Quick CB question.

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Andy Land

Andy Land

Well-Known Member
Posts
4,499
Location
... en route to Cuckoosville.
Hi guys,

Fitted a CB into the Defender and went to SWR it in tonight and it obviously has a problem as the needle barely dropped on the meter when reflecting the signal.
I have used a mount as shown on this link:
PSM-1 3/8 CB & HAM ANTENNA BODY MOUNT for 4x4 USE ETC. | eBay UK

I have mounted this on the back just below a side window and then fitted a 1.65M Thunderpole. Has anyone else had trouble with these mounts or is it likely that the body panel it's mounted on is not a good enough earth?

All RG58 cable is routed through the vehicle with no coils of cable and so I was expecting it to be fine.

Any ideas welcome ... ta.
 
Is that panel Ali or steel can't remeber if ally will need to run an earth cable to an earth at the lights or any bolt that goes through the chassis

if i'm wrong someone will say

Had to run earth cable on the disco when the mount was on the spare wheel mount

hth
 
i used to find if you have trouble getting a goot SWR , check the total length on the coaxal cable should be in fractions of the wave length ie;36ft .
so 18,9,4-6ft if you have excess cable loop it and tape it into long flat loops
[===========] like so ;)
 
landy1uk said:
Is that panel Ali or steel can't remeber if ally will need to run an earth cable to an earth at the lights or any bolt that goes through the chassis

if i'm wrong someone will say

Had to run earth cable on the disco when the mount was on the spare wheel mount

hth
Click to expand...

Hi fella ... yes it is thin ally, almost feels like plastic so I'm guessing this is the problem. Just had a quick Google and read that fitting an earth from the mount to the chassis or similar should sort it. I'll give that a go tomorrow. :)
 
buckshot said:
i used to find if you have trouble getting a goot SWR , check the total length on the coaxal cable should be in fractions of the wave length ie;36ft .
so 18,9,4-6ft if you have excess cable loop it and tape it into long flat loops
[===========] like so ;)
Click to expand...

Thanks Buckshot ... no excess cable at all, measured it all up and cut it to length.
 
Andy Land said:
Thanks Buckshot ... no excess cable at all, measured it all up and cut it to length.
Click to expand...

might be some of your problem . the earth lead to chassis idea is a good 1 too, try to get as near to center of the landy (will give you all around coverage if its closer to the back than front you will tx forward more than back and vis-versa) ;)
 
buckshot said:
might be some of your problem . the earth lead to chassis idea is a good 1 too, try to get as near to center of the landy (will give you all around coverage if its closer to the back than front you will tx forward more than back and vis-versa) ;)
Click to expand...

Well it is mounted on the back with the mount 1/2 way up so although not ideal it should be good enough for off road sites. Another thing is that I have my Hi-Lift jack mounted next to it which may affect it! I think I'll try the earth strap to chassis first and then if it gets lots of interference I'll try moving it. Ta muchly fella. :)
 
Andy Land said:
Well it is mounted on the back with the mount 1/2 way up so although not ideal it should be good enough for off road sites. Another thing is that I have my Hi-Lift jack mounted next to it which may affect it! I think I'll try the earth strap to chassis first and then if it gets lots of interference I'll try moving it. Ta muchly fella. :)
Click to expand...

that will definetly affect it if you have that close to the twig (bounces the signal directly back on itself )
 
Even just the bodywork close to the antenna will affect your SWR quite seriously - much more than the length of your co-ax. The co-ax will affect it a little, but should only be a negligible amount in what you describe.

Basically, you want the whip as far from any metalic object as is possibble/practical. You want the whip to be as near vertical as possible/practical. Aluminium works perfectly well as a ground plane but there does need to be an electrical connection between the ground plane and the co-ax braid, either through the mount or via an earthing wire to anywhere on the vehicle.

Any SWR reading below 2 is acceptable and should never cause any damage, 1·5 to 2 is reasonably good, anything under 1·5 is very good. People pay far too much attention to getting a dead needle SWR reading, but no-one would ever know the difference between a signal TX'd at 1·5 and one TX'd at 1:1. Don't tear yourself apart trying for perfection, any nearby object will throw your SWR off to some degree, even just your body standing nearby.
 
Right guys, the antenna is now mounted on the rear roof section so the coil is above the roof line. I have run an earth from the mount and riveted it to the centre roof pillar as it's steel. I now have a Ch 20 SWR reading of 1.8 - Ch 1 is roughly the same and Ch 40 barely moves the needle! I guess just a very fine antenna tweak is needed and I'm all done. Thanks for all your help fellas, hope to speak to some of you local guys on there some day :):)
 
Last edited:
Andy Land said:
Right guys, the antenna is now mounted on the rear roof section so the coil is above the roof line. I have run an earth from the mount and riveted it to the centre roof pillar as it's steel. I now have a Ch 20 SWR reading of 1.8 - Ch 1 is roughly the same and Ch 40 barely moves the needle! I guess just a very fine antenna tweak is needed and I'm all done. Thanks for all your help fellas, hope to speak to some of you local guys on there some day :):)
Click to expand...

Yep, you are almost there, set the rig to ch20 and adjust the tuning by about 3 mm at a time (in your case you want to lengthen it to lower the SWR on ch20.). Once the SWR drops to it's lowest level and starts to rise again, move the whip back down but by only 1mm at a time until you get the lowest reading you can. Then when you check ch's 1 & 40 they should be slightly higher SWR than 20 but both 1 & 40 should give you about the same reading.

You won't get it much better than that.
 
Clanforbes said:
Yep, you are almost there, set the rig to ch20 and adjust the tuning by about 3 mm at a time (in your case you want to lengthen it to lower the SWR on ch20.). Once the SWR drops to it's lowest level and starts to rise again, move the whip back down but by only 1mm at a time until you get the lowest reading you can. Then when you check ch's 1 & 40 they should be slightly higher SWR than 20 but both 1 & 40 should give you about the same reading.

You won't get it much better than that.
Click to expand...

Perfect Clanforbes, I'll follow your instructions closely and report back when I'm done. Many thanks. :5bhat:
 
Hi Clan ... reporting bask as promised ... now I either have a faulty brand new SWR meter or my rig is perfectly set up ... I say this because I didn't think it would go so low.
Basically Ch20 just moves the needle to 1.1 and both Ch1 and 40 don't go above 1.0
I have triple checked this, re-calibrated every Ch before reflecting the signal and these are the readings I get. My only problem is finding someone on there to get an idea of range. I assume people still use Ch14 as a meet and greet Ch just like in the old days? :eek:
 
Andy Land said:
Hi Clan ... reporting bask as promised ... now I either have a faulty brand new SWR meter or my rig is perfectly set up ... I say this because I didn't think it would go so low.
Basically Ch20 just moves the needle to 1.1 and both Ch1 and 40 don't go above 1.0
I have triple checked this, re-calibrated every Ch before reflecting the signal and these are the readings I get. My only problem is finding someone on there to get an idea of range. I assume people still use Ch14 as a meet and greet Ch just like in the old days? :eek:
Click to expand...

Isn't it channel 19?
 
Nice one Andy, no reason to suspect a dodgy SWR meter. If done carefully and everything is good these readings of very low SWR and dead needle are perfectly attainable, though not at all necessary, it does feel nice to get them that low though.

Ch 19 is pretty much recognised as the "breaking" channel these days. Although it was officially designated as the mobile "breaking" channel - which has been pretty much mis-interpreted as the mobile channel. Though, as you rightly say, ch14 is the official breaking channel most commonly used in most areas of the UK, even though no-one uses it now-a-days
Very few people respect this sort of thing now and you will find that many of the bigger fleets (like Tuffnells Parcels Express night trunks) run on one channel so they can catch each other as the pass, or make contact from a distance as they aproach (TPE used 26 when I worked with them.)

Where I live, it's a very rare thing now to hear anyone on any channel. If you are anywhere near a motorway then you should be able to find someone mobile, but many won't answer you if they don't recognise your voice. Try lugging into a conversation for a while and if you catch a name, give him a shout making it sound like you know him. The other reasonably failsafe method to get a response, after trying to shout out to him a few times with no reply, is to call him a fnckiug ignorant arsehole (or something similar) then when he swears back at ya, you know your rig is working ok. But remember to say thanks as all you wanted was to make sure someone could hear you. Another method is to mention you are repairing a cb for a trucker mate of your's, can anyone tell me if they hear me?

Anyway 10-4 Good Buddy, happy cb'ing. 10-10 for now. I'm going 10 - 100.
 
Last edited:
Clanforbes said:
Another method is to mention you are repairing a cb for a trucker mate of your's, can anyone tell me if they hear me?

Anyway 10-4 Good Buddy, happy cb'ing. 10-10 for now. I'm going 10 - 100.
Click to expand...

LOL ... thanks fella, Roger that! I had been on Ch14 asking for a signal check but will try Ch19 instead. I did hear a couple of chaps with broad Black Country accents chatting away but didn't want to be rude until I found out the modern day etiquette (or lack of it) ;)

Anyway, thanks muchly for all your help and 10-10 till we speak again :)
 
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