I should know this....

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swanson37

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But my brain isn't functioning....
At the weekend I got a shock off the VHF when turning it on..
I had a quick gander round the back of it and the aeriel connecter has some serious corrosion around it....

That means that it is not properly earthed right???

Should I be earthing it to the hull anode or engine block or what??

GRP Hull boat

Cheers
 
Presumably the 12volt system is the highest voltage in the ship. It is possible to feel 12volts under some conditions ie wet hands and especially into a cut or graze where skin is thin. But don't worry about it. Although it should be at earth potential.
However if you have the ship connected to 240VAC then you are very likely to feel even slight leakage. Then worry about it.

You may for instance have a battery charger whose negative is connected to mains earth.(or is not supposed to be but is) The earth line may be carrying a fault voltage or current. That then would be a good case for grounding the earth to the engine etc via a galvanic isolator.
Yes the VHF negative supply should connect to battery negative which is usually connected to the sea via engine block negative return for stater and gen. However not always......

The antenna connector corrosion should be cleaned up. Check with a meter or lamp that connector is connected to battery negative. The corrosion may be caused by a voltage different to earth.

Sorry the whole story is complicated especially if your system has a negative not earthed (some elctrics are insulated and require a negative return for starter and gen) some have an insulated shaft coupling. More info needed. ...olewill
 
I don't think that the shock has much to do with the aerial connector corrosion.
Radio Frequency output happens when you press the Push To Talk switch, so unless you turned it on with the PTT sw pressed you get no RF O/P.
Look more to the earth return for the radio.
But also sort out the aerial connector corrosion as this could seriously affect the transmit power output and can damage the radio.
 
[ QUOTE ]
Presumably the 12volt system is the highest voltage in the ship. It is possible to feel 12volts under some conditions ie wet hands and especially into a cut or graze where skin is thin. But don't worry about it.

[/ QUOTE ]

Come on William, 12 Volts into the set can produce a lot more than 12 volts of RF out of the set. 25 Watts into 50 ohms is 1.414*(25/12)*50 = 147 peak RF volts (which is the voltage you 'feel'!) A bit of mismatch in the sytem and even that voltage can easily be exceeded.

However as Ladyinred says:

[ QUOTE ]
I don't think that the shock has much to do with the aerial connector corrosion.
Radio Frequency output happens when you press the Push To Talk switch, so unless you turned it on with the PTT sw pressed you get no RF O/P.


[/ QUOTE ]

I am interested to know under exactly what circumstances the shock was felt.

One ought to remember that a reactive load in the supply will also produce a spike of volts that can give you a shock when something is turned on and off.
 
[ QUOTE ]
25 Watts into 50 ohms is 1.414*(25/12)*50 = 147 peak RF volts (which is the voltage you 'feel'!)

[/ QUOTE ]That doesn't look right to me. Where does the '12' come from? I think the peak voltage is about
1.414*SQR(25*50). That's about 50V.
Go on - shoot me down...
 
Senior moment I think /forums/images/graemlins/smile.gif - assume you are talking of the rf output side as you introduce peak. So along lines already said power and resistance are known (25 W and 50 ohm, assuming match) so solve for V from P=V^^2/R (35v). The 12 doesn't come into it.

The feely effect from voltage is dependant on average (certainly so at RF) so really the 1.414 doesn't come into it from the shocking situation I would claim.

Would also claim if "tingle" from RF may (probably?) not be reported as a tingle but as pin prick like or burn - in my experience rf doesn't tingle.

For original poster don't know what would cause the tingle. Will only be rf (and maybe then to do with you coax connector) if you have the unusual habit of turning the radio on with the mike PTT pushed or it is jambed pushed on. The corroded connector should be fixed in any event and you will likely find that all the coax has to be too (usually the connector fails from water wicking down inside the coax from the antenna end or from some intermediate water leaking fault/connector. If the radio is wet inside from water coming down the coax then it is also possible it is shorted into transmit when you turn it on (and the radio will be likely stuffed).

As has been suggested - 12 v will give a tingle under some conditions and may have been that (and is indication of a fault in the radio - maybe from being wet inside) and if have 230v on board then make very sure you do not have a fault in that.

The set does not have to be separately grounded, just connect the DC positive and negative, the antenna connection (and assuming the antenna at other end is correctly installed) and GPS input if a DSC set.

John
 
Entirely possible, but you would probably have heard the change in noise if you got the belt off the aerial socket while transmitting.
When demonstrating power from the transmit on an Army Land Rover mounted C42 VHF set, we used to put a 6ft fluorescent about 6 inches away from it. Lit up like a Xmas tree, of course.
Put your tongue on a 9v battery, it tingles.......
 
Hi John and others. Radio Frequency power does not give shock. The usual feeling is one of burning. in my younger days I worked at a broadcast transmitter. 55kw and 10Kw into one antennan at 690 and 810khz. The field arounnd the tuning hut was enough to give a noticeable arc from bicycle handlebars to hand or from one person to another. It stings from the burn but no shock as such.

I guess we are still waiting for Swanson37 to tell us if he had 240V power on the boat at the time or maybe his silence is a bad indication....... olewill
 
Wow, a lot of information to absorb there....thanks all for your help.

I am still alive and kicking , thanks for your concern there olewill :-)

I don't have 240v on board. Shock was felt whilst turning VHF on whilst underway (sailing not motoring) I'd forgot to turn it on in the marina (I know I'm supposed to turn it on before I even head out but I'm still a rookie and there is enough to remember as it is). Shock seemed to be when my knuckles brushed against one of the screws holding the headlining panels in.
Radio wasn't jammed on transmit and as far as I know the radio still works OK, I did a radio test a month or so back and it seemed to be fine and it still receives fine I was listening to the weather forecast for Port Phillip Bay on Sunday...

I'll clean up the corrosion on the aerial connector and look into the earth for the unit istself I'll probably pop it open and check for water ingress whilst I am at it....it's a pretty ancient thing and my teak deck has been leaking a lot in the past
 
Do I find myself agreeing with you Will? I too have been involved in RF power tranmission and RF burns are particularly nasty. Agree that they are better descibed as burns and not shock, although I did wonder whether the original poster might not have made the distinction.
 
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