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686keydragon 06-05-2013 11:23 PM

CB: Another High SWR Thread
 
3 Attachment(s)
Hi everyone, I apologize in advance for the length of this post... I am having difficulty getting my SWR to acceptable levels on all channels. I am able to get channel 1 to 1.8 but channel 40 will not go below 3. I am new to CB but have read extensively through the forum and have troubleshot using firestick's, stu's, and several other websites to no avail. Based on all the other threads I have read I will post the answers to the common questions:

Antenna: 3' firestick II

Mount: stainless steel mirror mount that I mounted next to my drivers side tail light, the other side of the mirror mount is on the other side of the tub to help distribute the load. The mount is installed with the insulating shoulder washer in the correct location and it is not off angle.

Attachment 634968
Attachment 634969
Attachment 634971

Coax: The coax is from a truck stop and I don't remember the brand, but it came in a kit with the mount. Since I don't know the brand these are the specs: It is a RG-58A/U Coaxial Cable w/Molded PL-259 connector. It has 95% shielding and a stranded center conductor.
The coax is routed up the tub from the mount, over the lip of the tub, under the carpet, across the floor to the passenger side, along the tub to the roll bar, around the roll bar and under the passenger seat, under the lip of the console to my CB that is mounted on the passenger side of the console in the foot well. There are no loops in the cable.

CB: Uniden 520xl. The CB power is routed to the radio's constant power and the ground is to a separate ground.

SWR Meter: Road Pro

Troubleshooting performed:
*For all SWR testing I had the Jeep in an open field with no trees, buildings, power lines, etc. within 30 yards.
  1. I started by simply adjusting the antenna the typical way to get the lowest SWR by switching between channels 1 and 40 and adjusting. When doing so I ended up with an SWR close to 2.8 on channel 1 and 3.1 on channel 40. I then began adjusting to see how low I could get the SWR on channel 1 on its own and I was able to get it down to 2. No amount of movement of the adjuster screw lowered the SWR on channel 40 below 3.
  2. I began testing continuity on the mount stud, the mount itself with the tub and the negative terminal of the battery, the antenna, and the coax; all check out with no shorts and continuity where it should be.
  3. I then added a spring to see if the antenna was just not high enough. I then went through and re-adjusted antenna. It turns out that this did have a positive effect and I was able to get the SWR on channel 1 down to 1.8, but channel 40 was still above 3.
  4. I then removed the paint on the tub beneath the mount, removed the chrome coating from the tub side of the mount, and removed the chrome coating from the underside of the mount where the stud grounds. I then went through and re-adjusted the antenna, but I saw no change in SWR.
  5. I then began trying different cable routing to see if I was picking up stray RF and no cable relocation made any difference in SWR.
  6. I then relocated the mounting stud to the spare tire carrier (I ground the paint under the stud). I checked all continuity to ensure it was grounded and working correctly, re-adjusted the antenna, and had higher SWR than in the previous location. I then relocated the antenna back to the original location.
  7. I realize that this should have no affect on SWR, but just to be safe I then tried routing the CB's power directly to the battery. As you probably expected, this had no affect on the SWR.

At this point I believe that I have tested everything and I am down to individual components that may be causing the issue. Before I begin blindly replacing parts (that seemingly check out) I wanted to see if anyone else had any ideas. Thank you for any assistance that you can provide.

CJ7-Tim 06-06-2013 11:23 AM

Quote:

Originally Posted by 686keydragon (Post 15525721)
.... started by simply adjusting the antenna the typical way to get the lowest SWR by switching between channels 1 and 40 and adjusting. When doing so I ended up with an SWR close to 2.8 on channel 1 and 3.1 on channel 40.

SWR can depend on the length of your antenna, the quality of the antenna construction, and your choice of mounting location. Longer antennas are easier to tune and have a flatter SWR curve across the CB channels. Shorter antennas are more difficult to tune, and have a sharper SWR curve. Mounting an antenna down low, usually increases the SWR curve. In addition, you may have an insufficient ground plane, which will also negatively effect your overall SWR.

The ideal SWR is usually a balanced curve, close to the same on Ch 1 and Ch 40 and the lowest in the middle channels, about Ch 20. See this -> http://firestik.com/Tech_Docs/swr-curves.htm

2.8-3.0 is not great, but it usable. If you are always on one specific CB channel, you can tune for best SWR on that channel.

Filthy-Beast 06-08-2013 12:04 PM

your antenna is electronically too long. completely remove the tuning tip, put the cover back on and try again. Try another cable.

psouza1943 06-08-2013 05:13 PM

You do not have a problem with your installation other than your choice(s) of mounting location. Your VSWR readings are high because of near field effects. Too much tub, too close to the antenna. Tires, top, and everything else in proximity to the antenna will cause this. It can not be tuned out.

As has been said already, you will get acceptable results as you are now, if you can ignore the stigmata of not being "perfect". Odds are good you will not be able to notice the very slight reduction in efficiency.

I suggest you install one or more external star washers between your backing plate and body sheet metal to ensure a permanent "good ground" for the antenna. Your ground is probably fine now, but the star washers will keep it good, for a longer time.

Ground plane has been mentioned.....ha We are mounting on a Jeep, there is no good ground plane available, but the antenna can still be tuned to resonance.

Too late now, since you committed to that mount location, but higher and to the outside, would usually give you a better result. It's not worth worrying about....it will be fine as is.

Phil

686keydragon 06-08-2013 08:41 PM

Quote:

Originally Posted by Filthy-Beast
your antenna is electronically too long. completely remove the tuning tip, put the cover back on and try again. Try another cable.

Tried both they had negligible results.


Quote:

Originally Posted by psouza1943
You do not have a problem with your installation other than your choice(s) of mounting location. Your VSWR readings are high because of near field effects. Too much tub, too close to the antenna. Tires, top, and everything else in proximity to the antenna will cause this. It can not be tuned out.

As has been said already, you will get acceptable results as you are now, if you can ignore the stigmata of not being "perfect". Odds are good you will not be able to notice the very slight reduction in efficiency.

I suggest you install one or more external star washers between your backing plate and body sheet metal to ensure a permanent "good ground" for the antenna. Your ground is probably fine now, but the star washers will keep it good, for a longer time.

Ground plane has been mentioned.....ha We are mounting on a Jeep, there is no good ground plane available, but the antenna can still be tuned to resonance.

Too late now, since you committed to that mount location, but higher and to the outside, would usually give you a better result. It's not worth worrying about....it will be fine as is.

Phil

That's what I was afraid of... I should have picked a better mount location.

Thanks for all the feedback everyone. While this goes against my nature and will likely drive me insane... Based on the general consensus here I am going to leave it as is and call it good.

2002 TJ, 2.5L, 4" lift, 33" dunlop mud rovers, xrc8 winch, aussie locked d30, a-z fabrication ww2ww sliders, mounted mv-50 compressor, SirGcal sub box

Filthy-Beast 06-08-2013 09:14 PM

He should be able to get better than 3.1 on channel 40.

This install gets 1.4 on CH 1 and 1.8 on ch 40

http://www.bigaperture.com/photos/i-...-LWVtkxv-M.jpg


This install gets 1.2 on ch 1 and 1.4 on ch 40

http://www.bigaperture.com/Wheeling/...CIMG1291-M.jpg

psouza1943 06-08-2013 10:17 PM

Small differences can have seemingly large effects.

Phil

686keydragon 06-09-2013 01:36 AM

Quote:

Originally Posted by Filthy-Beast
He should be able to get better than 3.1 on channel 40.

This install gets 1.4 on CH 1 and 1.8 on ch 40

This install gets 1.2 on ch 1 and 1.4 on ch 40

What length and model of antenna's are you running? I am starting to wonder if I would get better results with a 4'. I would just prefer a 3' so I can keep it on and still park in my garage.

2002 TJ, 2.5L, 4" lift, 33" dunlop mud rovers, xrc8 winch, aussie locked d30, a-z fabrication ww2ww sliders, mounted mv-50 compressor, SirGcal sub box

Filthy-Beast 06-09-2013 05:28 AM

Red jeep is a no name bottom loaded metal whip. The black jeep is my favorite antenna, a 4ft Wilson Flex antenna.

http://www.bigaperture.com/Wheeling/...CIMG1289-M.jpg

CJ7-Tim 06-09-2013 07:18 AM

In my experience a Wilson Flex antenna is more difficult to tune than a Firestik II, more threads with high SWR issues involve Wilson Flex antennas than any other brand. YMMV.

I agree a longer antenna will probably give better results and will not be very expensive.

JPNinPA 06-09-2013 06:15 PM

You changed antenna length with no effect but improving ground made some improvement. Check continuity and resistance from the antenna tip to the center of the stud mount while it is in the mount. Do this then wiggle the antenna and check again. I'll bet there is an issue with the antenna or stud mount.

686keydragon 06-09-2013 06:24 PM

Quote:

Originally Posted by JPNinPA
You changed antenna length with no effect but improving ground made some improvement. Check continuity and resistance from the antenna tip to the center of the stud mount while it is in the mount. Do this then wiggle the antenna and check again. I'll bet there is an issue with the antenna or stud mount.

I have checked that as well and cant find any issues.

I picked up a new stud, cable, and a different antenna today. I am going to try replacing one at a time to see if I have a hardware issue.

2002 TJ, 2.5L, 4" lift, 33" dunlop mud rovers, xrc8 winch, aussie locked d30, a-z fabrication ww2ww sliders, mounted mv-50 compressor, SirGcal sub box

Jerry Bransford 06-10-2013 02:31 PM

First, you can save a lot of time by not bothering to tune it at both channels 1 & 40, just tune at channel 20 which is mid-band. When channel 20's SWR is as good as you can get it, 1 and 40 will be too.

One of the things a good low SWR depends on is a good ground plane which depends on the mount having a good RF ground connection to the tub. An RF ground is MUCH more demanding than a simple DC ground is that is only good enough for non-RF needs like light bulbs & DC power circuits.

To get the best possible RF ground, the mount needs to have a bare metal connection to the tub. Grinding a bit of paint off under washers etc. is not as good as grinding the paint/powder coating away between the mount and tub (or bumper, etc.). The mounts I install are either welded in place or they have a complete bare-metal to bare-metal connection.

My feeling right now is your mount may just not have an adequate RF ground connection to the tub. It will of course show continuity to the tub but even a skinny wire strand can do that. One more FYI, the Jeep itself is half of the antenna system, the other half of the antenna system is the whip antenna. So that RF ground connection to the tub or chassis is critical to the antenna working as well as it possibly can.

686keydragon 06-10-2013 03:15 PM

Quote:

Originally Posted by Jerry Bransford
First, you can save a lot of time by not bothering to tune it at both channels 1 & 40, just tune at channel 20 which is mid-band. When channel 20's SWR is as good as you can get it, 1 and 40 will be too.

One of the things a good low SWR depends on is a good ground plane which depends on the mount having a good RF ground connection to the tub. An RF ground is MUCH more demanding than a simple DC ground is that is only good enough for non-RF needs like light bulbs & DC power circuits.

To get the best possible RF ground, the mount needs to have a bare metal connection to the tub. Grinding a bit of paint off under washers etc. is not as good as grinding the paint/powder coating away between the mount and tub (or bumper, etc.). The mounts I install are either welded in place or they have a complete bare-metal to bare-metal connection.

My feeling right now is your mount may just not have an adequate RF ground connection to the tub. It will of course show continuity to the tub but even a skinny wire strand can do that. One more FYI, the Jeep itself is half of the antenna system, the other half of the antenna system is the whip antenna. So that RF ground connection to the tub or chassis is critical to the antenna working as well as it possibly can.

Yep, I saw that you had made similar comments on other threads so I ground all the paint beneath the mount when I first installed it.

Today I replaced the stud, coax, and antenna separately and tuned for the best possible swr. Replacing each item made it possible for me to get better readings. The stud got me a couple 10ths, the coax got me a couple more, and the antenna did quite a bit. The new stud is a firestick stud, the coax is Wilson, and the antenna is a 4' k40.

With the new stud and coax I was still not able to get channel 40 on the firestick to much below 3. Once I added the new antenna I was able to get down to 1.6 on 1 and 1.6 on 40. Ch 20 is at 1.3.

My assumption is that the 3' firestick just didn't get high enough above the roll bar. Now my only issue is that the 4' antenna is too tall for my garage. Fortunately it is very flexible so for now I have it tucked under my mesh top.

Thanks to everyone for their help with this project!

2002 TJ, 2.5L, 4" lift, 33" dunlop mud rovers, xrc8 winch, aussie locked d30, a-z fabrication ww2ww sliders, mounted mv-50 compressor, SirGcal sub box

JPNinPA 06-10-2013 03:58 PM

Roll bar should not have been a problem unless it was 2-3 in away. Sounds like the antenna is open as the adjustments did nothing for you initially.

Anyway glad to hear you got it fixed.


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