 | Home > In Car Electronics > CB SWR readings |
 
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| bob neville Member Since: 30 Apr 2009 Location: Marbella Posts: 3247   
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When you say the earth is good is that from the radio and the aerial ?
If they are both OK I would then check the coax - it sounds a bit like a short somewhere. Another trick I use is get an old metal coat hanger and straighten it out. Then cut to the length of the mast. Clean one end back to bare metal and replace the mast with the hanger. Then check the swr on ch 1 and 40. Lowest swr on ch 1 means the mast is to long, lowest on ch 40 means the mast is to short. You can then take a bit off the temporary mast if you need to shorten it or reposition it if you need to increase length. I have found the adjustment needed is 1 or 2 mm, 5mm can make a lot of difference. If you get good readings then you can shorten the real mast by the amount you took from the temporary mast or reposition the real mast to the extended length. Bob 2015 Jaguar XE240 R Sport - goes like ....... !! 2013 Defender 90 CSW - sold 2009 Defender 110 Double Cab - sold 2001 TD5 90 CSW - offroad project - sold to a forum member 2011 Porsche Boxster - for the sunny days ! |
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4th Aug 2014 9:54pm |
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| couplands Member Since: 31 Aug 2011 Location: Peak District & Cornwall Posts: 1826  
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Thanks Bob, I checked with a meter the resistance at the aerial to earth and also at the CB end. Didn't check the core of the cable, so may be that.
I was also thinking of using a bit of wire/coat hanger prior to cutting the aerial, but I'm so far off at the moment I need to fix that first. Cheers Simon |
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4th Aug 2014 10:12pm |
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| jst Member Since: 14 Jan 2008 Location: Taunton Posts: 7709 
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there is no cont between core and shielding i take it?
what mount have you got? have you tried thunderpoles forum and Q&As? Cheers James 110 XS Utility 130 Puma Station wagon/camper (in the making) 90 Puma Hardtop |
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5th Aug 2014 8:29am |
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| couplands Member Since: 31 Aug 2011 Location: Peak District & Cornwall Posts: 1826
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No shorts as far as I can see. Will recheck it all later...I still think there must be a bad earth somewhere. I'll get some new cable/connectors too, just to eliminate the possibility of a duff cable.
cheers simon |
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5th Aug 2014 8:33am |
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| shaggydog Member Since: 12 Aug 2012 Location: Kent Posts: 3346 
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Worth a look at the cable.
Last one I bought came from ThunderPole and the soldered connections were terrible and had to be re done, not that it mattered because the cable was broken somewhere in the middle. They did however apologize and send me a new one. |
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5th Aug 2014 1:21pm |
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| couplands Member Since: 31 Aug 2011 Location: Peak District & Cornwall Posts: 1826
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This might be a stupid question....when talking about an earth for the CB unit, is this in addition to the positive/negative connections..? I have taken both of these back to the battery directly (via a fuse in the +ve cable), but just wondering if I need to separately ground the CB case..?
Just looking for reasons why the signal is rubbish..! Cheers Simon |
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6th Aug 2014 1:15pm |
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| jst Member Since: 14 Jan 2008 Location: Taunton Posts: 7709
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you shouldnt need to earth case sep.
did you check Tp forum? there is cont between the shielding, case and body i take it but not with the core? Cheers James 110 XS Utility 130 Puma Station wagon/camper (in the making) 90 Puma Hardtop |
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6th Aug 2014 2:05pm |
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| couplands Member Since: 31 Aug 2011 Location: Peak District & Cornwall Posts: 1826
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Yes, checked no shorts between core and shield.
Looking at the TP forum, it seems my issue might be related to the fact that the spring on my aerial is below the roof line (I have it mounted off a bracket fixed to the rear door hinge). Seems the spring needs to be in clear air. Here's the bracket:-  Click image to enlarge Will try lifting it up, although the reason I dropped it below the roof line was to offer some protection from trees etc. cheers simon |
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6th Aug 2014 2:54pm |
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| couplands Member Since: 31 Aug 2011 Location: Peak District & Cornwall Posts: 1826
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It was definitely the position of the aerial close to the body that is giving loads of reflections and sky high SWR readings.
I've jury rigged it higher up and can get the SWR to around 2.5 on channel 40. So, where do I fix it permanently....I was thinking on the roll cage, centre above the windscreen. Any better ideas..? Cheers Simon |
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6th Aug 2014 4:20pm |
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| Dave T Member Since: 07 Jun 2013 Location: Glasgow Posts: 487 
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I have one of these on the back
http://www.thunderpole.co.uk/fixed-body-mo...t-kit.html You can also get gutter mounts 2015 RRS Autobiography SDV6 1994 Def 90 300tdi |
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6th Aug 2014 5:03pm |
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| CFB Member Since: 13 Sep 2007 Location: Bradford, West Yorkshire Posts: 803
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Only way I have ever got a perfect SWR is using a body mount straight through the roof! OK on an old off-roader but wouldn't be so keen on a newer one. Andy
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6th Aug 2014 8:21pm |
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| couplands Member Since: 31 Aug 2011 Location: Peak District & Cornwall Posts: 1826
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^^^ Ive ordered some exhaust clamps so I can bolt the aerial mount to the front roll-cage bar. I'm going to get this CB rig set-up as perfect as I can, plenty of clear space around the aerial and super grounding through the roll-cage.
Wish me luck.. 
cheers simon |
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6th Aug 2014 8:24pm |
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| jst Member Since: 14 Jan 2008 Location: Taunton Posts: 7709
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i have one with the body mount - works well even though part of it is below roof line around 1.8SWR. and another with a through mounting on a roll cage (plate welded on) that give 1.5SWR and works really well. Cheers
James 110 XS Utility 130 Puma Station wagon/camper (in the making) 90 Puma Hardtop |
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6th Aug 2014 10:39pm |
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| couplands Member Since: 31 Aug 2011 Location: Peak District & Cornwall Posts: 1826
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Today I made the new mount for the aerial. Not had chance to wire it up, but hopeful it will get a better SWR reading than before....
Click image to enlarge Click image to enlarge Click image to enlarge Cheers Simon |
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9th Aug 2014 8:51pm |
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