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| x-isle Member Since: 26 May 2011 Location: Midlands Posts: 1327   
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This is a strange one, I've been having problems getting a good SWR reading from my CB antenna, turned out to be a dodgy co-ax.
So I made a new cable up and rigged it all up temporary using a roof rack mount with a temporary earth strap to the chassis and did an SWR check. It was just below 3 which I'm not concerned about as it was temporary setup using a 10m cable of which some of it was running across the drive. However, the strange this was, if I disconnected the earth cable, the SWR dropped to below 2.5??? That doesn't make sense to me? I'm sure it'll all be fine once I've cabled it all up right, but can't understand that removing the groundplane completely (the roof rack is powercoated and not earthed) the SWR improved? Craig Rogers 2007 Puma 110 XS 2011 Evoque Coupe Dynamic Lux www.craigrogers.photography |
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26th Sep 2013 8:32pm |
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| richardg Member Since: 17 Dec 2012 Location: Cumbria Posts: 30 
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Not quite a reply but I look after the VHF radios in 3 mountain rescue Landrovers and have spent a lot of time trying to get good VSWR readings from roof rack mounted antennas. The best I can get is about 2.1 and to do this we mounted the antenna on a small ally plate fixed to the front of the rack to form a ground plane. Must admit I have never checked the earth continuity between the rack and the body of the vehicle but the outer sheath of the co-ax will earth this anyway. Next time I am at the base I will check the reading. VSWR of 3 sounds a bit high to me, I would be trying to improve this for the VHF kit we use (around 160MHz).
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29th Sep 2013 7:56pm |
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| x-isle Member Since: 26 May 2011 Location: Midlands Posts: 1327
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The unit was back to the battery and the groundplane was back to the chassis.
I'm not really that concerned as it was all just temporary, it just didn't make sense that removing the groundplane gave a better SWR. Even if it was a bad earth, I'd expect the SWR to remain high, but to actually go up when connecting the groundplane just seemed odd. Craig Rogers 2007 Puma 110 XS 2011 Evoque Coupe Dynamic Lux www.craigrogers.photography |
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30th Sep 2013 5:32am |
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| SteveG Member Since: 29 Nov 2011 Location: Norfolk Posts: 660 
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This is your problem, alluminium doesn't make a good ground plane, and for an effective ground plane you need the sheet to be at least 30-35cm square. I used to mount my aerials on my defender bonnets for this reason and could get ~1.5 readings ok with adjusted aerial lengths. Another alternative is to buy a ground plane kit. Cheers Steve |
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30th Sep 2013 9:30am |
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| x-isle Member Since: 26 May 2011 Location: Midlands Posts: 1327
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Well I've now put a PSM-1 mount on the back and also ran a thick ground cable down to the crossmember and I now get around 1.5 on the meter, so pretty happy with that.
However, that is with my full length springer. If I put my mini springer on there the meter is up at around 10 and I can't tune it. I think it might be to do with the fact that the mini's coil doesn't clear the roof rack which is causing the issue. Thanks to Jim (4x4cb) for a quick service on the bits I've ordered from him, I've also emailed to see if one of the "Rubber Duck" aerials might work better than the Mini Springer as I'd like the option of a long and short aerial for the overgrown trips. Craig Rogers 2007 Puma 110 XS 2011 Evoque Coupe Dynamic Lux www.craigrogers.photography |
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30th Sep 2013 7:04pm |
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