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SWR Tuning/Antenna ground

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scanner90

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Feb 19, 2008
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Seattle, WA
Well I've got a realistic radio that was hooked up to a junk radio shack antenna and lets just say I was better off rolling down my window and shouting across the road at whoever I was trying to talk to.

So I bought a 5ft Wilson FGT antenna, some quality coax(forget the brand) it said something like mini-8 and 95% shielding, as well as a roadpro SWR meter.

I've got the antenna mounted on my front bumper which might not be the ideal spot but it's the best compromise for my use.

Anyway I hooked up the meter and couldn't get it to work right, it was pegging the needle on some channels and almost pegging on others. I tinkered w some stuff but never got it working right.

So after testing the mount and coax w a multimeter as well as other stuff I found out that if the mount wasn't grounded that it all worked fine. I got another nylon washer and installed it on the bottom of the mount so the outer part of the coax connector isn't grounded at all. I bolted everything back on the bumper and grounded the ground wire from the antenna.

This time the gauge worked just as described in the book and after adjusting the tuning stub I have an SWR between 1 and 1.3 on all channels.

My question is does this sound right? I'm new to CBs but I though the mounts were supposed to be grounded?? Maybe since the Wilson has the ground wire it's different? I'm hoping I didn't just "trick the meter" and still have a problem. It seems to get great reception(much better than the old setup) but I haven't tried transmitting yet.
 

Allan_Love_Jr

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Dec 4, 2005
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884
Location
Arlington, Nebraska
Well I've got a realistic radio that was hooked up to a junk radio shack antenna and lets just say I was better off rolling down my window and shouting across the road at whoever I was trying to talk to.

So I bought a 5ft Wilson FGT antenna, some quality coax(forget the brand) it said something like mini-8 and 95% shielding, as well as a roadpro SWR meter.

I've got the antenna mounted on my front bumper which might not be the ideal spot but it's the best compromise for my use.

Anyway I hooked up the meter and couldn't get it to work right, it was pegging the needle on some channels and almost pegging on others. I tinkered w some stuff but never got it working right.

So after testing the mount and coax w a multimeter as well as other stuff I found out that if the mount wasn't grounded that it all worked fine. I got another nylon washer and installed it on the bottom of the mount so the outer part of the coax connector isn't grounded at all. I bolted everything back on the bumper and grounded the ground wire from the antenna.

This time the gauge worked just as described in the book and after adjusting the tuning stub I have an SWR between 1 and 1.3 on all channels.

My question is does this sound right? I'm new to CBs but I though the mounts were supposed to be grounded?? Maybe since the Wilson has the ground wire it's different? I'm hoping I didn't just "trick the meter" and still have a problem. It seems to get great reception(much better than the old setup) but I haven't tried transmitting yet.
If the SWR is down to 1.1:1 or say. Then your fine.
 

kb2vxa

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A dummy load will give you a 1.1:1 SWR so the acid test is a radio check, right? Oh and BTW, how do you get an SWR reading without transmitting? (;->)
 

mrweather

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Joined
Dec 19, 2002
Messages
1,251
By isolating the coax shield from the vehicle ground your coax cable is effectively becoming the counterpoise for the antenna.

The fact you saw a low SWR which means you got lucky with the length of the cable.
 

kb2vxa

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Not quite but the explanation of common mode current is too complicated to get into here. In any case now you see why I mentioned the dummy load, we also use the term to describe an inefficient antenna system that may look good to the meter but just won't radiate a decent signal.

"I tinkered w some stuff but never got it working right."
Being a fixed tuned antenna there is nothing to tinker with. It seems to me you assembled the mount incorrectly, placed it on one of those cheap plastic bumpers or one not electrically continuous with the vehicle frame. That's what the grounding wire is for, providing electrical continuity from the mount to the fame if there is none to begin with.

Once you have the mount assembled correctly and properly grounded look for the cause of your problem, likely the coax assembly. Check DC continuity with an ohm meter making sure both the center conductor and the shield are continuous end to end and they are not shorted together. If all else fails look to a defective antenna, it happens.
 

DX949

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Not quite but the explanation of common mode current is too complicated to get into here. In any case now you see why I mentioned the dummy load, we also use the term to describe an inefficient antenna system that may look good to the meter but just won't radiate a decent signal.

"I tinkered w some stuff but never got it working right."
Being a fixed tuned antenna there is nothing to tinker with. It seems to me you assembled the mount incorrectly, placed it on one of those cheap plastic bumpers or one not electrically continuous with the vehicle frame. That's what the grounding wire is for, providing electrical continuity from the mount to the fame if there is none to begin with.

Once you have the mount assembled correctly and properly grounded look for the cause of your problem, likely the coax assembly. Check DC continuity with an ohm meter making sure both the center conductor and the shield are continuous end to end and they are not shorted together. If all else fails look to a defective antenna, it happens.

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