Odd SWR Issue With Jeep Wrangler

[kb1elv]

I'll try to keep this brief, but I've discovered an interesting issue with my 2013 Jeep.

I have a stainless steel antenna bracket mounted to the rear gate of the vehicle (Teraflex 4997101). I'm aware this is a compromise location but for the sake of this discussion let's look past that. Attached to this I have a 6ft fiberglass antenna which shows no deflection on ch 19, and 1.1:1 or so on either end of the dial.

From one of the 2 bolts on the mount I have run a jumper of tinned copper strap to the tailgate with paint ground to bare metal. From there I have another jumper run to the metal body in the same fashion.

For some reason, when I lock the doors (which includes the tailgate) the SWR shoots up to 2:1!!!

I have swapped or tested every component. New stud(s), 2 different external SWR meters, new coax (including jumper to SWR meters), different radio, different antenna (Wilson 2000 w/10" shaft). What could it be?

 

[mmckenna]

Does it go up and stay up as long as the doors are locked, or does it just jump up to 2:1 when you hit the lock button and then return to normal?

 

[kb1elv]

That part is intermittent. It jumps between the low SWR I have when unlocked and the high SWR. If it even happens it is immediately after locking the doors. From there it comes and goes unpredictably and for either a short or long time. The jumping around is unpredictable but always solved by unlocking the doors.

 

[mmckenna]

If the power locks control the rear gate, then there might be a grounding issue that changes depending on the position of the lock mechanism. Could be how the latch contacts ground depending on the way it locks. That would seem to explain the nature of it. The intermittent part of it could be a byproduct of how the latch mechanism works. Since it probably isn't intended as a grounding point, the connection between the tail gate latch and the striker on the body could vary.

I see where you have a ground strap installed between the rear gate and the body, but even changing the ground path can make things change. If I recall correctly, Wranglers have a rear gate that swings to one side. I'm guessing the extra ground strap you added is on the hinge side, makes sense. Could be that the locking mechanism is making/breaking a ground path on the other side of the gate.
And interesting test would be to install a temporary ground strap on the opposite side of the gate from the one you already installed. It would obviously make opening the gate impossible, but might help narrow down the issue.
It might sound like a long shot, but back when I used to have a CB in my truck I could change the SWR by opening/closing the doors. This was a NMO mount, permanent, in the center of the cab roof, Larsen NMO-27 antenna.
If that temporary strap fixes it, you might need to rig up some sort of copper finger wiper set up on the latch side of the rear gate to help improve the tailgate grounding. Sounds like it could be the changing ground path.

Other than that, there could be a separate ground issue somewhere else in the jeep. Hard to say, but that seems like a possible starting point. Maybe the driver/passenger doors, hood, frame to body, spare tire carrier, etc. However, since it seems to change depending on the power lock position, it seems like the doors would be a good place to start.
Also, while I'm sure you have a good DC power ground for your CB, make sure you add a ground braid from the chassis of the CB radio directly and as short as possible to the vehicle body. I've seen weird stuff happen with radios that relied on a really long wire for their ground.


Sounds like a fun challenge for a weekend. Let us know how it works out.

 

[kb1elv]

Thank you very much for the suggestions I will try them out later on in the week. You got all of the logistics correct, the gate does swing from one side and the strap is on the opposite side of the locking mechanism.

I find it amazing that something like this could happen, and like you I'm almost certain it is something in the linkage of the lock mechanism in the tailgate. I tried it with the passenger doors open and didn't see any perceptible change in the SWR. I have access to the linkages so I may attach one end of a wire to a bolt on that side and the other to various components to try and isolate the problem.

I posted in a Jeep forum to see if anyone else had seen this before, but I think most people there just set it and forget it, or don't even bother obtaining an SWR measurement. Again, appreciate the suggestions and hopefully I will get to the bottom of this soon.

 

[mmckenna]

I'm intrigued by this. The technical side of me really wants to dig into this and figure out what's happening. I'd love to hear what you discover.

 

[AC9BX]

I'm with McKenna, this is very intriguing.

I'm aware this is a compromise.... no deflection on ch 19, and 1.1:1

This is absolutely astounding and may be pointing to the problem. Never mind the antenna is electrically too short (this is commonplace and affects the efficiency, although 6ft is pretty good compared to others for CB), when you mount an antenna on a vehicle where you do not have a large enough flat area of metal to act as a good ground plane you aren't going to get good SWR. Mounted in a less than good location the impedance is often very low. This is compensated in coil loading. But because this is a far from ideal set up you should not expect a great SWR. I suspect the 2:1 is true and the lower values are wrong.

Yes you want it low but keep in mind 2:1 isn't that bad and the radio (assuming it's a modern unmodified unit) is good to 3:1 without serious concern.

I'd start by disconnecting the extra ground straps and such. Be sure there is a solid ground at the radio. Take both power leads to the fuse block or battery. The DC ground and RF ground are different and behave differently. Start with a known good DC ground. Connect coax to antenna and take a measurement, doors locked and unlocked. On a vehicle the metal body panel acts as a counterpoise for the antenna. Because it's not shaped as a good ground plane for you it is nothing more than a hunk of metal and that's okay. It does not need extra grounding unless you are having RF issues such as when you key the radio other electronics go crazy. You could be causing more harm than good. You may have created a ground loop or some similar issue causing the radio to float around a bit which can be intermittent.

You may find the radio DC ground is best tied to the chassis near the radio, but usually at the battery is best. You may also find the radio chassis is tied to this ground internally, it may or may not be. You may find the radio needs a strap to the vehicle chassis, or not.

I suspect when the doors are locked there's a switch sensor (even without an alarm) that knows it's locked. When this connection breaks while the system is locked it knows that door has been opened. Using negative logic is commonplace, done for various reasons such as logic compatibility or power saving for a particular logic system. That is, a wire is normally high (has voltage) when the switch is open and is shorted to ground when closed. When closed the gate has a ground reference it doesn't have when open. This is low current connecting to some computer, logic, or microcontroller device could appear intermittent when you apply RF. Then you have this coax shield between the antenna and radio. It now has 2 ground references, different at each end, and may not be functioning as a shield at all. RF can do some crazy things in this case and your meters can lie to you.

You may find you need the jumper from the mount to the gate to make the gate work as a good counterpoise. This should be as short as possible, ideally connected right next to the mount. Likely the jumper to the body is where this is problematic.

 

[kb1elv]

Well I've got my fingers crossed, but this seems to have fixed everything.

I put a rubber band around the mic and I crawled into the back of the jeep with the keys and a piece of wire. I tried a bunch of combinations, but touching the wire to the latch striker (in photo) yielded the only significant results.

I removed the latch and ground down the paint behind it. There is a metal shim that goes between the body and the latch hardware so I wedged the strap between the two and ran it to the body the same way I did on the tailgate hinge side. I screwed the strap down in the area pictured as there are no wires/structural metal/double metal panels in this area. As you can see the strap is only 6" so I'm not concerned with it being any shorter.

Thanks everyone for the thoughts/ideas. I will let you know if it acts up again.

 

[mmckenna]

Interesting fix. It would seem more likely that there wasn't a good ground through the latch, rather than between the striker and the body. Just putting s star washer under the striker plate might have worked, too, but it looks like you got it.

That's one of the drawbacks of using the door/gate as part of the groundplane. RF ground and DC ground are different, so making sure you've got a good path for RF in all directions from the antenna base is important.

 

[kb1elv]

After the paint was ground off I didn't have the problem 100% solved. The smooth outward facing surface of the stiker plate was covered by something non-conductive, but the bolts had continuity through the plate to the "loop" area that the latch hits as well as the metal shim. The strap was more or less a brute force method so I never have to go back there again.

The bonding ended up being completely hidden by a plastic panel I pulled to do the work. Definitely an interesting fix. I think the latch ended up losing conductivity so between removing all the paint and adding the stap I shold be set until I get rid of this thing.

Thanks again for the advice.

 

[JayMojave]

Hello ELV: Good postings and good advice here I'd say, makes you glad we can use the Forum for such problems, and see the fix good going.

I have a old 1977 Ford F250 4X4 Truck that I keep running. Using various radios and such I always had a ignition noise in the radios and the interference was stronger than tolerable.

A ground connection made to the right hand wheel well, going to the fire wall, then going to the hood cured all the ignition noise problems. I used braid wire from old RG213 Coax. I also removed the ignition modular and bonded / cleaned the paint off from the mounting surfaces, and installed aircraft nut plates so that the mounting screws could be tightened down better than automotive type quick assembly screws.

I was contemplating installing shielding on the wiring, added inline chokes and bypass capacitors at the ignition module, but didn't need it.

Driving down the road with no ignition noises in the HF and CB Radios is sure a pleasure.

Jay in the Great Mojave Desert Most of the Mojave's Desert "Historical Landmarks" are just holes in the dirt, so if you have seen a hole in the dirt, you have it all.

 

[kb1elv]

Jay, just goes to show how some unsuspecting things can cause some very weird problems. I just got reintroduced to that yesterday..

So the issue came back... I couldn't believe it. I was way too invested to not figure it out.

Keyed the mic again, crawled back there with a wire. Same thing as last time, touching the wire from the tailgate to the catch dropped the SWR right back down. I knew it had to be in the tailgate this time, there couldn't possibly be a better connection from the loop to the body unless it was a continuous piece of metal. Just for kicks I removed everything I had done previously and double checked, re-installed and still at 2:1. Locking/unlocking did nothing.

I thought maybe some grease in the tailgate latch could be getting warmer in the day, cooler at night and moving around a bit. I took a rag to clean it off, no dice. I took a dremel to barely skim the latch down to cleaner metal. It was at this point I realized the latch in the tailgate is covered in a plastic skin!! It looked exactly like metal from the outside but when I dremeled it a bit it became obvious that it wasn't. I cut it away from the latch. And all is good.

I think when I removed/reinstalled the catch for the first time it misaligned just enough to trick me and get a good connection.

 

[cmdrwill]

Did you bond across the hinge?

 

[kb1elv]

Did you bond across the hinge?

I'm not sure I could do that without interfering with the operation of the mechanism or bonding the tailgate shut.

 

[JayMojave]

Hello ELV: Ok it sure sounds like the tailgate is making a intermittent RF connection effecting the antenna SWR.

Its not easy to give suggested help and such over the keyboard, how about a picture of the antenna installation. I am interested to how close the antenna is to the rest of the vehicles body.

Its also a possibility that the intermittent SWR may not have a significant field strength difference between the low SWR and high SWR. Good Luck.

Jay in the Mojave

 

[Project25_MASTR]

I've always had good luck with hood channel mounts on Jeeps. The Firestik mount works on JK's…I used one to run a Larsen NMO2/70 with 1/2" antenna mount on the bracket. Now I just use the plain ol' $4 flat bracket that can be found at any PCTEL/Laird/Pluse retailer's website (for NMO installs). Works just as good on JKs as the Firestik mount.

 

[kb1elv]

I have attached a few pictures. I am considering the issue resolved for now, hopefully nothing else pops up!

The first is of the general configuration, the second is of the mount itself, third is the bonding at the gate/body and final is the latch I had to remove a plastic skin from. Once I got rid of the plastic coating on the latch everything worked great. Tested with my friend earlier today from inside a dense city area and was impressed with the reports.

I had considered NMO mounts up front but someone I ride with had one repeatedly ripped off by trees/brush when out on the trail. Probably the best location/setup though as its tough working with fiberglass or a soft top and getting a good ground plane.

 

[Project25_MASTR]

I had considered NMO mounts up front but someone I ride with had one repeatedly ripped off by trees/brush when out on the trail.

Generally has to do with antenna selection. A lot of cb guys will try the nmo mount and (foolishly) put an NMO to 3/8-24 adapter so they can run a common cb antenna (generally fiberglass) which has a 3-4 times higher wind load (and 6-7 times higher moment at the mount) instead of a quality commercial grade nmo lowband antenna. The mount isn't designed (they are generally brass after all) to handle that kind of stress.


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[kb1elv]

Generally has to do with antenna selection. A lot of cb guys will try the nmo mount and (foolishly) put an NMO to 3/8-24 adapter so they can run a common cb antenna (generally fiberglass) which has a 3-4 times higher wind load (and 6-7 times higher moment at the mount) instead of a quality commercial grade nmo lowband antenna. The mount isn't designed (they are generally brass after all) to handle that kind of stress.


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This was someone running a 2 meter larson 5/8 wave. The antenna would survive just fine, but the mount would rip or pop out when the antenna got hung up on branches.

 

[Project25_MASTR]

Never had a problem. I keep a nmo27 on a hood channel mount of my XJ. Bouncing around the Rockies hasn't bothered it.


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