VHF Interference

[wiggy]

I have a VHF ariel mounted on pushpit to supply data to ais engine. works fantastically without engien running but reception is worse with engine on. The cable runs about halfe a meter or less from engine, how can I reduce interference? or do I have to move cable route?

 

[philip_stevens]

You could try fitting suppressors to the alternator for a start.

Get the type suitable for the engine type. Strange as it may seem, there are different types for diesel or petrol engines.

During the days of Decca, there was a special suppressor called a DeccaMate that got rid of just about all "noise" from the alternator. I had trouble with my Decca and it sorted the problem of interference.

 

[William_H]

I would suggest the first move would be to if possible extend the cable so that you can increase even if only for test the distance from engine especially the alternator and ignition coil if it is a petrol engine.
Another worthwhile test is to run the AIS engine on a battery that is not being charged by the alternator even though the alternator is charging the main system. This can identify if the interference is coming through the aerial or the battery power wiring.If you still have interference or can't move the cable. Interference can come from the alternator. It may be easy to remove the drive belt for test with engine running. If it is the alternator. some give interference from an inbalance in the forward volt drop of the rectifier diodes. There are 6 plus possibly another 3 for excitation. you get even more interference if a diode has died but then you get limited charging current as well.
So a filter or suppressor is used which is a capacitor from the output terminal of the alternator to ground. This is usually in the region of one microfarad. Yet a value of even one hundredth of that may fix the problem. A capacitor is like a tiny battery which charges and absorbs the spike in voltage then discharges it into the systenm after the spike passes. The bigger the capacitor the bigger the absorbtion capability. However the kind of spike which might interfere with VHF getting into the aerial is extremely short and fast.
The interference may also be on the battery line into the AIS engine ( not via the aerial) This interference can be quite low frequency which requires a larger capacitor. (either on the alternator or on the battery line near the radio)
Unfortunately a large capacitor is made by wrapping many turns of foil i nto a small cylinder the effect is to make a coil (inductor) inside the capacitor which has a resistance to high frequencies . The answer is to add a small ceramic capacitor in paralell with a large value capacitor. One better solution is to use a feedthrough capacitor made for the job where the main wire from the alternator is cut and goes into one terminal of the capacitor and out the other end. the body is earthed.
The standard capacitiors are quite cheap from Maplins RS etc. The feed through types are a little more expensive.
This is all a very tricky subject. I remember spending many hours trying to fix interference problems on light aircraft especially appering on HF AM being more susceptible than FM good luck olewill

 

[bruce]

if there are plug wires present, they may be failing and no longer shielding the spark from the antenna lead.