High SWR on roof installed 3/8 through hole NMO mount

[WSJK800]

Hello, I recently installed my new GMRS radio and getting high 3 to 3+ SWR readings using the Midland Ghost and the Midland 32" whip antenna in an open area. When I connect the magnet antenna that came with the radio, I have a 1.3 SWR. The NMO mount is 3/8 through hole and mounted in the center of my Tacoma roof. The radio is powered direct to battery both positive and negative. The base of the NMO mount has ground continuity with the outside threads of the PL-259 female on the radio. The mount has good continuity with the shield/outside and center conductor on the PL-259 at the end of the cable with no shorts. I do get a short between shield and center conductor when I screw on either antenna. I'm thinking maybe the coax was compromised somehow during installation and I'm facing pulling the headliner down again and installing a new mount. I'm in San Diego if anyone works on these things and would be happy to pay a professional to get it right. Any advice would be appreciated.

Here's my setup:

Arcadian MABVT8U: 3/8 Thru-Hole NMO Mount with 17 Ft. RG-58/U Cable

MicroMobile® Ghost 3dB Gain NMO Antenna - MXTA25

MicroMobile® Whip 6dB Gain NMO Antenna - MXTA26

MXT575 MicroMobile® GMRS 50-Watt Two-Way Radio - MXT575

[WRUE951]

32 minutes ago, WSJK800 said:

Hello, I recently installed my new GMRS radio and getting high 3 to 3+ SWR readings using the Midland Ghost and the Midland 32" whip antenna in an open area. When I connect the magnet antenna that came with the radio, I have a 1.3 SWR. The NMO mount is 3/8 through hole and mounted in the center of my Tacoma roof. The radio is powered direct to battery both positive and negative. The base of the NMO mount has ground continuity with the outside threads of the PL-259 female on the radio. The mount has good continuity with the shield/outside and center conductor on the PL-259 at the end of the cable with no shorts. I do get a short between shield and center conductor when I screw on either antenna. I'm thinking maybe the coax was compromised somehow during installation and I'm facing pulling the headliner down again and installing a new mount. I'm in San Diego if anyone works on these things and would be happy to pay a professional to get it right. Any advice would be appreciated.

Here's my setup:

Arcadian MABVT8U: 3/8 Thru-Hole NMO Mount with 17 Ft. RG-58/U Cable

MicroMobile® Ghost 3dB Gain NMO Antenna - MXTA25

MicroMobile® Whip 6dB Gain NMO Antenna - MXTA26

MXT575 MicroMobile® GMRS 50-Watt Two-Way Radio - MXT575

well, assuming your magnet mount used as a reference is using a different cable. I would start looking at the cable or connectors.   And try moving your ghost antenna to the mag mount and check that.  If you see the same poor SWR than the antenna is suspect..  Process of elimination is the game.  Try reading in one set of circumstsnces than anoter as i suggested.  It's not always easy to chase poor swr problems..  

[WSJK800]

Update:

I went to open area yesterday and took Steve's advice. All antennas have a 3+ SWR using the trucked mounted NMO. I connected the magnet base with the unity gain antenna that came with the radio and the SWR was 1.1 and the Ghost had 1.124 SWR. The only thing it could be now is the through hole NMO/cable. I'm going to put a new PL-259 on and if that doesn't work then I'll have to drop the headliner again and install a new mount/cable.

[WRDJ205]

On 8/22/2025 at 11:00 AM, WSJK800 said:

I'm going to put a new PL-259

Is there any way that you could have gotten water intrusion to the cable via the connection?

[nokones]

Connecting the ground lead directly to the battery is not the best procedure. It is best to connect the ground lead to a factory chassis grounding points on later vehicles to prevent electrical noises being introduced by the vehicle's electrical system and modules into the radio.

However, that wouldn't be the cause of your high VSWRs. I would whack off the connector and install a new amphenol connector. Be cognizant of the little Itty bitty wire strands when installing the new connector.

Just for giggles, disconnect the cable from the radio and remove the antenna from the NMO. Get a multimeter and at the coax connector, check and see if you have any resistance between the center pin and the outside connector. You should have "OL" or zero ohms meaning no continuity between the two conductors. If you have any reading, then there is a direct short. 

If you get zero or "OL" then check the center pin and the center pin contact on the NMO for continuity and then check the outside of the connector with the outside contact point of the NMO for continuity. You should have continuity end to end and if not, you have an open circuit.

Unless you have an antenna analyzer to check where the open circuit may be, it would be best to whack off the coax connector and recheck for an open circuit again. If you have an open circuit, return that cable assembly. Arcadian is easy to work with and they will take care of you.

[nokones]

35 minutes ago, WRDJ205 said:

Is there any way that you could have gotten water intrusion to the cable via the connection?

Probably not since he/she lives in San Diego.