Interference on VHF

[itsonlymoney]

Suffering from a whining white noise type interference on my vhf when engine is running, the noise increases with the revs. I imagine that I require some type of noise filter or suppreser but what type, where from and where to fit. Or could it be summat else ie. knacked ariel.
Ian

 

[Piers]

I would look at the prime potential source - the alternator(s). Is suppression already fitted?

 

[itsonlymoney]

Dont know.

 

[Salty John]

Sounds like alternator whine. Could be your batteries were very low and the alternator was working very hard to charge them so the whine is more obvious. Could be you have a diode failure in the alternator. Also, check that your connections, including battery connections are clean and tight. Try taking power to the radio directly from the battery and see if you get the same whine. Better to find the cause than try to mask it with filters. Good luck.

 

[sailorman]

@ least it wasnt " Radio Check PLEASE" /forums/images/graemlins/wink.gif

 

[steverow]

Classic case of Alternator Interference.

Simple checks to see how the interference is getting into the radio:

With the engine running, disconnect the antenna from the back of the radio.
If the interference goes away then it is getting in probably on the antenna cable.
If this is the case then sometimes moving the antenna feeder will help. Try and get it as far away from the alternator as possible.
If that doesnt help then check your antenna grounding. If your system is Negative ground then check that there is a path (with a multimeter) between your radio chassis, antenna cable and the alternator casing or engine block.
If your engines are Isolated, ie floating ground then the situation is a little more complex. PM me for details.

If the whine is still there with the antenna out then it is coming in on the power cable. You will need to fit two components.
1. an Alternator suppressor Capacitor, fairly high value,
2. An in line Choke fitted in the radio power feed line, if there isnt one already.
3. In extreme cases it may also be advantageous to fit an additional de-coupling capacitor of about 10uF from the power feed direct to the radio chassis, just to clear up anything that the other two components miss.
Components 1 and 2 will be available from a Car radio shop or Maybe Halfords (on a good day with a flollowing wind and a half intelligent sales person). If you need the additional decoupling capacitor then get it from Maplin.

In really really really difficult cases, it can be necessary to have make up a filter network circuit which usually consists of two high value inductances and two capacitors, but dont worry this is a professional job, and I only ever had to do this a few times in all the years I was in Land mobile radio.

Hope this all helps

Steve.

 

[itsonlymoney]

Very interesting and informative steve.
I will print off your post and have a go next few days. I will let you know how I get on.
Ian

 

[itsonlymoney]

If anyone is interested in the outcome I fitted a noise filter to the radio power supply. No noise at all now, clear as a bell. Thanks for everyones input.
Ian