, CHIRP technology modifies the transducer’s
output to transmit a modulated pulse. This separates
targets that are closer together using a different
frequency to those further apart, with each target
showing up separately on the monitor. This range
or pulse sweep is known as the bandwidth, hence the
term broadband sonar.EXPERT’S TIPS FOR GETTING DEEP
Mark Reid – Furuno
Mark is an experienced gamefisherman and an expert
on fishfinders and their use. As a veteran of national
boat shows and one of the key personnel at Furuno
Australia, he is well practiced with helping anglers
maximise the capabilities of their fishfinders.
Mark offers the following tips to find more fish in
deep water:- Run in ‘Manual’ mode with no interference filters.
- Only use filters if getting electrical noise on the
screen. - Run pulse length on ‘Long’ for deep water.
- Set your ranges to suit the depth by making sure
you use the full screen. The bottom image needs to
be in the lower section of the screen. - Set all clutter to zero before slowly increasing the
Gain until you get a bit of blue ‘snow’. Once the ‘snow’
or ‘noise’ is even throughout the screen, turn the
This screenshot is from a Simrad NSS Evo3 using an S5100 sonar module and a 3kW R509LHW
transducer. The left-hand side of the split-screen view shows from the bottom up to 250m in
a Low-CHIRP mode. There is a large school of bait, or multiple predators, marking at about
325m. Below that, in the deeper water, the band of light-blue shading shows what scientists call
the deep scattering layer, consisting of plankton, squid and small creatures that roam from the
depths during the daytime to the surface at night. This layer is a good sign when looking for
deeper-water predators.
The right-hand side shows just the top 200m of the water column, with a 200 kHz frequency
wide beam giving greater coverage and more detail. This is helpful for keeping watch for surface
action, even while you’re searching the bottom in deep water.44 facebook.com/BlueWatermagazine
Sounding Out The Final Frontier