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Paff

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Just a bit of info others might benefit from...

To cut a long story short, I have been having a right old time trying to SWR a new CB radio. I mounted the antenna on the roof rail and made a little earth lead from mount to rail allen bolt.

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I have gone through two different antennas and could not get the SWR out of the red. I visited a friend today with an analyser that you connect you antenna directly to. We still had trouble getting the SWR down and he was saying maybe the antenna was no suitable for 27MHz. Then by sheer chance he leaned over touched the antenna mount and the SWR was perfect :augie

It turns out that the roof rails are not earthed to the body :doh So I will have to make up two earth leads and run them into the covers where the coax is and earth direct from mount to body either by a suitable nut/screw or by drilling a new fixing.

This had me going for two days and I nearly gave the guy who sold me the antenna an ear full, glad I didn't :eek:

Just thought I'd pass this nugget on...

If anyone else knows how to earth out the roof rails easier than my way, please do tell ;)
 
Now you mention it it's obvious but I wouldn't have thought of it first. All I new was the aerial always works best on the roof .
I guess to earth the rail you'll need to undo one foot and remove the rivnut locator and substitute it with a nut and bolt through the roof skin. I'd do the back one as you can just get to the underside with minimal haedlining removal . Good luck :cool:
 
It was proven when I used a mag mount and the SWR was nearly 1.1:1 :thumb2

I was thinking to run an earth lead from the mount and to follow the coax in just in front of the rear door and onto the metal work under the trim. But now you mention the rail fixing at the read door, I'll take a look tomorrow.

Just beats me why the damn rails are not earthed :doh
 
I had to earth mine to the body when I had a mount like that on the roof rails.
I fed my coax cable with an earth along side it and bolted it to the body through a hole behind the driver side door seal.
 
Also remember the allen bolt inside the bracket tightens a plastic coated cable around the bracket that is bolted to the roof.

Do you think then that the end caps of the rails are in fact earthed to the body? So maybe an earth link from the allen bolt to the end cap (drill/screw)?

Can anyone confirm how the rails fix to the roof? Surely the rivnuts earth?
 
Also remember the alan bolt inside the bracket tightens a plastic coated cable around the bracket that is bolted to the roof.

That's the way it is , well remembered ;)
So confirming there is no direct earth. The foot is insulated on rubber gasket then pulled tight with the wire loop onto an alu hook thing rivnutted to the roof.
 
Just something that you might not be aware of, the anodised surface of aluminium is in fact an insulator. Think about this when you look at how the roof rail is assembled.

A mag mount gives good SWR but then the mag base is not in direct contact with the metal of the roof. It just has a large surface area, so acts like a capacitor, which at high frequencies, looks like a short, so achieves a good equivalent earth.
I would also be doing a continuity test, starting at the aerial coax at the radio connector, and through to the base of the aerial and then to local ground.
Maybe also increase the thickness of the earth between the radio chassis and car chassis, don't just rely on the 12 volt power plug.:thumb2
The other thing to note, check your aerial isn't one for the 2m band.
They look similar.

Rustic
 
Rustic - Yep, def a 27MHz antenna and coax checks out OK from rig end to antenna/mount. I'll take another look tomorrow...
 
Well had another look and still up sh!t creek, I just don't know what the heck is going on. I checked the continuity again from antenna mount to body (via a door catch screw) and the mount is earthed -:doh

Fitted the halfbreed to the mount and still could not get the SWR out of the red. I even clipped a jump lead from mount to man step bolt on the chassis, still in the red. So I fitted the half breed to a mag mount in the middle of the roof and SWR is now around 1.3:1 across the UK band :eek:

I just don't understand it as I have an identical mount on the drivers roof rail for my Amateur radio VHF/UHF band and the SWR is under 1.5:1 everywhere, what gives?

So mag mount it is then, not what I wanted but it works...

I might try the CB antenna on the Amateur mount just for curiosity's sake.
 
I had the same prob on my Disco2 in the end I ran a thick earth wire from the antenna mount along the coax into the cab & earthed it there & managed to get it to 1.4:1 not great but better than it was yet on my old T2 I had my antenna mounted on steel roofrack mounted on a set of thule roof bars & got 1.1:1-1.2:1 across the range:augie
 
...in the end I ran a thick earth wire from the antenna mount along the coax into the cab...

I clipped on a jump lead that I made from 1/2 inch starter cable when I was an apprentice mechanic in the 70's from the mount and to the chassis. That didn't change anything, so a smaller cable won't. I've had enough now, I leave the mag mount on. If I feel bored one day I might look into it again. Strange that the other mount on the other rail is OK :confused:

I made a nice tidy job running the coax behind plastics, under carpets. Now the coax from the mag mount just runs in from the roof and under a floor mat, then tucks under the center console trim :rolleyes:
 
There is a chance the earth to the chassis isn't very good as Nissan used thin wire for the earths:doh infact I use thicker speaker cable:lol
 
Heres photos of the Front mounting for the rails which is identical to the rear and also the centre. Neither of these are very well earthed. Remember 27mhz is HF so a good earth to the vehicle body is a must. Ideally a clean earth terminal to the aerial mounting with a nice thick wire going to a good earth on the body of the car.
 

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So why didn't the 1/2 inch copper wire from the antenna mount direct to chassis work?

Nice photee's :thumb2

My LWB has not got a center mount on the rail...
 
I'm guessing you need clean metal so there's less resistance. Like you I've run UHF VHF equipment without issue but then perhaps there was enough earth with just the rail it's self. I know that my ATAS120 seems to work on my rail mounting though ideally I know it really needs a good earth connection.
 
Looking at the construction of those rails there will be very little earthing going on simply due to the amount of joints between sections. Also the cables on the ends that go over the hooks are insulated.
 
Heres photos of the Front mounting for the rails which is identical to the rear and also the centre. Neither of these are very well earthed. Remember 27mhz is HF so a good earth to the vehicle body is a must. Ideally a clean earth terminal to the aerial mounting with a nice thick wire going to a good earth on the body of the car.

Do you think that the allen bolts on the bit that fit on the roof is a good earth?

Just thinking if so, I could make an earth lead from one bolt, run it through the rail to the mount. But I'm still confused as to way when I used a jump lead to earth the mount nothing changed?
 

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