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New CB Radio install - very high SWR

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Old 12-30-2012, 12:48 PM   #1
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New CB Radio install - very high SWR

I'm having a bear of a time tuning my CB install, and was hoping you guys might be able to point me in the right direction. I'm getting a very high SWR (3.5+) on all channels, and my radio's range is about 100 feet before it drops off completely. Obviously this is no fun. My setup is a Uniden Pro505XL mounted in the dash. It's not the most high end radio but it has great reviews pretty much everywhere, and should certainly perform better than this.

It's powered off of a clean wire behind the radio, and has a short ~6" ground to the dash behind the radio. I've checked both with a multimeter and the power is constant and the ground is solid.



From there I have a 12' coax that runs from behind the dash, down the doorsill and out the back of the cab. There's about a ~6 foot coil of it under the seat, but everything I've googled tells me that it's normal to have some overlap wrapped up. There are no kinds/bends/cuts in the wire. I have good ground on both ends, and no continuity between the center pin and the sleeve (which I believe is the way it should be), so the cable isn't grounding out.

From there is runs out to the bed corner behind the cab. Here's my mount. The cable is tight and the mount itself has solid ground, as do both coax connectors. There's an additional ground wire with an eye connector on it (you can see the wire coming off of it) that connects to the underside of the mount between the lock washer and the mount.



That ground wire runs down to the frame, where I wire-wheeled it off and fired a screw into it. I've checked it with the meter as well and there's continuity between my battery negative terminal and the mount on both sides.



I've gone through two antennas so far - one's a Firestik 3' that's not tuneable, which I initially thought was the problem. I swapped it out with a 4' K40 tuneable, and ended up with the exact same readings.



Then I tried the meter with NO antenna attached, and the reading is the same - 3.5-4 across all channels. Everything I've read tells me that this is usually a bad coax or a bad ground to the mount, but from everything I can tell both my grounds are good and this cable is fine.

Any ideas as to what I can try next? I have the radio custom mounted into my dash and it would be a bit of a PITA to swap it out and try another one, and I don't want to just arbitrarily throw money at it before I've asked around and got some insight from people who know more about this than I do.
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Old 12-30-2012, 01:01 PM   #2
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Sounds like you ruled out the antenna and ground wires. So, if the grounds are good I would suspect the coax. Had that problem in the past. If you have another cable, just route it straight from CB to antenna and test SWR again. Will at least rule out the coax cable.
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Old 12-30-2012, 01:15 PM   #3
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I don't have one handy, but that's my next step. I have been Googling some more though and I'm not sure that my stud mount is correct. I DO have continuity between the stud and the mount itself. Is this not the way it should be?

(Pardon the crude drawing)



I'm reading this:

http://www.rightchannelradios.com/tr...na-stud-washer

And the stud that my antenna screws on to is definitely grounded. Do I need to do a better job of isolating the stud?
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Old 12-30-2012, 01:29 PM   #4
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Sounds like some troubles. Make a constant. If youve done everything you feel you can do than its time to uninstall EVERYTHING. Set it up outside of the truck. This will insure you can look everything over and pretty much start from scratch. If you can get it working outside at a consistant rate you know it has something to do with the way it is installed. Also what possible interference could you be getting from an outside source. Drive a few miles and test it out in a different area.
Just some other options.
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Old 12-30-2012, 01:36 PM   #5
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Thanks. I have tried it outside in a wide open area, same result unfortunately.

Setting it up outside of the truck is a good idea, but it's freakin' freezing out, haha.
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Old 12-30-2012, 01:37 PM   #6
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Take the star washer off the bottom and put it on top of the plastic insulator under the nut.
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Old 12-30-2012, 01:49 PM   #7
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Deflection off from all that STEEL beside it ?????

We have to run our antennas as far out on our mirrors due to our "Dump Buckets" for this reason...

Do you have enough coax available to "dummy rig" it too the top of all that steel ??? I know it will be too high BUT it will give you a chance to correct what is going on....

If those are working lights on that bar,try trouble-shooting using their ground...sometimes TOO MUCH of a good thing back fires.....Try to keep it simple without having to re-write your whole process....

Very good advice on Checking coax; it is not pinched or crimped anywhere ???

GOOD LUCK....
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Old 12-30-2012, 02:31 PM   #8
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I can't help but feel as though I'm missing something glaringly obvious here, so apologies in advance if this is a stupid question.

I flipped the spacer around, and that isolates the center conductor. Without the bottom bit screwed on (the coax connector) I have no continuity between the actual antenna mount and the ground, which (I think..) is what I'm looking for.

However as soon as I screw in the bottom half that the coax screws into, I do get continuity between the center conductor and the mount again. If the center is shielded correctly. I can't help but think that either I need another shielding washer between the bottom half of the connector and the mount, or my connector itself is the issue:



Same here - I do have continuity between the threaded antenna piece and the black bit, which is my mount.



Thanks again for the replies. I always had mag mount antennas in the past, so this was never an issue.
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Old 12-30-2012, 02:59 PM   #9
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Quote:
Originally Posted by maineah View Post
Take the star washer off the bottom and put it on top of the plastic insulator under the nut.
This did the trick - thank you! I had the plastic insulator upside down, but just flipping it the correct way didn't do it - I still had cont between the ground and the antenna mount. Putting that washer in the way isolated it, and I was able to get a buddy of mine a few miles away on the highway with no problem at all from my driveway.

Cheers all, appreciate the replies.
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Old 01-02-2013, 11:12 AM   #10
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Ive had the same issue time to go try this out...
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Old 01-02-2013, 11:46 AM   #11
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Guess mine isn't going to be as easy of a fix, my mount is different:


Passenger side:
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Old 01-02-2013, 09:09 PM   #13
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Gotta love them radios! I am running an ntenna in bed too.
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Old 01-03-2013, 06:04 AM   #14
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Remove your connector set a meter to ohms check between the antenna stud and ground if it shows resistance there is something wrong with your mount assembly. Also by adding the spring you have effectively made the antenna longer there by lowering the resonant frequency. Try it without the spring and see where it is if it’s ok check the instructions for the antenna as far as tuning because it will have to be shortened in order to work with the spring. Where you have it is sufficient grounding the bars are grounded to the body. You can add a ground wire if it makes you feel better but that's not your issue.
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Old 01-04-2013, 10:02 AM   #15
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Quote:
Originally Posted by maineah View Post
Remove your connector set a meter to ohms check between the antenna stud and ground if it shows resistance there is something wrong with your mount assembly. Also by adding the spring you have effectively made the antenna longer there by lowering the resonant frequency. Try it without the spring and see where it is if itís ok check the instructions for the antenna as far as tuning because it will have to be shortened in order to work with the spring. Where you have it is sufficient grounding the bars are grounded to the body. You can add a ground wire if it makes you feel better but that's not your issue.
I'll have to give this a go, I had checked the grounding on these things and as you said, it was grounded (which surprised me), I also have these set up as the radio antenna with the little box by Firestik:
http://www.amazon.com/dp/B000RF26FK/...SIN=B000RF26FK

I've been meaning to try and by pass this and test the SWR but that requires tearing apart the dash and its snowy and cold out and I don't have a garage... so it'll have to wait. I'll have to check the ohms soon prolly this weekend while working on painting the sliders.
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Old 01-17-2013, 03:15 PM   #16
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I've never used one of those matching units. The problem I foresee with them is that some, if not a great deal, of your power is going to be lost as heat in the matching unit because, while the matching unit presents the ideal 50 ohm impedance to the radio, the mismatch is still present between the matching unit and the antenna - especially if the antenna isn't resonant for 27 MHz.

As for the antenna mount - the ONLY place you should see a "short" or no resistance is between the mount itself and the vehicle/ground. Your meter should show open between the stud that you screw the antenna into and the body/frame/ground of the vehicle. Even if the mount is installed correctly and everything checks out on a DVM, you can, obviously, still have a high SWR. On my 07 Tacoma Double Cab, I had a mount made at a metal shop - very similar to many of the right angle mounts you find for sale except that the horizontal piece was about 6" long so I could get the antenna away from the back of the cab because I prefer to mount my antennas as close to the center of the vehicle as possible to get the best omni-directional pattern from the antenna. Since the Tacoma bed is a composite material and is electrically isolated from the frame, I used two copper braid straps that I connected to the each bolt of the mount and ran them to the frame on each side of the truck. Braid is generally preferred because RF flows on the surface of metal and braid offers the most surface area. This can be easily made by taking some old coax and simply stripping back enough of the vinyl jacket to expose the shield on each end and soldering a lug on to to make your connections. Now, you would think the match would be great...but it wasn't. What I ended up having to do was bond the metal rails together that run along the top of the bed. This was done using small lengths of coax as just described. I removed all the rails. On the piece that runs behind the cab, I sanded around the hole for the screw on each end. On the ones that run the length of the bed, I only sanded around the hole at the end closest to the cab. When I re-installed the rails, I added my strips of coax to the ends and made sure all three rails were electrically connected. Doing so dropped my SWR from 3:1 down to 1:1 across the entire band and then some.
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