Roof bars. will they conduct electricity?

[muddyjames]

I have bought a cb radio and hope to mount the aerial onto the roof bars, you know the ones. they go front to back on both sides of the disco and the other pieces clip into them.

I am hoping that they will be a good earth for the aerial. I am guessing they are bolted onto the disco as you can rock the car and they stay put! So in theory they should be ok.

[TimM]

Hmmmm, not sure.

I have a feeling they are Aluminium, and I have visions of seeing some sort of padding between the roof and the bar.

Why do you want to mount it there?

[thermidorthelobster]

I think they're steel bars with ally end mouldings, but that's not really relevant as both steel and ally conduct electricity very well of course.

The question is how they bolt through.  There is foam padding between the ally pieces and the roof, but I imagine the bolts which hold them to the roof are not insulated from the roof.  But I've never taken the head lining out, so I'm not sure.

Stick a multimeter on and you'll find out pretty quickly.

[TimM]

I think they're steel bars with ally end mouldings, but that's not really relevant as both steel and ally conduct electricity very well of course.

That sounds about right, but I thought Aluminium wasn't a very good groundplane (for the CB).

I could be wrong though  :?

[muddyjames]

Why do you want to mount it there?

Coz I bought a quickly removeable aerial holder as I want to take the whole cb radio indoors when not in use or put it into another car if needs to be. I dont want the aerial on show as light fingers may want to pinch it and or break in and steel anything elseinside they like the look of.

[NeilWilson]

Stick a multimeter on and you'll find out pretty quickly.

Just checked on mine and the bars are earthed (or if you prefer, negative terminal connected).

Neil

[muddyjames]

cool wicked. thats a good start. just hope my roof mount grips now!

[Tyke]

You need good bare metal contact when dealing with earths on radio equipment. We are dealing with RF currents at very high frequencies and what is fine for a DC system may offer such a high impedence to the flow of RF current that a normal DC earth may be all but useless.

Doubt that the roof bars would provide a good enough earth for efficient working of the radio. This could show up later as very high SWR readings when you are setting up the system and in severe cases, operating with high SWR's can cause heat build up in the radio set and effectively burn out the output transistors.

Don't worry too much though, just take advice from the guys who have them fitted and working ok . there's loads on here.

All bit complicated if you are not into radios but here is a site that may help you understand a little better.

http://www.k2bj.com/Ground.htm

[NeilWilson]

You need good bare metal contact when dealing with earths on radio equipment

True enough, although lots of people get away with mounts on roofracks with rubber gutter feet ends and various other mechanical contraptions.  You can counteract some of the SWR with suitable aerial tuning.  Any earth is going to be better than none at all.  I have mine mounted on the spare wheel carrier.

Neil

[muddyjames]

how do you mount yours Niel? I thought about that but my spare wheel has the grey plastic trim on it and i cant be bothered to take that on and off after every trip or unless it will mount on and then leave on and be hard for light fingers to take of I may leave it thee.

[NeilWilson]

Small bracket, close to the door but not not too close to foul the rear wiper.  Then the cable through a hole in the door and around the top side of the trim to the CB on the dash.  

Based it on an article in http://www.expeditionexchange.com/cbantenna/

Neil

[Tyke]

You can counteract some of the SWR with suitable aerial tuning

Thats true mate. I didn't want to start frightening people but as long as we are aware of what we are doing we usually get by. There may be a loss in efficiency of the set due to the high SWR's but considering cb's are generaly used on 4x4's for local comms then we aint looking for finely tuned DX type transmissions.


Just to get things back into perspective, I used to contact the States regularily on the older, illegal,  27Mhz AM with a quarter wave mobile antennae stuck in the middle of a biscuit tin when the skip was good during the peak sunspot cycles - was doing better than the local Ham on his 150w HF rig  :lol:


So we dont have to worry too much about fitting them to vehicles  

[NeilWilson]

That is a pretty good skip by any definition, what was the rx quality like ?

Neil

[hobbit]

Aluminium is a better conductor than steel, that is what the expensive wire is made from

[Damonski]

I recall from reading various other posts that fitting those is not the easiest job in the world  :(

[landraver]

Aluminium is a better conductor than steel, that is what the expensive wire is made from

Thats right Kev, in fact, weight for weight, its a far better conductor than copper!

Jeff

[muddyjames]

so i could be in with a chance then?

[Range Rover Blues]

A CB ariel needs a decent ground plane ie a large flat bit of steel.  Aluminium doesn't work! and the CB needs to be connected electrically to the ground plane,  If you mount the CB on the roof bars it will try to use the roof as ground plane, not the bar.

[Jake]

I used to have a aerial on my roof bars.
I connected a wire to the bottom of the aerial and ran it to a grub screw, just behind the roof bar.
Not the best idea, drilling into your roof but the cb worked great.
 :?
 :D

[Tyke]

pretty good skip by any definition, what was the rx quality like

Good mate - S6, R7'ish - on good days hitting the red and into 9-10  

A CB ariel needs a decent ground plane ie a large flat bit of steel. Aluminium doesn't work!

Hence all the advice about hooking everything down to the chassis/block to maintain a good RF ground.


Well I must admit, you've got me going again now, might just pull the old rigs out and hook 'em up again. Might even renew my vhf 2m license for mobile ops.

[!Expletive Deleted!] - I can see some spend coming on - better not tell SWMBO - ended up with a 27' antenna and a 4 element Yagi beam on the house roof last time :twisted:

[muddyjames]

Right then. I have had an idea. Plan 2 as it were.

What if I were to make a hole in the bracket that hooks onto the roof bars and put a bolt through it and attatch a piece of wire. Then make another hole in the roof gutter and bolt on a solder less connector like these jobbies http://cgi.ebay.co.uk/electrical-solderless-terminals-connectors-200pcs_W0QQitemZ7589487790QQcategoryZ41499QQrdZ1QQcmdZViewItem and then when I take the aerial off I can unplug it and then plug it back in again easily and quickly. Yes down side is that it may pull off by a twig or 2 but I cant think of anything else that would be able to mount to the gutter and be quick to undo. If it isnt quick to undo I wont ever end up using the radio as I am a self confessed lazy arse!

My other idea is. I am wanting to put in a 3 way cigarette lighter socket in the map holder above the sun visor to power the radio as I think the radio will go in the elastic map holder when in use. What if I were to attatch an earth wire to a cigarette lighter plug so I can just plug it into the socket? Obviousley I wouldnt wire in a live wire to the plug!

What size wire would I need to earth it?

[hobbit]

Actually I just use a little mag mount on the roof, and run the cable through the door rubber, only use it for laning reallyu, so plenty of range there

The rack protects it mainly bit if challenged by an oak tree the mag will fall over.

Depending on where you want the signal to generate, ie you are in the front and want a strong signal behind you a front mounted areial is best, and vice versa if you are at the back

[muddyjames]

Everyone said to me buy a cb radio they are easy to use and great fun, yeah dead easy. Now I have got one all this stuff comes out of the wood work like licenses and mounting points and earths and signals forwards or backwards of you.

I DONT KNOW????????

I just want an aerial that I can take on and off very quickly that I can use for when I do lanes so I can chat to other people no matter where I am in the convoy and possibly to chat to lorry drivers when on a long journey and I am bored!

[TimM]

As hobbit said then, mag mount.

Put your power cord onto a cigarette lighter plug, have a mag mount for the aerial.

1) Open door
2) put CB down
3) plug into cigarette lighter
4) place aerial on roof
5) shut door

Removal reverse the above, it a 20 second job (in or out).

[muddyjames]

I was going to go with a mag mount but Matt_h used to have a mag mount and every time he went under a tree it fell off and spent more time putting it back on the roof than he did talking! I want a quick easy way to mount it and so it doesnt fall off. If the cigarette lighter for the earth will work that sounds like my solution.

[NeilWilson]

Remember though this is for CB, not for intercontinental usage.
You could always try bolting it on as you want, measure the SWR & see if you can get a decent reading then make it a semi-permanent fixture as required.

Neil

[Tyke]

Don't let us trouble you MuddyJ old chap - it's only a cb when all said and done.

The guys have told you what works and what is quick to set up mate. Try it first, then make it more permanent if required. If it dont work well then get back and tell us. Someone will be sure to put you right  

[TimM]

every time he went under a tree it fell off

I used a mag mount for ages, and yes it did get knocked off by trees sometimes, it depends on how strong your magnet is    mine used to get knocked off occasionally, but if I came to a particularly tree lined section I moved it to the back bumper, which was fine for range to nearby vehicles  :D

[Wanderer]

I wouldn't use anything as an earth that went steel ally. It'll cause the ally to rot in no time.

Have a look under your rear wheel arch where the rear quarter panel (ally) bolts to the rear mud flap mount (steel) it's always corroded and that's just down to the two materials touching each other.

I always run a good earth up to the roof rack for the lights and antennae.

[muddyjames]

I wouldn't use anything as an earth that went steel ally. It'll cause the ally to rot in no time.

Have a look under your rear wheel arch where the rear quarter panel (ally) bolts to the rear mud flap mount (steel) it's always corroded and that's just down to the two materials touching each other.

I always run a good earth up to the roof rack for the lights and antennae.

So what are sugesting I do? Not to mount the quick release bracket onto the roof bars? Remember it wont be a permenant fixture hopefully.