depth finder interference

[jim rosenthal]

I have two depth finders on Blue Note- the first is a digital depth finder that just reads out in numbers. I think it's operating frequency is 235kH or something like that. It was made by Standard Horizon. The second is the fishfinder unit on my Northstar 6100i. it can be set to 50kH, for deeper water than I usually encounter in the Chesapeake, or 200kH, which is what it is set to right now. Usually they are both switched on.

Took the boat out yesterday and it seems like one or the other of these units is reading wrong at any given time. I have not been under the boat to see how clean they are, but I am wondering if they interfere with each other. To the best of my memory, they are not on the same side of the keep (I'm going to check in a day or two)

Any thoughts? Do marine ultrasound systems commonly confuse each others' echo returns? I don't know enough about this to know for certain.

 

[captddis]

Yes. If two nearby transducers are on the same KHZ, they will confuse each other.

 

[Avenger]

Right. It will usually show on a graphic display as a series of curved or diagonal lines. However, if it wasn't a problem before it doesn't really make sense that it should be one now.

 

[jim rosenthal]

They are not on the same frequency, but they are close...

 

[Boatsb]

Close enough for interference. If the wires or transducers are close one will screw up the other quite often.

 

[jim rosenthal]

I'm going to have to sort this out. I am not certain they ever worked at the same time. I am fairly certain the two transducers are not on the same side of the keel, but obviously the echoes of one can be read by the other. Time to get someone on the boat who actually knows what they are doing....

 

[Avenger]

Let's don't do a HOF overthink on this. It ain't rocket surgery.

In an area of known depth run one unit at a time and see if they're correct. If they are turn them both on at the same time and see if the readings change.

As I said before, if they did cross-read the Northstar would probably show an intereference pattern. If the screen shows clear it's probably not seeing the other unit.

 

[jim rosenthal]

I'll try it again. I know about what the depths are on average where I come in and out of my home port. I'll take some readings this weekend. And look for the pattern you mentioned. I suspect there aren't any user-serviceable parts in all this, so either everything is working or it isn't.....

 

[Avenger]

Transducers clean?

From what you describe if it was interference you'd have seen it a long time ago.

 

[Maynard Rupp]

Jim, some cheaper depth sounders do not have a means of setting the transducer to waterline, (or transducer to bottom of keel), offsets. Most people want to read the depth as you would if you lowered a ruler over the side and let it touch the bottom. Some folks want to know how much water is under their keel. Higher end units allow you to set this offset as you desire. The transducer to bottom of ocean is a useless number eh? Cheaper units will only read the depth from the transducer. Maybe these thoughts will help.

 

[rsmith]

Transducers get weak and go bad with age. Its kind of like depthfinder cataracs. You dont know it until you put a new one on. Most people just think the whole unit is bad and junk it.I think about 10 years is about it.

 

[jim rosenthal]

Seems as if it is the interference between 200kHz and 235 kHz. With the Northstar set on 50kHz I think the problems go away. We'll see.

 

[Boatnut]

You could try the following.

Have you tried turning them on seperately, and are they accurate when only one is running?
Do the readouts change when one is running and you turn on the second unit.
Are both of them giving incorrect readings?

I have heard several knowledgeable people say do not try to run 2 sounders simultaniously.

 

[jim rosenthal]

I did do those things. Interestingly the problem doesn't happen all the time, but when I have both running at nearly similar kHz rates, the problem does occur. Changing the Northstar to 50kHz seems to solve it, but low-frequency like that isn't really much use in shallow water. I have to try it again, which I will in the next week or so, but I think you are correct in that I shouldn't have both running at the same time.

 

[Avenger]

We ran into the problem on my father's boat back in the days of "flasher" type depth finders. We installed a color fish finder which was pretty bleeding edge compared to the paper recorder we were using before. It kept getting these hash-marks in it. Shut off the flasher and the problem went away.

We just left it that way. Either you needed depth so you didn't run aground or you were looking for fish and used that for depth. We felt no need to do both at the same time so the solution was the off switch.

 

[Trojan]

Jim I run three depth finders at once and have no problems. You could have a frequency problum. It could a harmonic of one of the other units causing the problum. Check the frequency you trying to use when you have the interferance and see if there any multiples that coinside. Check that the transducers are on differant sides. Transducers don't get bad they eather work or they don't. All the brains are in the control box. Transducers are nothing more than an antenna. It could be your using an old unit and a new unit and the interferance is on the top side not in the water end. Check your grounds on the units top sides. RF and DC signals travel differant roads. Try adding a snap on toride to the power lines. ( Radio Shack)That might help. Check the coax cable routings. Keep them away from each other. Check and see that there is no chaiffing of the coaxal cable or a bad splice or a dirty connector on the back of the units. Any place a stray signal could get out. It could be simple as above or it could be that your units are not compatable. If you have a friend with a spectrum analyzer he could easly check this out in short order. Good luck.

bill