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Author Topic:   VHF interference
surfkast posted 08-02-2006 02:23 PM ET (US)   Profile for surfkast  
Hello,
Anyone have some ideas on this? I'm getting static on my VHF when I turn on my Gps/ff. I have a Standard Horizon PS1000 and Garmin 198c. On all channels, I need more squelch level after I turn on the GPS/FF.

They were both wired to the same switch, so I just ran the VHF direct to the battery. Doesn't appear to help much, but I need to do a water test. I also notice if I put the VHF antenna down a little more horizontal, I get less interference.

Also, the transducer cable is coiled in the center console. Should I move this excess cable to the rear of the boat? Any ideas appreciated.

davej14 posted 08-02-2006 03:55 PM ET (US)     Profile for davej14  Send Email to davej14     
Does the VHF pick up interference without the Sonar cable connected? If so then that isn't it. Try to make sure ALL cables from the two units take separate physical paths. I would also check the connector on the VHF antenna to make sure it has a good shield connection to ground.

Couple of other questions:

Where is the PS1000 antenna cable relative to the GPS and the sonar cable?
Where is the VHF antenna relative to the GPS.
Is the GPS connected to the PS1000's NMEA0183 port?

surfkast posted 08-03-2006 09:21 PM ET (US)     Profile for surfkast    
Things I've tried already:

-disconnected the sonar
-disconnected the DSC to the gps
-moved vhf 5 feet away
-put vhf straight to battery
-moved excess sounder cable to back of boat.

The vhf antenna is on the left side of the console, the gps is on the right side. I'll try putting the gps straight to the battery to see if that helps.

I called Garmin and they said "wrap all wires in aluminum tape." I asked which wires, they said just wrap all of them.

jimh posted 08-04-2006 12:11 AM ET (US)     Profile for jimh  Send Email to jimh     
Most SONAR devices excite the transducer with a sharply rising pulse of voltage. When I worked for a very high resolution ultrasonic detector company, we called this pulse the "main bang". It is just a big pulse of DC coming down the cable to the transducer. The transducer is piezoelectric, and it converts this electrical energy into ultrasonic acoustic energy. The resonance of the transducer is what determines the frequency of the acoustic energy emitted; the exciting pulse is really just a big DC pulse.

A sharply rising pulse like that also contains a broad spectrum of radio frequency energy. It is very likely that there is some radiation of this energy from the cable connecting the SONAR and the transducer, particularly if this cable is poorly shielded. Some manufacturers use a coaxial cable to connect their transducers to the SONAR, and these are usually well shielded. However, I have seen some cables where there was not much shielding at all, and I suspect that those devices will emit a fair amount of radio frequency energy.

This energy may not be directly in the VHF Marine Radio band, but it may be so strong that it overloads the receiver and produces a noise modulation on the desired signals. In any case, you want to keep it from propagating to the radio antenna and getting into the radio receiver.

From your description of the problem, it sounds like the interference is coming into the radio via the antenna. This would be a reasonable outgrowth of having the cable between the SONAR and transducer lack shielding. Also, having it coiled up may be enhancing the effect by introducing some resonance into the circuit.

My recommendation is:

--confirm that the primary path of the interference into the radio is via the antenna. One way to do this is to disconnect the antenna from the radio. If the interference goes away, it was coming in on the antenna

--route the antenna cable and SONAR cables so as to isolate them from each other. If they cross make it at a right angle. Keep them as far apart as possible. I would remove the coiled SONAR cable from the vicinity of the radio antenna.

--if the SONAR cable is not well shielded, experiment with adding a shielded covering. You can use the outer braid conductor of some old coaxial cable for this purpose.

jimh posted 08-04-2006 12:14 AM ET (US)     Profile for jimh  Send Email to jimh     
Also--make sure your radio transmission line is properly terminated at the radio and the antenna. It is often seen on home-brew marine installations that there is quite poor connection of the coaxial cable shield at the radio end. This could lead to all kinds of problems with interference.
surfkast posted 08-04-2006 07:01 PM ET (US)     Profile for surfkast    
Jim,
Thanks for your detailed replies. I will try the things you mentioned.

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