Autohelm interferes with VHF

[Gypsyjoss]

Hi Guys
New problem.
My Autohelm ST4000 is creating interference on my ICOM M601 VHF, but only on channel 16!

The ST4000 is not linked to anything else and is supplied from the switchboard. The VHF is also stand alone and supplied direct from the domestic battery.
The vhf aerial cable and the ST4000 electronic compass cable are very close together, but they always have been, I think.

Any bright ideas before I phone Raymarine from Greece, please?

Cheers.

 

[robmcg]

Add a suppression ferrite at one or both ends?

 

[AngusMcDoon]

Another cunning feature of some Raymarine autopilots is to go into standby mode when an incoming mobile phone call is about to happen. How they ever get their stuff through EMC testing I really don't know.

 

[William_H]

Interference can arrive at the VHF set either via the 12v power wire or the aerial.
You can disconnect the aerial to see if this reduces or stops the interference. If it does not stop the interference then it is coming in via the 12v supply.
Even if it is coming in via the aerial it is likely being radiated by the 12v wiring of the AP. As said this can be minimised by an inductor in series with the 12v supply to the AP and by bypass capacitors on both sides of the inductor to ground.
A suppression ferrite as robmcg suggests is an iron powder type of magnetic material. If it is in the form of a tube it can increse the inductance of the power wire simply by feeding the wire through the ferrite. If the ferrite is in the form of a ring then the power wire can be looped though the ring several times each loop increasing the inductance. You see this kind of thing on computer peripherals. (keyboard printer etc. Sometimes in the cable sometimes inside the box. Here is your best source of ferrite. Electronic stores also sell the ferrite in various shapes.
The capicitors should be both around 10 microfarrad electrolytic capacitor and .001 microfarrad ceramic capcitor. Both at least 50 volt rating. These 2 capacitors are wired inparallel to negative or earth wire. Connect as close to the AP motor as possible. Fit the ferrite in the supply wire again as close to the motor as possible. Repeat this pair of capacitors on the supply side of the ferrite. If needed.
So what you get is a bypass to suck away the high frequency interference at the point where the dc goes to the motor. You then have a high impedance to high frequencies produced by the inductance ie ferrite on the cable to stop it getting out then yet another capacitive bypass of anything that gets through the inductance.
You can extend the feed wire to the AP so you have enough to go around the ferrite several times. You can use more than one ferrite and you can use an enameled copper wire to permit many turns on the ferrite. Just remember the wire on the ferrite must carry the AP supply current. This should stop interference getting to the VHF via the 12v line but also stop it radiating into the air and so to the aerial.
Lastly you can do all this again with the power wire feeding to the VHF set. This will stop anything getting in via the 12v supply. The set probably has some filter capacitors on 12v in line anyway.
If this interference has just appeared you may have a motor in the AP getting bad contact at the commutator. Which may get worse until it fails. good luck olewill