High SWR, all new gear.

[Dpatt711]

New Uniden PC78LTX, coax, Wilson 2000 antenna, and stud mount, however I'm getting SWR of about 3.0 even, 1 & 40. My mount is the mirror mount of a Mack tri-axle. Checked barrel to barrel (cont), core to core (cont), barrel to core (no cont), radio end barrel to lower stud (cont) radio end barrel to upper stud (no cont), and lower stud to chassis ground (cont 1.3ohms). Other identical trucks with identical mounts (the mounting brackets are oem welded on so consistent truck to truck)) are achieving SWRs of <1.5. My meter is a Kalibur SWR meter.
Any ideas why I'm having SWR issues?

 

[prcguy]

Lack of ground plane. Are the doors and cab fiberglass or metal? Is the antenna way out on the edge of the mirror or mounted close to the door? If the doors and cab are fiberglass that's a problem and you may have to add a small antenna pointing upside down from the ground side of the mount to make a dipole. If the doors are metal and hopefully the cab, mounting the antenna right at the door on the mirror mount as close as you can get to the door sheet metal will improve the ground plane and allow the antenna to tune. Or possibly 9ft of wire attached to the antenna mount ground side and snaked with the coax through the door and into the cab somewhere but that can get complicated as RF will be flowing on that wire.

Grounding as in a wire to the battery is not a ground plane. The antenna wants to be mounted in the center of a large flat sheet of metal. Mount it hanging in the air on a big mirror mount and it will not cooperate.

 

[Dpatt711]

Lack of ground plane. Are the doors and cab fiberglass or metal? Is the antenna way out on the edge of the mirror or mounted close to the door? If the doors and cab are fiberglass that's a problem and you may have to add a small antenna pointing upside down from the ground side of the mount to make a dipole. If the doors are metal and hopefully the cab, mounting the antenna right at the door on the mirror mount as close as you can get to the door sheet metal will improve the ground plane and allow the antenna to tune. Or possibly 9ft of wire attached to the antenna mount ground side and snaked with the coax through the door and into the cab somewhere but that can get complicated as RF will be flowing on that wire.

Grounding as in a wire to the battery is not a ground plane. The antenna wants to be mounted in the center of a large flat sheet of metal. Mount it hanging in the air on a big mirror mount and it will not cooperate.

This is what I initially assumed but like I said, identical trucks (we're talking sequential serial numbers identical here) have great SWRs even when I throw my own antenna on. Doors are metal and the antenna is about 7" from the door. It also sticks about 3ft over the cab and cab shelf.

 

[k7ng]

I'm trying to make sure I understand: You swapped antennas between a 'good' truck and the 'bad' one and the VSWR is bad in either case?

 

[TomLine]

Wilson 2000

Kink in cable ?

 

[Dpatt711]

I'm trying to make sure I understand: You swapped antennas between a 'good' truck and the 'bad' one and the VSWR is bad in either case?

I took my antenna (since I figured different height antennas might behave differently) threw it on another truck identical to mine and the SWR was around 1.5, so the issue definitely seems specific to my truck and not just the geometry itself.

 

[Dpatt711]

Kink in cable ?

I don't think so, just for shiggles I tried running a different new cable straight through the window and the issue persisted.

 

[N0ZQR]

Can you provide a photo of the mount?

 

[Dpatt711]

Can you provide a photo of the mount?

 

[TomLine]

Instruction sheet attached. Mentioned something about coating on mirrors? Maybe there's clear-coat on the chrome? Anyhow, here's the manual. Perhaps a temporary ground wire the base would reveal something. Also said to not coil the wire.

 

[prcguy]

Can you measure the length of coax in the other trucks and compare to yours? Since the ground plane is minimal on these trucks the outer shield of the coax becomes part of the ground plane and its length can affect SWR and performance. It might be possible to custom tune a length of coax to smooth things out in these cases but I prefer making the antenna happy with a second short antenna grounded and pointing down so the antenna is "complete" then choking off the coax with a ferrite 1:1 balun to stop RF from riding into the cab on the coax shield.

 

[shadow2019]

the little plastic washer needs to be on the top of mount. if it is not you will have high swr. I can't tell by the pic. I made the mistake once and swr was high. Just something to check. black washer in pic

 

[Dpatt711]

the little plastic washer needs to be on the top of mount. if it is not you will have high swr. I can't tell by the pic. I made the mistake once and swr was high. Just something to check. black washer in pic

Yeah the nylon washer is installed correctly and properly isolating the top side from ground

 

[PlayinMySkip]

Ok, you measured the SWR...good job so far. Yeah, keep the coax length to a minimum. Did ya ever think that you needed to trim the antenna to bring the SWR down? Can't just hook a radio up to an antenna, say the SWR is 4.0 and leave it at that.

 

[TomLine]

Ok, you measured the SWR...good job so far. Yeah, keep the coax length to a minimum. Did ya ever think that you needed to trim the antenna to bring the SWR down? Can't just hook a radio up to an antenna, say the SWR is 4.0 and leave it at that.

The manual says high swr across all channels, as he's experencing, suggests grounding issue. If it just needed tuned, the swr would vary across the band.

 

[prcguy]

The OP measured for shorts and opens from the radio end to the antenna end and reported its ok so probably not a shorted or grounded mount. If the ground plane is really lacking the match will be bad everywhere and not necessarily different across the 40 CB channels.

The manual says high swr across all channels, as he's experencing, suggests grounding issue. If it just needed tuned, the swr would vary across the band.

 

[900mhz]

Disconnect the PL-259 at the mirror mount and hook up a 50 ohm dummy load. Repeat test. This will verify the cabling from the radio to the base of the mount. There should be no reflected power if all is well...i.e. 1:1 SWR.

 

[Dpatt711]

I'm going to try an electrical grounding strap since that's the easiest to test (I don't have a dummy load or the components to make one) and run it to a few different ground points on the body & chassis. Hopefully that works.

 

[TomLine]

I'm going to try an electrical grounding strap since that's the easiest to test (I don't have a dummy load or the components to make one) and run it to a few different ground points on the body & chassis. Hopefully that works.

Wondered if it's the radio ground, not the antenna. Could there be another RF source, tracking thing, something going on?

 

[prcguy]

I would attach 9ft of wire to the mirror mount ground side and stretch that out sideways for a test. If the SWR is still really high its not the grounding or lack of ground plane.

I'm going to try an electrical grounding strap since that's the easiest to test (I don't have a dummy load or the components to make one) and run it to a few different ground points on the body & chassis. Hopefully that works.