VHF and atmospheric static

[alteredoutlook]

Hi, I fitted a new vhf ariel to my boat at the beginning of the season and now whenever there is a build up of static electricity in the atmosphere the ariel plug in the back of the VHF crackles really badly (i can recieve a static shock by touching it).

The build up of static then wrecks all the signals going between my other electrical instruments and i lose depth, speed etc.

Additionally the last thunderstorm that approached me off the mull of Kyntyre caused a huge build up of static that could be audible coming from the top of the mast.

How can I stop this?


Do I need to fit a 'spiked' ligntning conductor/dissipator that is higher than my ariel which I then ground.

Would the conductor have to have its own cable and grounding plate or would just attaching it to the aluminium mast and a simple ground down thru the mast/mast support (its a deck stepped mast) and keel bolts to a (painted)cast iron fin keel suffice?


many thanks

 

[pmagowan]

That sounds worrying. Is it just a new aerial or any other nex equiptment. Have you the right type of aerial, what type? Have you changed the coax cable? If you are hearing static coming from the top of the mast I would have thought you are making quite a good lightning rod. What type boat?

 

[alteredoutlook]

The boat is a sigma 35.
I cant remember the ariel manufacturer - it was a 'marine' one, though.
The ariel is fitted to the top of the mast and is fed down the mast to a connector in the cabin headlining (connecting to the original coax cable) before going on the ICOM VHF transreceiver.
The connectors all appear 'solid'.

I have fitted no new electrical instruments.

The wind/speed/depth instruments are 20 yr old stowe/robertson dataline equipment

thanks

 

[Billjratt]

If the problem only appeared since you fitted the new aerial, then that's where you would expect the fault to lie.
If you don't have the equipment to properly test it, perhaps a professional should have a look.
It sounds like a possible ground fault to me.

 

[Bosun Higgs]

It isnt anything to do with a build up of static except perhaps during a thunderstorm. Somewhere you have a bad connection - no way should you get a shock from the back of the radio.

Do you have mains leccy on board or an inverter running?

 

[alteredoutlook]

No Mains leccy running (This happens at sea - I havent got a leccy cable long enough!!). Inverter not running either.

 

[William_H]

Static

OK working along the lines that it really is a static build up on the aerial from atmospheric conditions then there is potentially a problem. Many antennae havea DC connection to ground however some don't. The type suitable for fibreglass cabin top mounting usually are about 1.5 metres long in a fibreglass tube and they usually do ground the aerial.
The other type are 1/4 wavelength 17 inches or sometimes longer often made of stainless steel and require a connection of the cable braid to the mast. This type usually don't have a DC connection antenna to ground.
Is it possible you have changed from a grounded type to a non grounded type? This may explain.
However the outer barrel of the aerial connector at the radio should be at ground potential. So the non grounded antenna may have the outer of the coax and then the barrel of the connector not connected to ground until you plug it into the radio. This insulation could conduct the static build up down the outer of the cable to give you a shock. The good news in this case is that the outer barrel of the ant. cable can be earthed without any problems to the radio and this should be the first thing you try. ie a cable from the barrel of the aerial cable to the keel bolts.
In conditions usually very dry when lightning is about it is not uncommon to get a large static build up. The word static here referring to static electricity not to be confused with "static noise" on a radio. The build up of static an present itself as Saint Elmo's Fire a blue glow around the mast top and other weird scary things.
The usual fix is a lightning conductor ie a metal rod being the highest point and grounded to the sea.
The tip of the rod is sharpened to a point which actually concentrates the static to the point but does diissipate it at a lower voltage so earlier than a rounded end. The problem is that the conductor can attract actual lightning which will discharge at huge currents such that really heavy cable is needed to go to the keel. This to take the current away from lighter conductors that may take the voltage/current if the main conductor fuses. NB don't use stay wires as SS has high resistance and will melt at relatively low currents. Al; mast is OK however.
I hope that the conditions that caused your concern do not reappear soon or too often. I guess most of us just hope lightning does not strike and we go home and hide under the bed if a lightning storm is imminent. good luck olewill PS one of the first things to do is disconnect the aerial from the radio.

 

[Salty John]

I'd check that the radio itself is properly earthed, and that no other item of electrical equipment is shorted to earth.

The aerial should be internally grounded to avoid static build up, most are. You need to check that the outer braid on the coax is sound at all connections.