[ARCHIVED THREAD] - CB Grounding issue. Updated. (Page 1 of 2)
Posted: 6/17/2016 4:20:49 PM EDT
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So when I have my antenna unplugged from the CB itself it is grounded properly. the mount isnt in contact with the antenna and making it ground out, But when I plug the antenna into the radio. MY SWR reading is over 2;1. and the antenna is no longer insulated from from the mount and shows it is grounded via mutimeter. My radio is hooked up to the fuse box and grounded to the dash, I don't Remeber how the radio was wired to in my other truck. Everywhere online says I should wire the radio to the battery.
SCott |
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Yes you should power the radio from the battery
The vehicle is used as a ground plane for the antenna, the more metal under the antenna the better. This is why you get continuity between - and antenna shield, it's the way it should be. Find a local ham with an antenna analyzer to help you tune the antenna. |
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You're multi-meter "seeing" an inductor inside the radio which goes from the center pin of the antenna jack to ground. If the "hot side" antenna was truly at RF ground you would see an infinite SWR meaning it would be pegged and not at 2:1.
Check SWR at ch 1 and then on ch 40. If it's higher on ch 1 than 40, then it needs lengthened. If it's higher on 40 then it needs shortened. |
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You're multi-meter "seeing" an inductor inside the radio which goes from the center pin of the antenna jack to ground. If the "hot side" antenna was truly at RF ground you would see an infinite SWR meaning it would be pegged and not at 2:1. Check SWR at ch 1 and then on ch 40. If it's higher on ch 1 than 40, then it needs lengthened. If it's higher on 40 then it needs shortened. See the thing is that the antenna is tuned. 1 and 40 are the same so mind boggled |
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Then the antenna doesn't have a good enough "ground plane" or is getting reflection from a nearby metal structure. To confirm my theory, disconnect the radio's power and antenna and meter the coax jack on the radio.
I bet your meter shows the center to be grounded. ETA -- Does it show exactly the same SWR on 1 and 40? ETA2 - What is the SWR on ch20? |
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Then the antenna doesn't have a good enough "ground plane" or is getting reflection from a nearby metal structure. To confirm my theory, disconnect the radio's power and antenna and meter the coax jack on the radio. I bet your meter shows the center to be grounded. ETA -- Does it show exactly the same SWR on 1 and 40? ETA2 - What is the SWR on ch20? Center is grounded |
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Do not test the antenna with it connected to the radio.
I can't tell you if that particular model is, but some antennas have DC connectivity between the antenna and ground. Best solution is for someone with an antenna analyzer to look at the antenna and installation. You could well have a problem with the cable, connector or mount. Your flat 2:1 SWR across the band seems odd for sure. |
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Do not test the antenna with it connected to the radio. I can't tell you if that particular model is, but some antennas have DC connectivity between the antenna and ground. Best solution is for someone with an antenna analyzer to look at the antenna and installation. You could well have a problem with the cable, connector or mount. Your flat 2:1 SWR across the band seems odd for sure. This kind of antenna is just a center loaded whip. There's no matching coil at the antenna base like some have. OP stated when antenna is disconnected from the rig that there is no continuity between center conductor and ground on the antenna. The DC ground the OP sees is part of the low pass filtering in the rig's final output section. Some of these antennas are very broad banded and it's not unusual for them to have an SWR under 3 to 1 for a megahertz or more. While this broad banded-ness doesn't help efficiency, it makes the radio happy especially for radios with "funny channels". 
I do find it odd that the SWR is so consistent across the band, but I've dealt with this type of thing before. A model number of the antenna or pic would help. |
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you need to find a local with an antenna analyzer
something like an MFJ-259 it's like an SWR meter with a built-in transmitter that can transmit a small signal below, and above the CB band once you find the SWR dip, you'll know how to adjust the antenna (you need to do this every time a mobile antenna is installed or relocated ) |
i feel like a idiot. Pics to follow
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that looks like a decent setup
do you have a DDM/volt ohm meter ? if so, remove the radio from the coax, and check continuity from the antenna mount to the antenna shaft ( below the coil) or go to the radio end of the coax, and check continuity frm the center conductor to the shield/out coax there should be none/ open/ infinity ohms ......like KD7DX said, try adjusting the whip make it as short as possible-measure swr ch1/20/40 then make it as long as possible and measure again 1/20/40 if the SWR is lowest at ch1, the whip is too long if the SWR is lowest at ch40, the whip is too short this is were an analyzer can help,...because your SWR might dip at 25 or 29 mhz, and you'll never know it with a 27mhz CB . |
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Have you tried adjusting the whip sticking out of the top of the coil? No As Derek45 said above, adjust all the way down and check SWR at 1,20 and 40. The adjust all the way up and check again. It should make some kind of difference. The whip is held in place with some allen type set screws. Wish I was there to help you out. I worked on CB's back in the 80's and 90's.
ETA - Sometimes the whip is extra long from the factory and may need quite a bit of trimming to bring the SWR down. |
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Did you scrape all the paint off from under the mount where it bolts to the fender? If so, then there's something going on with the mount or coax. Even though the coax/mount shows no continuity between center and shield, the coax may have a "bad spot" in it. Common with cheap coax. The coax isn't pinched between the hood and fender is it?
ETA - Did moving the whip up or down make any difference in SWR? |
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If you got small changes, which way did it go? Did the SWR on ch 40 go up or down, even to the slightest degree.
ETA - Some of those style antennas may need 3 or 4 inches cut off the whip for a match on 27mhz. They make them extra long for people who run on the illegal channels below ch 1. |
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As an experiment, wrap a piece of bare copper wire around the whip tip and extend it upward effectively making the whip longer. Start by adding about 3 inches to the whips length. Wrap it tightly so it makes good contact with the whip.
Maybe the antenna came with the wrong (too short) of whip. |
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Solid copper? Whta gauge? anything, solid, standed, anything to electrically make it 3" longer. wind it tight if the SWR drops more, than you need a new whip ....like I said before, an analyzer would tell you in 10 seconds. I'm not in TX, but if you were local, I'd be able to help more 
Edit...I type too slow,....but I'm grilling steaks, so that's my excuse 
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I know it sounds funny, but use something to prop up the wire. A thin wood dowel or other non-conductive rod taped to the whip will work. Remember, this is just an experiment. there may still be something funky with the coax or antenna itself.
Just trying to eliminate some things. ETA - How long is the coax? |
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I know it sounds funny, but use something to prop up the wire. A thin wood dowel or other non-conductive rod taped to the whip will work. Remember, this is just an experiment. there may still be something funky with the coax or antenna itself. Just trying to eliminate some things. ETA - How long is the coax? 18ft. |
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Ok, that's fine and what I expected. it's looking like there's something funky going on somewhere. Do you have another antenna you can screw into the mount and replace the antenna itself?
Even a cheap fiberglass CB antenna? ETA - One last thing you can try before condemning the system is to attach a ground wire to the mount itself (I see it has an extra screw on the bracket) making a counterpoise and extend it away from the truck. This is making a dipole of sorts. Make the wire 102 inches long. If the SWR goes down or changes drastically one way or the other, you have a ground plane issue and a different mount or way to mount the antenna is needed. |
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If you got small changes, which way did it go? Did the SWR on ch 40 go up or down, even to the slightest degree. ETA - Some of those style antennas may need 3 or 4 inches cut off the whip for a match on 27mhz. They make them extra long for people who run on the illegal channels below ch 1. Not on that antenna. I ran one of those antennas until the setscrews seized and the whip snapped at the base (it was a good antenna, I like the Tram 10/11m antenna that replaced it more though). My first thought, try the fender mount closer to the cab and see if that changes SWR. I saw something very similar when I looked at SWR of a CB antenna mounted on my spare tire carrier of my Jeep. Moved to a fender mount close to the cab and it solved the issue (VHF/UHF got moved to the carrier). |
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Not on that antenna. I ran one of those antennas until the setscrews seized and the whip snapped at the base (it was a good antenna, I like the Tram 10/11m antenna that replaced it more though). My first thought, try the fender mount closer to the cab and see if that changes SWR. Quoted:
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If you got small changes, which way did it go? Did the SWR on ch 40 go up or down, even to the slightest degree. ETA - Some of those style antennas may need 3 or 4 inches cut off the whip for a match on 27mhz. They make them extra long for people who run on the illegal channels below ch 1. Not on that antenna. I ran one of those antennas until the setscrews seized and the whip snapped at the base (it was a good antenna, I like the Tram 10/11m antenna that replaced it more though). My first thought, try the fender mount closer to the cab and see if that changes SWR. He had the antenna mount closer to the cab at first. I can remember some of those oil filled antennas needing 3 or 4 inches cut off to be resonate on 27mhz. This was a long time ago though. If the counterpoise doesn't make any difference then he's got a bum coax assembly IMO. Hard to troubleshoot from 1000 miles away.
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Then you need to go shorter.
ETA - What was the SWR on 1 and 40 with the short Wilson whip? Quoted:
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So for shits and giggles i grabbed the whip outta my wilson and chopped 6 inches off and shoved it in. Made my antenna way short. 1 was higher than 40 Then you need to go shorter.
ETA - What was the SWR on 1 and 40 with the short Wilson whip? 1.1 and 40 was like 1.7 |
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1.1 and 40 was like 1.7 Quoted:
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So for shits and giggles i grabbed the whip outta my wilson and chopped 6 inches off and shoved it in. Made my antenna way short. 1 was higher than 40 Then you need to go shorter.
ETA - What was the SWR on 1 and 40 with the short Wilson whip? 1.1 and 40 was like 1.7 Ok, now shorten it up by trimming or adjusting 1/2 inch at a time until it's 1 to 1 on ch 20. This will make it about 1.3- 1.4 to 1 on 1 and 40 with ch 20 being 1 to 1. 
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Ok, now shorten it up by trimming or adjusting 1/2 inch at a time until it's 1 to 1 on ch 20. This will make it about 1.3- 1.4 to 1 on 1 and 40 with ch 20 being 1 to 1.
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So for shits and giggles i grabbed the whip outta my wilson and chopped 6 inches off and shoved it in. Made my antenna way short. 1 was higher than 40 Then you need to go shorter.
ETA - What was the SWR on 1 and 40 with the short Wilson whip? 1.1 and 40 was like 1.7 Ok, now shorten it up by trimming or adjusting 1/2 inch at a time until it's 1 to 1 on ch 20. This will make it about 1.3- 1.4 to 1 on 1 and 40 with ch 20 being 1 to 1.
Dont wanna touch nothing honestly. Just shot skip to far east texas And cali |
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Dont wanna touch nothing honestly. Just shot skip to far east texas And cali Quoted:
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So for shits and giggles i grabbed the whip outta my wilson and chopped 6 inches off and shoved it in. Made my antenna way short. 1 was higher than 40 Then you need to go shorter.
ETA - What was the SWR on 1 and 40 with the short Wilson whip? 1.1 and 40 was like 1.7 Ok, now shorten it up by trimming or adjusting 1/2 inch at a time until it's 1 to 1 on ch 20. This will make it about 1.3- 1.4 to 1 on 1 and 40 with ch 20 being 1 to 1.
Dont wanna touch nothing honestly. Just shot skip to far east texas And cali Copy that! I feel vindicated now! (bolded text above) I knew my memory wasn't that bad....yet.
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