Electrically conductive paint

[martyn668]

Hi all,

My CB aerial is mounted to a small mild steel bracket welded to my roll cage. Works perfectly fine, but at the time, I didn't think about rust (doh!) The bracket obvisouly has to be bare metal to get a good earth to the aerial, and I want to avoid hacking it off and welding on a stainless one (my welding skills leave a little to be desired). I wondered about painting it in an electrically conductive paint to prevent corrosion, and found this at Maplins.

My link

Anyone any thoughts on this as a solution?

A bit of googling brought up all sorts of ideas like electroless nickel plating and so on, but all that would involve removing the bracket, dipping, etc., and welding it back on, so I might as well go stainless in that case. I'm after something cheap which I can preferably do in situ.

Thanks for your ideas/comments.

[Night Train]

I would just grind back the rust to bear metal, bolt on the aerial and then paint the whole of the bracket to prevent rust from forming and water getting into the joint.

[tweetyduck]

that paint isn't very good and its a very small amount.

easier to connect an earth by soldering to the aerial and wiring the earth to a bolt somewhere. Don;t think you have an easy solution. Surely the coax onto the aerial gives an earth also?

[TSD]

Smear the contact area with Vaseline (petroleum jelly) before you bolt it together. It keeps out air and water, so stopping corrosion, and doesn't wash out.

I guess you can paint over it if you wipe off the excess with a solder rag.

[pete3000]

Surely the coax onto the aerial gives an earth also?

Nope. relying on the coax for an earth will electrically just add the length of the coax to the base of the whip element until it gets back to the radio. Then the swr will feedback into the set and cause all sorts of nasties. It needs to be earthed to the roll cage (preferably) or some other ground plane/large metal plate (roof etc) as close as possible to the whip base.

Pete

[martyn668]

Thanks for all the (cheap and easy) suggestions. Should be able to manage one of those.

Excellent

[David Sparkes]

There is an assumption here that the rollcage has a good earth.

Is it welded to the chassis, or just bolted to brackets welded to the chassis?

I guess as the first post says the installation is working fine at the moment the assumption must be proved, but it's worth bearing in mind.

I also assume the CB is more demanding of a clean and noise free earth than any lamps that might also the bolted to the roll-cage.

Cheers

[Night Train]

The aerial just needs a large ground plane, IIRC, and the roll cage would provide that. The cage would just need to be earthed to the battery either with an earth strap or through its connection to the chassis.

[ThreeSheds]

On the cheap and nasty front - Vaseline is suggested above, which is excellent stuff being totally impervious to air and water, unlike common types of grease - or so I was told many years ago by an immensely clever engineer. The only other non-setting substance that will prevent rust long-term is 'waterproof' or 'marine' grease according to the same source. The only down-side is that in on a hot day both Vaseline and waterproof grease can become quite thin and easy to pressure-wash off.

Personally I would say disassemble, grind clean, assemble, and then give it a real good multi-coat paint application.

On the subject of why the earth at the base of the antenna, it is my understanding that there are two reasons for earthing at the antenna base: One is using the metalwork to provide a "ground-plane" which modifies the radiation pattern meaning that you will get better range if the antenna is in the middle of a large flat conducting base like a roof. The cage will not provide this enhanced range (gain) and so the other reason for grounding at the antenna base is the SWR which apparently is improved dramatically by having a good earth at this point. (Bad SWR can damage the output transistors of you transmitter). The two things - SWR and gain, are inter-related and on something a strangely shaped as a Land Rover the actual effect of any change (even opening the doors!) is difficult (impossible) to predict and although the best setup is to have the antenna mounted in the centre of a flat metal roof where everything is well grounded, it's really just a matter of doing the best you can and fingers crossed! (Although you can measure the SWR of course).

One last thing - be aware that alternating current with a frequency of 27,000,000 hertz like you are stuffing up your coax does not behave in the same way to wires and grounds as 'normal' electricity (mains or 12 volt DC) that we are all used to - it is perfectly possible to build an antenna which has a direct earth path (a few inches of copper pipe) between the conductor and braid, and as long as the rest of the antenna is constructed correctly and coax is routed correctly, then the whole assembly will be seen by the transmitter to be a 50ohm load...

Sorry for the waffle!

Roger

[martyn668]

I have no idea of the electrical connection between the rollcage and the chassis. There are lights mounted to it, which use it as a ground, but as you say David Sparkes, they may not require such a good earth as a CB aerial. However, it was SWR'd when it was newly mounted to a clean non-rusty connection, and the reading was good (can't remember form memory, but it was fine.)