VHF interference problem

[mbird]

We have had a bit of an intermittent interference problem with our VHF reception. It seems to happen most on CH68 and CH73 though I have also heard it occasionally on CH14. It starts as a low whine which builds in pitch a bit like a jet engine starting. I was wondering if it was something on the boat as I could not squelch it out, and then by pure chance discovered that if I moved my hand under the flybridge helm it stops!

I am thinking it is interference from my Seatalk network, as turning off the power to my log/depth/autopilot stops it immediately. Given that I have had a bit of an issue with the 7001 autopilot head-unit on the flybridge where it kept making a high pitched beeping noise, I think I have narrowed it down to that unit.

Has anyone come across this before?

[diesel falcon]

Nope....but my vhf sends my depth finder crazy, and the chartplotter makes the vhf blip when aquiring satellites

[DelticMemories]

Worth checking the coax cable and connectors between your transceiver and antenna are undamaged and properly terminated. If the screening is damaged, there is water in the coax or a dry joint in the connectors it can mean electrical noise from other devices cause interference.

[mbird]

That is worth checking I think, thanks Deltic. The Antenna was new this year, and there is only on connection at the plug into the VHF, but I could get a new plug and re-older it I guess just to be on the safe side.

[puddlejumper]

If you can reroute antenna cable away from other units pwr cables

[DelticMemories]

Yes puddle, good point I didn't mention - antenna cables and power cables are best routed separately and away from each other. My experience in this field is entirely automotive so there may be some other diagnosis peculiar to marine based applications.

[Antares_9]

Oh bugger, RFI can be a most difficult thing to diagnose, especially remotely (basics apart) but I'll start digging myself a hole.

Are all the data cables that run close twisted pair shielded (or at least twisted pair) Mark and are they grounded ? I include NMEA 0183 in that as I assume all seatalk cables are proprietry Raymarine sourced ones, is your radar switched on standby? what happens to your handheld if you poke it around the suspect area? does this happen in one general location or wherever you happen to be and finally where is your mobile whilst all this is happening? Start by simply switching or disconnecting individual instruments in turn.

Try disconnecting the PL259 at the set and see what happens, then connect your emergency antenna (you do of course have one ) and wave it around suspect areas in an attempt to isolate the source, try another independant power source as it may be that, though the symptoms do suggest radiated RFI rather than conducted RFI. Finally if you do replace the PL259 get a crimped one, in my view (and that of ABYC, NMEA et all) is that solder has no place in a marine environment, certainly long term.

Good to see you made it to Shotley finally by the way.

[DelticMemories]

Out of interest are PL259 the most commonly used connector for marine radio? I have a preference for N-Type for VHF and UHF but use PL259 for HF.

[mbird]

Thanks for all the suggestions chaps.

I know it is definately something on the seatalk network that is causing the interference, as the instruments are all powered from the same switch on the main panel, and when this is off, the interference disappears.

The general area is around the flybridge autopilot head unit, as if I wave my hand under the dash in it's visicinity, the noise stops as I guess I am effectively blocking the RF. It does also happen on the handheld too.

All the NMEA/Seatalk cables are new to the plotters, but all the existing instruments were already in place using standard "Autohelm" cables. Another reason I suspect the autopilo head unit is that it the later ST7001 model, which has had it's Seatalk cable butchered a bit to connect into the older Seatalk system.

I hadn't thought of the radar, so I shall play some more at the weekend to try to isolate it.

ps, David, that was our fourth visit to Shotley. We were there last year, and twice in May as a stop-over en route to and from St Kats.

[Antares_9]

PL259 is industry standard for marine VHF, AIS etc, though of course any 50Ω one will do the job, just every marine VHF I've ever seen is so equipped and there's little point in changing the chassis recepticle for personal preference. You may want to try a bit of Baco Foil (other aluminium foil is available) as a shield as a get around Mark, I must have been asleep or off somewhere, either that or early onset.

[mbird]

Very good tip about the foil David. That may help me pinpoint the cause too!

[DaveS]

Mark

The source of the interference is almost certainly the result of digital logic within the Seatalk Network.

By their very nature digital devices operate using 'square wave' signals( a 1 or 0 in a binary sequence). In simple terms, the data in switched on and off at high speed to create a unique digtial code binary code for each command sequence.

The unwanted side-effect of digital signals, is that they are rich in harmonics, stretching way up into the radio frequency spectrum. The faster the bit-rate, the higher the harmonic frequencies.

All modern digital equipmnet, including radio devices, have to meet the requirements of a European Harmonsised Standards, know as European Norms (EN). Such as EN55022 for digital equipment.

The CE Mark on such equipment, means that it is "supposed" to meet ALL relevant EN's, meaning that that should not radiate (conducted or otherwise) radio frequency interference.

If the interfence is radiated from cable connected to the Seatalk, then fitting Ferrite Sleeves over the cable, where they leave the Seatalk, may well be enough to suppress the interfernce.

You will have seen these Ferrite Sleeves fitted to such things as the DC cable of Latop Power supplies, and computer cables etc. Their purpose is to suppress 'common mode' RFI signals flowing on the cable. I could well hav a couple of these laying around if you wanted to try them. Or, I think the likes of Maplin sell them.

If twisted-pair data cable (CAT5 etc), becomes 'unbalanced', due to poor terminations etc, then some of the 'differential mode' digital signals carried on the balanced wire pair, gets mode-converted to 'common-mode' signal, and the wire will then radiate this, just like an aerial.

Ther fact that you are hearing it as whining noise, suggests that the interfering signal is somehow being modulated by a lower frequency signal (in the Hz to kHz range), perhaps a switched-mode power supply. The signal you are hearing is not the interfering signal, but the frequency of the modulating signal, that is superimposed on the interfering which is in the VHF marine band.

As the the interference appears to stop, when you move your hand around (hand capacitance) the potential source, would suggest radiation from the offending unit itself. In which case, some simple screening might help.

On the other hand, if if the interfering signal was being radiated from a cable or cables in the vicinity, you would likely notice the same hand effect.

Its difficult to digagnose an EMC problem, without actually being there to assess it, but hopefully some of these 'pointers' might help.

Dave

[mbird]

Thanks for that explanation Dave, which I confess to understanding about half of! Unfortunatley I didn't get time to investigate this weekend and the boat desperately needed cleaning after our holiday. 6 hours of compounding and polishing left me with little enthusiasm for anything other than cold beer!

I will pop to Maplin and get a couple of ferrite sleeves. I know all the new kit I have installed has them, but the old stuff doesn't. I hope to have a bit of time this coming weekend to look into it further, so I'll report back next week. Hopefully the combination of some ferrite sleeves on the cables and aluminium foil screening will sort it.

[Antares_9]

When you use the ferites make sure you loop the cable twice Mark, they are much more effective that way than simply routing it through them.

[mbird]

I purchased some small ferrites from Maplin, which were the click-together ones that just snap around the cable. I don't have enough cable to spare on the back of the ST7001 where is has been butchered in the past to loop it through one of the bigger donut type.

Whilst at Oulton at the weekend, I removed the helmpanel on the flybridge again and tried to re-create the situation that gave us intereference on CH68 before. Initially there was nothing, but then a hiss started accompanied by the familiar sound like a turbine spooling up. I disconnected the shield wire to the ST7001 and sure enough the sound ceased immediately. As all the other instruments remain on, to me this proves the problem is the ST7001.

I attached the ferrite cores to the various cables under the dash, which seemed to have no effect. The ST7001 has two Seatalk sockets on the back, so I removed the plug and used the alternative socket. This did seem to have a positive effect, and so I then wrapped the cable junction with aluminium foil (Tesco own brand, not Bacofoil!) and all seems to be good now. Time will tell if the problem is solved as it does seem to be intermittent, but early signs are good.

One thing I did notice was the way the seatalk cable is arranged up onthe flybridge. At the lower helm, the instruments (log, sounder autopilot head unit) are daisy-chained so they are all in series. However, the seatalk cable from the lower helm then travels up to the flybridge. Here there is a junction, where one spur goes to the ST7001 and the other spur then has the GPS display and multi-repeater in series with each other. Would putting the ST7001 in series with the other two make any difference I wonder? The problem is that all the instruments are the old Autohelm type with the round 3-pin seatalk connectors, but the ST7001 is a later Raymarine with the flattened D-shape plug; hance the cable butchery...

[Antares_9]

Makes no difference if ST is daisy chained or radial Mark, the daisy chain is simply for ease and neatness of insallation / service when you have an array of instruments. Did you check if the screen drain cable was connected to the vessels RF ground, and where, it sould only be connected at one point otherwise you can get loop feedback, it can sometimes be the case where an "add on" has been done and the owner thinks they are doing the right thing by connecting it, not realising that it is already grounded automaticaly when connected to the rest of the ST network.

[mbird]

Thanks for the reply David.

With regard the screen/drain, I don't think the boat has an "RF ground". As far as I am aware, the older Autohelm network simply has a red +ve, bare -ve and yellow data wire, with the +ve and -ve being connected to the main power panel "log" switch. I don't think there is anything else I have seen.

[Antares_9]

Some boats don't Mark, it's usually done to ground all the screening, my reason for mentioning it is that it can sometimes cause more issues than it solves and many installers simply ground the screen to

DC 0v anyway.

[mbird]

DC 0v anyway.

Smart a**e

[ExUserGone]

Have you tried powering the suspect device from a seperate battery to see if the interference is coming through the power cables rather than down the coax?

[mbird]

Have you tried powering the suspect device from a seperate battery to see if the interference is coming through the power cables rather than down the coax?

Not yet, but I'm pretty sure I've isolated the dodgy unit now. It's definately the autopilot head unit causing the problem. I have added ferrite cores to all the seatalk cables in the viscinity and then wrapped the connection in aluminium foil. Early tests seem encouraging, but with an intermittent fault like this only time will tell I guess!