Coddy Posted December 29, 2004 Report Share Posted December 29, 2004 Hi I wonder if anyone can help, my Navman Chart Plotter seems to interfer with my VHF radio on channel 16. I get the noise as if you have the squelch turned too low. Turning it up makes no differance. Turning the plotter off and the noise stops. The ariel lead and plotter leads are some way away from each other, the sets are apart from each other only the power supply lead is together at one point. The noise does not always stay on, it is when the plotter is checking its bearing I think. This happens by the way with the engine off. Anyone any ideas? Coddy Quote Link to comment Share on other sites More sharing options...
duncan Posted December 29, 2004 Report Share Posted December 29, 2004 firstly the ploter is continuosly checking it's position so I am not sure what's meant by checking bearing? are the units linked by an NMEA (DSC VHF?) - if so possible interference there if the VHF circuit requires NMEA -ve as well and it's not connected. I have GPS and VHF ariels next to each other, as do many (most?) so can't imagine that's the problem either best suggestion for isolating the problem would be to connect the VHF to a seperate independent power source (spare battery and couple of crocodile clips) - if thte interference goes away it's in the power feeds and you can try fitting some suppressors to the VHF power feed. If it doesn't go away then I am somewhat stumped....... all the best Quote Link to comment Share on other sites More sharing options...
Coddy Posted December 29, 2004 Author Report Share Posted December 29, 2004 Thanks Duncan, I thought about changing the battery lead to the other battery but did not think of croc clips (Doh!) What I meant about checking the bearing is as you say checking its position. It is a strange one I agree, as it does not always happen. The radio is an old model, non-DSC, so it might be that the unit is more sensitive than new models to interference. Better electronics and components in modern equipment. Now to persuade the FPO to spend some more money just after Xmas! Thanks again Coddy Quote Link to comment Share on other sites More sharing options...
Bob F Posted December 29, 2004 Report Share Posted December 29, 2004 Yep, Duncan's approach sounds spot on (at least it's what I would do). What radio do you have? Is the interference only on channel 16? Is the radio in duel-watch mode? BF Quote Link to comment Share on other sites More sharing options...
Bob F Posted December 29, 2004 Report Share Posted December 29, 2004 Dave, I've been thinking about your problem a bit more. If you only get the interference on channel 16 then it suggests you are picking up some EMI (Electromagnetic Interference). A GPS is a passive device, in other words, it is just a receiver, and so doesn't actually transmit a signal to the satellites, it just sits there listening and obtaining data FROM them. But, it does contain a CPU (computer) that will generate a small amount of EMI (apparently at the same frequency as channel 16 in your case). As Duncan suggested, try temporarily using two different battery supplies to the VHF and plotter, but if it is EMI, it will probably be cured by grounding the Plotter more effectively. If there is any metal casing on the plotter, try connecting a wire from the casing to ground. A ground cable should be at least 1.25in and kept as short as possible. Good luck. BF Quote Link to comment Share on other sites More sharing options...
Coddy Posted December 29, 2004 Author Report Share Posted December 29, 2004 Yep, Duncan's approach sounds spot on (at least it's what I would do). What radio do you have? Is the interference only on channel 16? Is the radio in duel-watch mode? BF Hi Bob Radio is an old Swiftec I think non DSC Seems to be only channel 16 Yes on dual watch it happens The plotter is made of plastic so there is nothing to ground from or to. There is a shield wire in the power/data cable which is used as the negative lead, I think I need to check that it is not all connected to the same connector. I will do some tests while you are all out catching Cod tomorrow! Thanks again Coddy Quote Link to comment Share on other sites More sharing options...
Coddy Posted December 30, 2004 Author Report Share Posted December 30, 2004 Hi Duncan Had a chat with the guy from JG Electronics in Weymouth today. He explained that it is not unusal to get interferance from GPS's and Plotters. It is all down to the power leads from the Plotter/GPS as you suggested. What you have to do is put a ferious magnet over the power lead and test the units. You may have to move the magnet to get the best position. He did suggest that I move the power leads as far as possible away from each other first. The other thing he suggested was that the SWR may be out and I would need a meter to check it against a dummy load. Does anyone have one I could borrow for a small fee and advise on how to adjust the ariel length? Coddy Quote Link to comment Share on other sites More sharing options...
Bob F Posted December 30, 2004 Report Share Posted December 30, 2004 Coddy, No, I dont have a SWR meter (Standing Wave Ratio), and it seems a bit of an over kill to consider chopping up your VHF aerial. Not sure why a mis-match in the SWR of your Radio would cause the interference in your radio, especially as you are sure the interference is coming from your plotter??? Perhaps Paul D could shed some light on this?? The SWR is a measure of any mis-match between the transmitted signal from your radio and the aerials impedance. If the aerial is not properly matched it can cause a reflection of the transmitted signal back into the radio, causing interference. But as far as I know, this is really only a problem when you are transmitting. With your problem you are not transmitting, you are just listening on ch16. Generally, the half-diople whip aerials that we fit to our boats (white plastic looking aerial) have the correct 50 ohm impedance, and the coax cable from the aerial to the radio should also be 50 ohm, so any impedance mismatches are usually caused by a poor connection somewhere. I would not cut the aerial up, but look at your connections instead. Has your problem been there since the plotter was installed, or has it developed recently? If recently, it could suggest a connection has deteriated and needs cleaning or replacing. Good idea to try the ferrites on the power leads. Bob F. Quote Link to comment Share on other sites More sharing options...
Paul D Posted December 30, 2004 Report Share Posted December 30, 2004 Bob is correct re: SWR. This is only applicable when transmitting, but as far as I am aware there is nothing to adjust on marine aerials ( on other aerials you used to be able to adjust the length to get the correct impedence ), but this is irrelevent, since does not affect reception to the same extent. The ferrite rings are a very good idea and I would put them in the power lead to the radio ( you just wrap the power lead through the ring say 6 or so times. ). The ideas is that these absorb any radiation which could be being picked up via the power leads. Quote Link to comment Share on other sites More sharing options...
Coddy Posted December 30, 2004 Author Report Share Posted December 30, 2004 Thanks guys I will try separating the power leads first to see if that helps. Bob, not sure when the interferance started but I suspect it was when I installed the plotter. Problem is it is intermittant which suggests to me bad connections. I am in the process of fitting a switchable battery switch so this would be a good time to re-route the cables. I did wonder about the SWR etc. but it was many years ago I had a CB and have forgotten most of what I learnt back then. Me thinks I will leave that side of things alone. Coddy Quote Link to comment Share on other sites More sharing options...
Swainiac Posted December 31, 2004 Report Share Posted December 31, 2004 Now quotes like these illustrates why a muppet like me joins a band of boat fishermen like you guys!!!!! I'll stick to tree surgery!!! Rich Quote Link to comment Share on other sites More sharing options...
Bob F Posted December 31, 2004 Report Share Posted December 31, 2004 Don't start believing we know what we're talking about, though... As my old tutor used to say "never trust an expert!" BF Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.