 | Home > In Car Electronics > cb radio SWR ratio |
 
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| jst Member Since: 14 Jan 2008 Location: Taunton Posts: 8093   
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i use the same setup on my 110 and 90 mount wise.
Have you tried a separate earth from the shielding fitting on the mount to an earth point. Does the springer contact the body work anywhere? Does the top of the mount sit level with the top of the roof or a fraction below, ie how much of the antenna is shielded by the roof fwd. Check there is no continuity between the shielding and the core. ensure head unit has good own separate earth feed, as sometimes they will use cable shielding earth! my setups are coming in at 1.2 on ch 20. range will be crap at SWR5. its meant to damage the head unit as you elude to, in the past i have lost/snapped antennas and run them with >SWR10 and head units all still work some 5 yrs later. Cheers James 110 2012 XS Utility 130 2011 M57 bespoke Camper 90 2010 Hardtop 90 M57 1988 Hardtop |
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2nd Dec 2014 9:27pm |
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| landy andy Member Since: 15 Feb 2009 Location: Ware, Herts Posts: 5740 
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Do you have a high SWR on high and low channels?
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2nd Dec 2014 9:27pm |
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| davew Member Since: 02 Jan 2012 Location: North Yorkshire Posts: 888  
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At that sort of level you can damage the radio, burning out the output stages. You can normally get away with if you transmit in short bursts but long transmissions will overheat the output stages.
When you say you've checked the earth, do you mean you've checked the aerial base is properly earthed or the radio ? Is the aerial supposed to be pre-tuned ? If not you may need to adjust it. Check the SWR at both ends of the frequency range channel 1 and channel 40. If the SWR is noticeably better at one end than the other then your aerial needs tuning. Check the resistance between the inner and outer part of the aerial plug at the radio, it should read open circuit, if not then you have a short somewhere either at the aerial or at one of the connectors at either end of the coax. I assume you're using the correct 50 ohm coax ! http://www.yorkshireoffroadclub.net/ |
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2nd Dec 2014 9:35pm |
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| Lou Sparts Member Since: 15 Apr 2012 Location: Kent Posts: 1501  
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If your mount is mounted low down and the aerial is not above the roof line you will be receiving your transmitted signal back as it will be reflected by the bodywork and will result in you getting a high SWR
As a general rule if SWR is higher on ch40 than ch1 then aerial is too long if vice versa then aerial too short. 2005 Td5 90 XS Steve |
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2nd Dec 2014 9:37pm |
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| Buz Member Since: 24 Jan 2014 Location: Forest of Dean Posts: 238 
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I had problems with mine, i ran a piece of cable from the chassis to the earth side of the aerial and that made the ground plane bigger, by all accounts ally is a good conductor but not very good as a ground plane. Also, when SWR'ing it, get somewhere away from buildings/trees etc so the signal has a good chance of getting out. Mine is now 1:1 on channel 1 and 1:3 on channel 40.
Originally I trimmed the whip but it seems the ground plane was the issue, i borrowed a mates whip and all ok....time to buy a new one it would seem. |
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2nd Dec 2014 10:41pm |
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| Hufflepuff Member Since: 25 Oct 2014 Location: Hampshire Posts: 727 
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Thanks for all the quick replies! The aerial is one of the fairly standard springer types. The mount is on the rear edge of the roof, so quite high up. About half of the spring part is at the roof line, and its not in contact with the body.
I have tried using a different cable to rule that out, and attached a ground wire to a shiny bolt on the rear NAS bumper which had a low resistance to other metal parts of the car, so I think its a good contact to the chassis - but the SWR ratio stays at 5 on transmit. I did also get one of those electronic ground units. With that attached, the SWR drops to 1.5, but I don't have a way to use that outsude of the vehicle. I have not tried tuning the aerial, my assumption had been that tuning would only offer a small improvement. I also have not checked if the CB is grounded itself, that's a good thought! I will check channel 1 and 40 in the morning, another good idea, thanks! Chris |
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2nd Dec 2014 10:57pm |
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| landy andy Member Since: 15 Feb 2009 Location: Ware, Herts Posts: 5740
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With my ariel in the same position as you I always had to trim at least 6" off to get it to tune correctly. If you don't fancy cutting the whip it's self (which is quite tough, bolt choppers are good
 ) straighten a metal coat hanger and cut that for test purposes.
Andy |
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2nd Dec 2014 11:16pm |
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| landy andy Member Since: 15 Feb 2009 Location: Ware, Herts Posts: 5740
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Possibly of some use
http://www.4x4cb.com/public/page.cfm?Page=795 Was for me |
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2nd Dec 2014 11:17pm |
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| Shax Member Since: 05 Oct 2010 Location: London Posts: 391 
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I've never had a "pre-tuned" antenna that didn't need a good old trim to get the SWR down...
And my HAM antennas are even worse, only really useable on a single band without an ATU (antenna tuner).. ... ,-------, .. I [__][_]|__ .. I __ |"_|"__| .. "(o)====(o)" ^^_-^-_^-^_^^^^^--^^^^ PAUL G7ALW 14.200 USB 26FB458 / 27.275 SSB |
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3rd Dec 2014 10:30pm |
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| Hufflepuff Member Since: 25 Oct 2014 Location: Hampshire Posts: 727
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OK just come in from the great outdoors (bbbrrrrr). I am utterly stumped by this antenna malarkey.
I have checked, double and triple checked the connections. There is: (a) No short between the inner and outer layers on the coax-cable with the aerial in place. (b) There is practically 0 resistance between the outer layer on the coax cable, and any earth point on the body. (c) There is practically 0 resistance between the inner layer of the coax cable and the antenna mount screw hole. I have tried adjusting the aerial length - I could only increase it as it started at its smallest adjustment on the grub screw - this made the SWR worse. There is not a lot of difference between channel 1 and 40 - 1 is *slightly* lower - but we're talking a very small amount. I then found that although I had set the CB unit to the 'U' country code for frequencies, you ALSO have to press the FM/AM button to enable 'UK' mode. This has made the SWR significantly worse - now about 10. I've tried a different aerial - this gave a worse result. Then I unmounted the aerial, attached it to a different coax cable and mount kit - and got him to stand about 2m away from the car in the street. There was no ground plate, so I didn't expect a good signal - but was getting desperate. This is where something odd happened - he didn't realise but he was clutching the aerial with his palm wrapped around both the upper (inner coax connected) and lower (outer coax connected) parts of the aerial mount. The SWR was 1.1 to 1.2!??! As soon as he held just the upper or lower part of the antenna mount - the SWR was back to 10. So my question is - should there be a specific resistance between the inner and outer parts of the coax when the aerial and antenna are connected? I temporarily dropped a 100 ohm resister between the bolt threads at the aerial end to see the effect - and ended with a SWR in the region of 1.5. I can't find any reference to this on the web, so I think it must be incorrect. Maybe I really do need to chop the aerial length down. I'm reluctant to do this without - as its obviously a one-way operation and it doesn't explain why either of my aerials aren't working (the other one is of these: http://www.4x4cb.com/public/item.cfm?itemID=2102 so can't be cut down). Its departure day tomorrow anyway so I've failed to get it working. It receives fine though - picked up plenty of transmissions at lunchtime today. |
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3rd Dec 2014 11:38pm |
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| Fifth Horseman Member Since: 08 Mar 2011 Location: Lanarkshire Posts: 326 
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The whip part of the aerial definitely needs to be shortened. As stated if the VSWR increases on the higher channels (higher frequency) then the whip is too long.
The UK CB frequencies are higher than the EU ones which explains why VSWR is even worse on UK channels. I think you will find most 'pre tuned' aerials are manufactured for EU/US frequencies and therefore need to be shortened to work on UK freqs. TBH pretty much any aerial will be too long as supplied. Higher frequency = shorter wavelength. The required aerial electrical length is directly related to wavelength. The ideal length for a whip antenna is 1/4 wavelength - for CB frequencies this is about 11 meters (not really practical on a vehicle) so to get the antenna length to manageable proportions loading coils are used to give the correct electrical length whilst maintaining manageable physical proportions. Other factors such as ground plane type and quality also effect aerial efficiency. The low reading when the aerial base was being held is mainly due to the holders hand acting as a resistor and absorbing some of the RF energy. With an antenna perfectly matched to the frequency in use all the energy is radiated as an RF signal, when the antenna isn't a perfect match then some of this energy is reflected back to the source (radio), by adjusting the length of the antenna we can minimise this reflected energy. Sorry this is slightly rambling, I do a lot of work with radio comms systems and have forgotten more aerial theory than I can remember... |
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4th Dec 2014 8:08am |
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| Hufflepuff Member Since: 25 Oct 2014 Location: Hampshire Posts: 727
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Thanks Fifth Horseman - that's encouraging news that this is all the problem is. I'll take a set of wire cutters with me and see if I can be brave enough to chop it with the other members of the expedition for guidance.
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4th Dec 2014 10:01am |
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| Fifth Horseman Member Since: 08 Mar 2011 Location: Lanarkshire Posts: 326
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Just remember, small cuts and check each end of the band after each cut. The link Landy Andy gave is a pretty good guide.
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4th Dec 2014 10:08am |
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| landy andy Member Since: 15 Feb 2009 Location: Ware, Herts Posts: 5740
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I am sure you are, but, SWR must be set with the vehicle in a clear area, and unit in the mode that you will be using it in
Andy |
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4th Dec 2014 8:14pm |
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