Chassis Grounding Points

[Dougstdig]

I had my CB antenna setup tested yesterday evening and I have a bad ground. I have a ‘21 JTRD. My mount is a fender mount on the drivers side. The CB shop didn’t have a tech available and one isn’t available during my non-working hours.

The fella cautioned me on using Body ground points, which I confirmed by using one near the hinge of the hood near the fire wall on the drivers side. It doesn’t provide a good ground. The routed coax was tested with a dummy load and it tested 1.1 to 1.

So I don’t have to tear apart a bunch of stuff, is anyone aware of a good place to attach a grounding strap to the chassis on the drivers side?

…yes I performed a search and didn’t find anything…

Sponsored

 

[Radio Guy]

Antenna grounds and DC/power grounds are two different things. The antenna needs a lot of sheet metal under it as a "ground plane" for it to tune and work properly like the center of a large flat roof. Deny the antenna a ground plane and it will complain by giving you a bad match or VSWR. I suspect (and hope) your CB shop is calling the lack of ground plane a "bad ground" as most CB shops do not employ actual antenna engineers.

Connecting a wire from the antenna base to the battery, which would be an ideal ground for power will do nothing for the antenna, it wants lots of flat sheet metal right under it and not a skinny thin wire going to a battery. In fact, you could take the hood off your vehicle, set that on saw horses next to the vehicle, mount your antenna in the middle of the isolated hood and the antenna would be very happy and have a good match with no other connections to the isolated hood.

The fact you got a perfect 1:1 match with a 50 ohm load at the end of the coax is good, meaning your coax is in good shape and the connector(s) are put on ok, but that has nothing to do with a ground plane to make your antenna happy.

Some antennas will work better than others with a minimal ground plane, what make and model is yours? Smaller shorter antennas will usually have more trouble with a compromised ground plane vs a longer one, its partially about how much capacitance the whip has to the surrounding vehicle body.

You can do a test to see if more ground plane will help like take a VSWR measurement as is, then take about 9ft of wire and connect to the grounded side of your antenna mount and stretch it out sideways away from the vehicle like an antenna "ground radial". If the match gets much better then you have a couple of choices.

Find a better place to mount the antenna, or go with a model known to work with a compromised ground plane. Or if you have easy access to the mount where the coax connects experiment with a 25pf to 50pf capacitor with very short leads across the hot and ground of the antenna mount. This can replicate the needed capacitance between whip and vehicle body and allow you to get a good match making the radio happy, but it won't affect the reduced range due to the compromised antenna mounting location. If it works you will have to find a good way to attach and weather proof the capacitor, which ideally is a dipped silver mica type and a 100v rating is more than adequate for a stock CB.

 

[Dougstdig]

The antenna is a 55” Husler center load whip (Hustler (HQ-27). I also got the magnum version for the thicker main shaft for less flex…to run on the street. At highway speeds the non mag version is all over the place.

I got 8 ft of ground strapping I’ll run from the mount off to the drivers side after work so see the result.

I also replaced the Stryker 94, previously shown in another post with a Ranger X9.

I also changed the coax. The attachment to the bracket is now a 90 degree with a proper end on it. The owner of the CB shop on Saturday indicated a ground issue so it was swapped out. I really hope I don’t have to reroute the cable again. My trim panels around the roll bar will be pissed…lol.

 

[Wholand]

Not sure if this helps you any but it maps out all the factory ground locations.

 

[Dougstdig]

Not sure if this helps you any but it maps out all the factory ground locations.

Great…thank you

 

[Radio Guy]

The antenna is a 55” Husler center load whip (Hustler (HQ-27). I also got the magnum version for the thicker main shaft for less flex…to run on the street. At highway speeds the non mag version is all over the place.

I got 8 ft of ground strapping I’ll run from the mount off to the drivers side after work so see the result.

I also replaced the Stryker 94, previously shown in another post with a Ranger X9.

I also changed the coax. The attachment to the bracket is now a 90 degree with a proper end on it. The owner of the CB shop on Saturday indicated a ground issue so it was swapped out. I really hope I don’t have to reroute the cable again. My trim panels around the roll bar will be pissed…lol.

The grounding at your current antenna mount should be sufficient in that its making contact with the sheet metal right at the mount, which is all you can hope for. If that checks ok with an ohmmeter to another known good ground point then any additional grounding at the mount should do absolutely nothing for the VSWR.

Your antenna probably wants more flat sheet metal right under it and the major ground plane right now is the hood, which has virtually no ground current flow back to the antenna. Even if you were to put ground braid across the hood hinges its nowhere near the antenna feed point.

Your antenna mount has raised the feed point way up and away from the sheet metal and there are other mounts that simply stick out the side just a little with the antenna mount much closer to the hood. Those mounts would work slightly better because they keep the antenna feed point closer to the sheet metal. When you raise an antenna feed point just a few inches above the ground plane you compound the problem and it will be harder to match the antenna. Just remember, the people marketing the type of antenna mount you have are not antenna engineers, they are jeep parts fabricators. Its about the same as an antenna engineer trying to design a suspension component or new pistons.

If you have time to do the flying 8ft ground radial out the side, that should be very informative.

 

[Dougstdig]

Well…. As I indicated I would, I followed through with the recommendation of @Radio Guy.
-The first 3 photos are of the current setup followed by the corresponding SWR reading.
-The next few photos show a longer ground strap stretched out followed by SWR readings. I threw the strap over the hood with the same result.

 

[Radio Guy]

In both sets of pics it shows a better match on channel 1 than 40 which would indicate the antenna is too long. It looks like the match might be very slightly better without the long strap and if the antenna is actually too long adding the strap would make it worse.

Are there any set screws that would allow shortening or lengthening the whip? If so make an adjustment and see if shortening makes it better or lengthening makes it worse. Its possible that cutting the antenna shorter will put you in a useable VSWR range but you gotta prove that shortening it is the way to go before committing to the cut.

 

[Dougstdig]

I’ll check it out tomorrow and report back…Thanks for your help.

 

[Dougstdig]

Tonight I messed with the antenna again. The tip was bottomed out. When I lengthened it to the max I was just short of 4…proving length was part of the issue. Never thought I’d be saying that…. Anyway, I whittled it down just over 2 1/2 inches. On 19 I’m right at 3. Got a radio check, and a fella 5 miles north said I was sitting right next to him. Granted, the X9 has been tuned and is throwing some power, I could be doing much better and reception would be much better. I guess I’ll have to figure out another setup.

 

[Radio Guy]

Tonight I messed with the antenna again. The tip was bottomed out. When I lengthened it to the max I was just short of 4…proving length was part of the issue. Never thought I’d be saying that…. Anyway, I whittled it down just over 2 1/2 inches. On 19 I’m right at 3. Got a radio check, and a fella 5 miles north said I was sitting right next to him. Granted, the X9 has been tuned and is throwing some power, I could be doing much better and reception would be much better. I guess I’ll have to figure out another setup.

Most CB antennas are a little long out of the box to accommodate different installs. If you monitor ch1 and ch40 SWR while trimming off 1/2" at a time you can usually tell if its going to continue to get better and its possible it needs to be shorter.

Its when ch1 dips to the lowest SWR then starts to go up a little when you stop cutting and evaluate things again. If it dipped nice and low like 1.2:1 then you can usually get ch20 to reach the same good match with 1 and 40 being a little higher. If the best it gets on the low dip is say 2:1 on ch1 and 40 is much hither then you will have to consider other mounting locations with more ground plane or do the capacitor trick, etc.

 

[Dougstdig]

Most CB antennas are a little long out of the box to accommodate different installs. If you monitor ch1 and ch40 SWR while trimming off 1/2" at a time you can usually tell if its going to continue to get better and its possible it needs to be shorter.

Its when ch1 dips to the lowest SWR then starts to go up a little when you stop cutting and evaluate things again. If it dipped nice and low like 1.2:1 then you can usually get ch20 to reach the same good match with 1 and 40 being a little higher. If the best it gets on the low dip is say 2:1 on ch1 and 40 is much hither then you will have to consider other mounting locations with more ground plane or do the capacitor trick, etc.

Thanks again for the reply. Do you have any photos of the capacitor mod?

Sponsored