VHF Interference

martinriches

martinriches

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I asked the coastgaurd for a radio check the other day''unreadable due to background hum'' was the answer.I tried again a few hours later when moored up and everything swiched off ''loud and clear'' the answer.Thinking this may be due to interference from new instruments I fitted during the winter I tried again yesterday with the engine running but all instruments off ''readable but with background noise''.I don't have a problem recieving and as the radio is 15 years old maybe it is time to buy a new one. Has anyone else had simular problems with background hum? Any ideas what could be causing it? I would be grateful for any advice

Martin
 
Could be interference off the engine.

Is engine petrol or diesel?If former might be plug leads.

Lack of supression on alternator.

Could also be a cable routing problem

equipment near radio could be inducing hum.

Screen on aerial cable may be compromised.

You need to go through a process of elimination
ideally with a friend with a VHF set on a little used channel.

I douby HM Coastguard would appreciate it being done with them repeatedly.

Start with everything switched off and transmit and receive. Switch one item on at a time and see what happens to the signal. I would suggest the other station is some way away in line of sight say 1 mile.

Hums are often caused by interconnection problems, is the radio connected to anything else other than the battery?

David
 
The most common problems with VHF 'hum' are a) that the fridge is running. Make sure it's off. or b) when the battery charger is connected to shorepower.

Hope this helps.
 
Thanks for the detailed reply. I rewired the boat last winter so the problem could be there. I used the same 12v curcuit for other instruments would that induce hum? It worked OK last year so I think it proberly isn't the engine. Could a instrument interfer even when it was swiched off?
It is a diesel engine and I don't have a fridge neither was I connected to shore power at the time.

Martin
 
This kind of background hum can also be for some other reasons than DC power, but

What a radio wants when it transmits is a DC feed that doesnt drop in voltage very much : thick wires to a healthy battery will help. Those cheap 1, 1+2, 2 , off battery switches can make the problem worse.

The alternator whine will vary with engine RPM and shouldnt be a problem if the batteries are low resistance.

All these different arrangements for powering the VHF are trying to smooth out the power to the transmitter of the VHF . When it is recieving it will draw much less current so the DC supply doesnt have to be as good.

The microphone circuit can be picking up the RF signal from the transmitter power stage. This can be caused by antenna degradation : breaks in the antenna cable, shorts, corrosion or water down the cable. All of these can make the RF come out of the radio rather than the top of the aerial mast.

This degradation creeps up slowly. Its only when you cease to hear all but nearby transmitters you realise the antenna is not really connected any more.

The alternator whine and other battery voltage variations can appear as amplitude modulation of the transmitted RF . That gets picked up in the microphone cable , detected by transistors in the microphone circuit acting as crystal sets , and becomes part of the audio.

IN the worst case , the transmitter circuits go into low frequency oscillations and the VHF can also be found transmitting on a few other channels .....
 
And for my penythworth........ You need to borrow a hand held so you can hear for yourself if there is interference. Use an unused or little used channel.
But check the commonly used channels as the problem may vary.

If you are keen you can buy a resistor to solder to a connector to make dummmy load. Used in place of the antenna. This must be 50 ohms and capable of dissipating 20 watts. It can be 4 50ohm resistors in series paralell or any arrangement that will give 50 ohms. Small resistors will fiz very quickly and could damage the transmitter if they fail.

Anyway you can't fix the problem unless you can hear it yourself.

Once identified you need to connect the radio to a separate battery with good wiring and nothing else connected. See if you still get interference.
If no then you have a ships wiring problem. If yes then it is the antenna if you are using it or the radio. It is likely the microphone cable or connector.

Another alternative is to find a friend who will let you connect your radio ibn place of his for a test good luck... olewill.
 
I'm with Forethought even though thats something I dont usually have.

Instruments that are switched off cant cause problems assuming they are not some super modern gadget that doesnt really switch off but goes onto standby.

Aerial can cause probs particularly on transmission but this shows as a lack of power. No aerial prob I can thing of would inject a hum into the signal on its own.

It is likely that you are picking up a hum from something like your depth sounder because you have a bad earth, two wires passing close together, or bad aerial shielding.

How to get round it? Carefully remake your aerial connections at the radio. Then if the prob presists, go to Maplins and buy yourself some ferrites and wrap the 12 volt feed wires to the radio round them. Put the ferrites as close as poss to the point where the 12 v wires go into the radio. This should get rid of the problem by blocking any AC signal trying to get up the 12v leads but I would still do some work to seperate the various wires feeding instruments and radio and checking / remaking the connections particularly the earths.
 
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