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Author Topic:   Best SONAR Transducer Frequency
SJH posted 01-28-2011 11:12 AM ET (US)   Profile for SJH   Send Email to SJH  
I plan on updating my electronics this spring [perhaps with] Lowrance systems. Right now I have two (old) 5-inch [GPS receivers, chart plotter] and SONAR systems. I do mostly bay fishing, always [in areas where the depth of the water is] less [than] 200-feet and most times [in water whose depth is in a range of] 30- to 80-feet. I [keep the speed of the boat] under 40-MPH. I find that I can [purchase] two 5-inch systems at about the same price as the HDS-7. Lowrance has a new Elite 5X [down-scan image] SONAR with a 455-kHz and 800-kHz transducer. The other sonar systems have either the 83-kHz and 200-kHz or 50-kHz and 200-kHz [transducers]. Which transducer would work best for me? --SJH
contender posted 01-28-2011 07:14 PM ET (US)     Profile for contender  Send Email to contender     
If you are less than 200 feet I would go with the wide cone.
jimh posted 01-28-2011 09:38 PM ET (US)     Profile for jimh  Send Email to jimh     
You do not need a 50-kHz transducer. With an HDS-8 and the 83-kHz and 200-kHz combination transducer, I got good bottom echoes from 800-feet at 30-MPH. This is an indication of the power and signal clarity of the HDS system.

The ELITE model is a new type of SONAR which uses much higher frequency. It gives very high resolution, but that comes with narrow coverage area. It appears that the higher frequency SONAR gives much finer resolution and detail, which shows objects and bottom structure in greater clarity, but at the same time reduces fish echoes to very small dots instead of the more familiar arches seen on lower-frequency SONAR returns. If you are looking for details of bottom structure, the 455-kHz and 800-kHz transducer will be better, however the depth of water they can penetrate will be much less than a conventional lower-frequency SONAR.

SJUAE posted 01-31-2011 04:18 PM ET (US)     Profile for SJUAE  Send Email to SJUAE     
SJH--I fitted twin HDS-5 instead of a single HDS-7 jpg

with 83-kHz and 200-kHz combination transducer and have since added structurescan IMG_0021.jpg

As Jim notes the higher frequency is better for shallow water but detail will deminish relative to speed. If I recall correctly the structure scan is around 15-MPH tops for detail. I'm not familar with your newer model.

SJH posted 02-01-2011 10:08 AM ET (US)     Profile for SJH  Send Email to SJH     
Thanks for the input. I think I may stick to the 83/200. I've also seen the Lowrance is now packaging the HDS7 with side scan for a saving of a [about $200]. I've heard the the side scan is not good above 8-MPH or in rough water. Has anyone used [the Lowrance Structure Scan SONAR] in the ocean?
SJUAE posted 02-01-2011 01:58 PM ET (US)     Profile for SJUAE  Send Email to SJUAE     
Stan--I assume you mean structurescan :). Can't say I have used it above 5-MPH, as that is my kicker top end. At high speed only the 83-kHz and 200-kHz combination transducer is reading the bottom correctly

When you have the two transducers fitted (as in my picture above) you also get downscan overlay mode

Any transducer in rough water, if turbulance is getting under the hull, will give poor results if the water is aerated

If a large bow wave passes by side-on to me whilst trolling quite offten a lot of surface noise results, but that is all; the rest of the image is fine.

When I am trolling or pottering along I have the left HDS on downscan and the right HDS in split structure scan. At speed, I switch the left back to chart mode or engine info. Most of the time I am only in 20- to 30-feet of water, and it's intresting to see how the structure scan pierces though floating seaweed halfway down compared to the 83-kHz and 200-kHz transducer.

I cannot say I have come accross any interesting images worth taking a screen shot. For comparision, here is one I found during researching gillnetred-1.jpg

This was the comparision I did on HDS screen areas FYI latesthdsscreenratio.jpg


jimh posted 02-06-2011 09:22 AM ET (US)     Profile for jimh  Send Email to jimh     
In general, it appears to me that as the frequency of the SONAR emission is increased, the resolution increases, too. However, for finding fish, the lower frequency and lower resolution may be more useful, as they give a bigger echo on the sonagram. Fish at 50-kHz produce rather prominent fish-arch echoes. Fish at 800-kHz come back as little dots on the display.

If you are interesting in seeing detail in bottom structure, the higher frequency SONAR will give more detail. The range of the higher frequency SONAR, that is, the depth of water from which it can return good echoes, will be less than the lower frequency transducers. Inasmuch as your range of depth is so very limited, you will not be constrained by any depth limits with the higher frequency transducers.

jimh posted 02-06-2011 11:16 AM ET (US)     Profile for jimh  Send Email to jimh     
When operating a SONAR in water with a depth of only 20-feet, the portion of the bottom being scanned is typically rather small. SONAR transducers are specified with a particular cone angle, which is an expression of the sensitivity of the transducer with respect to signals arriving off-axis. While usually not explicitly stated, the angle measurement probably refers to an off-axis orientation at which the sensitivity decreases below a particular threshold. The threshold is generally not stated, either. We can assume the decrease in sensitivity must be fairly significant, at least enough to reduce detection of echoes below the point of usability.

A LOWRANCE 200-kHz SKIMMER transducer for an HDS unit with an 83 and 200-kHz rating is specified to have a cone angle of 120-degrees in the 83-kHz mode. This is by far the widest cone angle rating that I can recall for a SONAR transducer.

If operating in 20-feet of water, a transducer with a cone angle of 120-degrees would cover a width of the bottom of about

2 x 20-feet x tan60 = 68-feet

In contrast, if using the high frequency transducer, the cone angle will be much smaller, perhaps no more than 30-degrees. In 20-feet of water the depth of the bottom covered is only

2 x 20-feet x tan30 = 23-feet

[Revised calculation to reflect proper coverage--jimh]

SJUAE posted 02-06-2011 02:22 PM ET (US)     Profile for SJUAE  Send Email to SJUAE     

Something I had not considered before but your reply has prompted me

Consider 2 beer crates (filled with sealed empty bottles so hopefully we get a good return signal) they are positioned 10ft apart port and starboard of your transducer

With the 83-kHz and 200-kHz combination transducer you would only see one blip on the screen and would not know if its the port or starboard or both.

With the structure scan you should see two images but in 20ft of water if the crates were further apart you would not see either.

I cant say I have noticed these scanerios in real life but it's something I'm going to look out for when I come across a boulder or something

Maybe this is why sometimes I see a variance of up to 2 ft between the two displays of the 83-kHz and 200-kHz and structure scan due to cone size or averaging of water depth by the software or both.


Mr T posted 02-06-2011 04:07 PM ET (US)     Profile for Mr T  Send Email to Mr T     
I run a Raymarine A65 sonar unit and find myself using the 50 and 200 khz frequencies in all conditions. The unit uses an Airmar P58 and the cone angle on the 50 kHz side is a very nice 45 degrees, while the 200 kHz is 11 degrees.

Having both on, I can use the 50 in a "searching" style of usage, for bait, contours, and fish. The 200 gets used as the detail side. If it shows in the 50 and not at 200, I know I'm in the area. If it shows in both, It's very close to being right under the boat.

Even in the San Francisco CA bay fishing for halibut, we will seldome go deeper than 20-30 ft, but I run dual frequency the whole time. You see more area on the 50, but with less detail.

I do a lot of downrigger fishing. and often times the 200 kHz side will not show the downriggers balls and the 50 kHz will. This can be invaluable for lake fishing where the objective is to get as close to the bottom at times as possible, as the lakes we fish have a lot underwater humps that you want to "bump" with the weight, but not slam into. Often times this tactic triggers strikes from Mackinaw.

SJH posted 02-10-2011 07:58 AM ET (US)     Profile for SJH  Send Email to SJH     
Jim, you bring up good points on the angle and coverage of obtained by different frequencies. I see that Humminbird now sells a unit, model 788ci HD DI, if I read it correctly, has both 83/200 and 455/800 Hz sonar. Looks like the best of both worlds.

Very interesting....


jimh posted 02-10-2011 10:04 PM ET (US)     Profile for jimh  Send Email to jimh     
At the risk of repeating myself, let me again mention that the Lowrance HDS SONAR and its 83-kHz transducer are specified to have a 120-degree cone angle. This is by far the widest cone angle that I can recall, and this makes this particular combination very effective in shallow water because of the wide coverage.
SJH posted 02-11-2011 02:09 PM ET (US)     Profile for SJH  Send Email to SJH     
Good point taken. I've been taking a look at Lowrance units. For about the same money, it looks like I can get the same functionality. I like the idea of a wider beam.
Hoosier posted 02-11-2011 09:00 PM ET (US)     Profile for Hoosier  Send Email to Hoosier     
One thing to keep in mind is that all sonars measure range to the target. If you have a wide beam you may get a return from a target that shows, on the screen, as having a depth of x feet, BUT if the target is at the edge of the beam then it's depth is much less because you're looking at an angle.
LCnSac posted 12-27-2012 11:24 AM ET (US)     Profile for LCnSac  Send Email to LCnSac     
Dredging this old thread because it has some excellent discussions on the new finders/plotters.

I fish mostly in 12-20' of water and am usually looking for stripers. While the 120 degree cone looks like an excellent prospecting tool, I've read that the detail is lacking. Would that be the case in such shallow depths? Someone said that too much power can cause multiple image echoes in shallow water. True?

I am replacing an older Raymarine unit with a GPS/Finder and want to get the right instruments for my fishing style which never water over 25-30'. Thanks!


jimh posted 12-27-2012 02:10 PM ET (US)     Profile for jimh  Send Email to jimh     
If a SONAR transducer emits signals and responds to signals over a three-dimensional cone with a 120-degree angle, it is inherently not going to provide very much detail because all the signals coming back to it from all those angles are mixed together. If you want to get a finely detailed picture of what is exactly under your boat then you use a very narrow angle transducer, which will typically also be a much higher frequency sensor. This technique is often called "down scan" SONAR because it only shows what is right down under the transducer. But it shows the reflections in great detail, almost painting a picture of the bottom and you go along. This is great for getting a detailed image of the bottom structure under the boat, but it can be much harder to locate fish because the fish echo returns are very small; they appear almost as just little dots.

I would not worry too much about making the proper inferences about frequency and cone angle as a neophyte SONAR user, and instead I would let the manufacturers and their vast experience dictate what to use. To make it simpler, just get a Lowrance HDS unit. They are excellent modern SONAR units and their cost is quite reasonable.

Visit the Lowrance website and read all about their products, and you will get a good idea of what will work best for you. Lowrance has a lot of good product information to share with you.

Mr T posted 12-28-2012 12:37 PM ET (US)     Profile for Mr T  Send Email to Mr T     
It sounds to me as if you are another California delta fisher. (do it a lot myself).
As in my previous post, I used a Raymarine with a 50/200 kHz transducer on the last boat.
The current boat is now running a Lowrance HDS-8 unit with the side scan/down scan accessory via an LSS-1, both of which can be ran at 455 kHz and 800kHz and the standard 200 kHz “skimmer” transducer. After using this combination in the rivers and sloughs for about 6 months, I came to a few conclusions.
1. Down scan sonar is in many cases a subpar substitute for standard sonar. The viewing area when one is running in 7-20 ft. of water (where I do most of my striper trolling), is so small that it precludes it being of much use. I did find that the resolution of the picture is quite good, but I was only viewing a few feet of the bottom.
2. The standard skimmer 200 kHz transducer is in my opinion, nothing more than a depth finder. Although it is quoted as having a “wide” cone angle, it is still 20 degrees, so I find myself looking at 2 ft of bottom in 10 ft of water. In addition, the build quality of the transducer is low, as I have had to replace at least 4 of these over the years in using a total of three Lowrance/Eagle electronic head units. As an OEM unit transducer, they are there to get you a picture and they do, but it is not the best possible one, and I noticed that they did not hold up well after making any contact with debris, which is always present in my fishing.
3. The side scan function is a new tool for me this year and I found myself using it more and more over the course of the fishing season. Typically when trolling an area, or scouting it for fly fishing and plugging, I found the side scan to be very useful in locating points, drop-offs and other typical striped bass “ambush” points. While you can see fish using the side scan, it is not as simple as a down scan sonar picture, and the ability to judge size or even type of fish is essentially moot. It is very useful to be able to tell which side of the boat the fish are on using this function as well. On a side note, it has been an invaluable tool for sturgeon fishing. I have used it a number of times to map very shallow areas that you could not use regular down scan sonar in and it has paid off already in finding some shallow feeding fish.
4. Overall, the HDS unit with side scan has some high points and low points. For the California delta, it is a good tool, but does have some limitations
5. I also fish lakes and deeper water, and found that the sidescan and down scan functions were of much less utility there. Most of my lake fishing is trolling at depths of 20-100 feet with downriggers for Kokanee and King salmon, Rainbow, Mackinaw and Brown trout in waters up to 400 ft deep. In lakes I found that any settings I used on the HDS-8 unit simply did not give what I needed- the down scan gave some details, but not to the point of being able to discriminate fish type. The air bladder on a kokanee salmon, king salmon, and rainbow trout are shaped and sized differently, and I could not get the level of resolution I desired. It eventually caused me to research and finally purchase another sonar, this time a dedicated Furuno unit paired with an Airmar P66 transducer operating at 50/200 kHz. This gave me what I needed for lakes and also allowed me to do some side by side real time comparisons of the HDS picture versus the Furuno. What I found was an excellent correlation between the units, with lower resolution of detail on the HDS unit. The positive for the HDS unit is the ability to map sideways. The advantage for the Furuno is much better discrimination and detail.
The combination is in my opinion fantastic.
Overall, I would suggest looking at the HDS units – keep in mind however that even after adding the HDS head unit, the full potential is not opened until you drop the additional 600 dollars to get sidescan/downscan. At that point, you are in the 1400-1600 dollar range, which is a very competitive price point.
Also keep in mind power consumption. The LSS side scan accessory is power hungry. Make sure you keep an eye on your voltage. Running the HDS setup all day drains a surprising amount of power from your battery. I suggest isolating you starting battery from your electronics to be sure you can get home.

Hope that helps your decision process.

jimh posted 12-28-2012 02:10 PM ET (US)     Profile for jimh  Send Email to jimh     
I know that people who fish for money and a living like Furuno fishfinders. I don't think for angling for small pan fish in 25-feet of water you will need a professional grade Furuno fishfinder.
Mr T posted 12-29-2012 02:55 PM ET (US)     Profile for Mr T  Send Email to Mr T     
I agree that for most sport fishing situations there is no "need" for a commercial grade fish finder.
For me it was a case of having been through a large number of different electronics, and after a number of years the old mantra “buy nice or buy twice” kept coming back to me. In much the same way I came around to buying a whaler after years of thinking other boats were as good, I realized that the Humminbird, Lowrance, Garmin and Eagle units I have had in the past simply did not last like I expected them to, often having problems after 2-3 years. I do fish salt water a lot and I know that makes a big difference in lifespan of electronics. Looking around at most of the salt water boats around me, Raymarine and Furuno are by far the most popular units and it appeared to my eye that the decision to buy those brands was often one of reliability in addition to performance. I’m not suggesting the need to buy that level for the fishing described, I just added my perspective as an owner of a side scan sonar along with a more “traditional” down scan sonar. As an aside to this, the Furuno unit I placed on the boat alongside the Lowrance HDS unit was less than half the cost of the Lowrance.
If the target is small panfish in 25 feet of water, it’s not going to hard to find a sonar that can perform at a level needed to help succeed in catching fish. Most any sonar is up to that task. The CA delta is much more varied than that and the variety of fish and the sizes are quite disparate. Good electronics pay off in my opinion, resulting in more and better fish. To me that’s the payoff. I don’t eat them but I do enjoy catching them.
LCnSac posted 03-11-2013 04:05 PM ET (US)     Profile for LCnSac  Send Email to LCnSac     
Yes Mr. T, I know you from the Kiene board. Thanks for the comments.

I am Whaler-less for now, after owning 3. I have a 12' trihull skiff that is perfect for all but the Delta in weather. I sure miss the Whaler quality, but not the layout for fly fishing. When I bought the boat it had a Raymarine sonar and I hated it and sold it.

I am looking at the new Lowrance Elite series, and for $600 you get a 7" screen and an conventional ducer. For another $100, you get down scan with DSI. I think THAT is overkill, so I'm not going to buy electronics that cost more than my boat anytime soon;-)

I'm still mixed on the value of the down scan. It's not helpful for trout but *might* be helpful for Delta bass of all varieties. I parted out the Raymarine and my old 337c and raised $700 so that's my budget.

I'm open, but I don't see a reasonable alternative to the Lowrance unit for the pricing.

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