Radio Interference Problems

[BFB]

Hi all,

I am hoping someone will be able to help me out.

I hava a fibreglass boat fitted with a 27mhz radio and a humming bird wide screen fish finder. The problem I am plagued with is that the fish finder interferes with the radio stopping me from having both on at the same time. Sometimes turning the squlech all the way up stops the problems but also stops me receiveing all but the very strongest radio signals.

Any help would be greatly appreciated.

Bruce

ENS 1639

[Flightmanager]

It sounds like your wiring for both units is probably in the same loom . Try running the power wires and transducer cable completely separate from each other.

Ross

[Seagoon]

Thats good advice there from Ross - try to get your antenna cable away from everything else too. Because radio signals are so tiny they can get crapped on pretty easily by interferers. There is basically 3 places that interferers can leak into a radio like this:

1) the power supply - good power supply filtering is required - a decent radio should have this taken care of though...

2) the antenna - routing the antenna cable away from noise sources is important - if the interferer is not super strong, sometimes an attenuator can help knock it down just enough to stop it overloading or distorting the front end of the radio. If the interferer is in-band, well, your stuffed! I doubt that is the case here, youd have to be unlucky for a sounder to be generating stuff right on 27 megs, but it can be a lottery.

3) the actual internals of the radio - not usually a problem unless you have a *really* strong interferer like a tv tower - once again a good radio will help with internal shielding etc

I suggest you try one thing to help ID the problem so that you can nip it in the bud. Turn on the radio and the finder and adjust the squelch knob until the interference is *just* audible. Then, as Ross says, separate the cables until you hear a difference. See what sort of positioning looks the most promising then do that.

Also, how far is your finder display from the radio and antenna? PC LCD diplays notoriously radiate like stars, due to the high data rates and fast edges, and these high-end sounders that are coming out are probably not much different. Another experiment you can try is to detach the finder power cable and then connect it to the display and move the display as far away as possible from the radio & antenna. Dont hold on to the display when you do this test. Also try rotating the display to see if anything changes noise-wise.

If the cabling is already well segregated, then your problem may not be so easy to solve on your own. But that is probably not likely. You may want to consider something like cat number LF1294 from www.jaycar.com.au. Position it as close to the cable exit as possible, and wrap the cable through as many times as you can (the more the better in general).

These guys will help prevent noise from being conducted down the power cables of your stuff. Probably best not to put one on the sounder transducer cable as it may actually degrade the sounder signal a bit.

Hope that is helpful in some manner! PM me if you need a hand.

Edited February 2, 2008 by paint_stripper

[Jigholio]

I would add: star earth everything. It's the basic principle behind all low-noise electronics, and +1 for the ferrites, they are potent little things, cheap and no major PITA rewiring and yes, I am a tragic geek

[IWANNABROCK]

Also run your negative wire from radio straight to battery. we have to do this at work with the truck's we work on as the c.b radio's cop a lot of interference as well. this usually help's a lot.

[Seagoon]

Also run your negative wire from radio straight to battery. we have to do this at work with the truck's we work on as the c.b radio's cop a lot of interference as well. this usually help's a lot.

Yes thats right, and thats what Jigholio means by a star earth connection. Its called a star, because all the cables come in to one point, like a star :-)

The reason it makes a difference is because the battery is a very low impedance source of power, and as soon as you introduce a cable, you introduce resistance, and if the cable is shared by other devices, their spurious current draw through this resistance will create voltage noise at the other end of the cable. So if you run separate power cables to the battery, then you can avoid this. Of course this isnt always practical!

This also means of course that you can route the cable along an entirely different route. On my boat, i have partitioned it into two halves - sounder on one side, and everything else on the other.