Permanent antenna mount on my car ruined my SWR. Ideas?

[OkieBoyKJ5JFG]

I was using a Comet SBB-1 antenna on a mag mount on top of my car and had excellent SWRs, 1.0-1.5:1. This week, I had a hole drilled in the roof and a Larsen NMO-K mount installed in the same place where the mag mount was. Now, using the same antenna (and two other antennas) I'm getting SWRs of 7-10:1 up and down the 70cm and 2m bands. I don't know how it could have changed so radically by installing a roof mount. The coax is shorter, but I can't imagine that would do it. I didn't do the work myself and I didn't see it done, but the guy has installed radios before. What can be the cause of this?

 

[rf_patriot200]

Inspect the connection to your nmo mount for a solid ground and the connector on the end of your coax. Also HOW short is your coax ?

 

[OkieBoyKJ5JFG]

Inspect the connection to your nmo mount for a solid ground and the connector on the end of your coax. Also HOW short is your coax ?

It's about 6 feet.

 

[mmckenna]

I was using a Comet SBB-1 antenna on a mag mount on top of my car and had excellent SWRs, 1.0-1.5:1. This week, I had a hole drilled in the roof and a Larsen NMO-K mount installed in the same place where the mag mount was. Now, using the same antenna (and two other antennas) I'm getting SWRs of 7-10:1 up and down the 70cm and 2m bands. I don't know how it could have changed so radically by installing a roof mount. The coax is shorter, but I can't imagine that would do it. I didn't do the work myself and I didn't see it done, but the guy has installed radios before. What can be the cause of this?

With an SWR that high, it sounds like a connector issue. Did the person installing your NMO mount install the connector and test it afterwards? Is this the Comet SSB-1 or the Comet SSB-1NMO?

If you have a multimeter or a continuity tester, I'd recommend you check it.

1. Remove the antenna and radio. Remove all adapters or extension cables. That includes any adapters on the NMO mount.

2. Test for continuity between the outer shell of the connector and the center pin. There should not be continuity. If there is, you need to fix that.

3. Test for continuity from the center pin of the coax connector to the center button of the NMO mount on the roof. There should be continuity. If there isn't, you need to fix that.

4. Test for continuity between the outer shell of the coax connector and the outer ring of the NMO mount. There should be continuity. If there isn't, you need to fix that.

5. Check for continuity between the outer ring of the NMO mount to a known good body ground on the vehicle. There should be continuity. If there isn't, you need to fix that.

Or, take it back to the installer and have them check. It's fairly easy to screw up a connector install, it happens.

With an actual ground connection at the NMO base, as opposed to a magnetic mount, you shouldn't have any issues tuning the antenna for low SWR.

 

[OkieBoyKJ5JFG]

With an SWR that high, it sounds like a connector issue. Did the person installing your NMO mount install the connector and test it afterwards? Is this the Comet SSB-1 or the Comet SSB-1NMO?

If you have a multimeter or a continuity tester, I'd recommend you check it.

1. Remove the antenna and radio. Remove all adapters or extension cables. That includes any adapters on the NMO mount.

2. Test for continuity between the outer shell of the connector and the center pin. There should not be continuity. If there is, you need to fix that.

3. Test for continuity from the center pin of the coax connector to the center button of the NMO mount on the roof. There should be continuity. If there isn't, you need to fix that.

4. Test for continuity between the outer shell of the coax connector and the outer ring of the NMO mount. There should be continuity. If there isn't, you need to fix that.

5. Check for continuity between the outer ring of the NMO mount to a known good body ground on the vehicle. There should be continuity. If there isn't, you need to fix that.

Or, take it back to the installer and have them check. It's fairly easy to screw up a connector install, it happens.

With an actual ground connection at the NMO base, as opposed to a magnetic mount, you shouldn't have any issues tuning the antenna for low SWR.

Despite good recommendations, the installer was kind of an idiot, so I wouldn't be surprised if he messed something up.

I don't have a multimeter. Is there some particular type I need?

When you say it's pretty easy to screw up a connector install, do you mean the PL259 connection?

 

[mmckenna]

Despite good recommendations, the installer was kind of an idiot, so I wouldn't be surprised if he messed something up.

Well, there ya' go. There's a lot of YouTubes radio technicians out there that are good at watching videos, but don't understand what they are doing. Testing the install after completion should have been one of the steps.

I don't have a multimeter. Is there some particular type I need?

You need a multimeter or a continuity tester. The challenge isn't so much finding the tool, it's learning how to use it.

While not great, Harbor Freight sells some inexpensive multimeters that will do what you need.
A basic continuity tester might be easier to learn/use. The electrical section of your hardware store should have what you need.

When you say it's pretty easy to screw up a connector install, do you mean the PL259 connection?

Yes. Easy to mess it up if the installer is not super careful. It just happens and it's why they should be tested. I've done a lot of coax connectors over the decades, and I screwed up a connector the other day because someone was talking to me while I was installing it. Caught it before finishing the install and replaced the connector.

You could take it back to the installer and have them fix it. Just make sure they test it before you leave. This is install 101 level stuff.

 

[rf_patriot200]

It's about 6 feet.

WHY did he shorten your coax that much ?

 

[OkieBoyKJ5JFG]

WHY did he shorten your coax that much ?

Because that's all I needed to reach under the seat.

 

[OkieBoyKJ5JFG]

So...I tested the outer shell of the coax and the center pin and definitely had continuity. I replaced the connector and must have gotten it right because there is now no continuity. That got the SWR down to 5, so that clearly wasn't the only problem. My test leads aren't long enough to reach from the connector to the NMO mount, so that will have to wait until tomorrow when I can get longer leads.

 

[K4EET]

<snip> With an actual ground connection at the NMO base, as opposed to a magnetic mount, you shouldn't have any issues tuning the antenna for low SWR.

While the SWR is quite high and quite possibly an install error causing that, has @OkieBoyKJ5JFG actually tried re-tuning the antenna to see if the SWR changes and hopefully comes down? If the SWR stays the same at all antenna adjustments, then it is probably an install issue. Also, what SWR meter is being used; the one built in to the radio or an external meter? An external meter would be preferred. Finally, make the SWR reading while sitting inside the vehicle, all doors shut tight, and preferably with the vehicle in an open parking lot with no metal objects within 50 feet of the vehicle.

 

[OkieBoyKJ5JFG]

Okay...get ready to laugh...

After I changed the coax connector and tested the SWR...I forgot to put the antenna on. I replaced the antenna and tested again and got SWRs mostly in the 1.5 +/- range all the way up and down the 70c and 2m bands. As I got to the upper end of 70cm and the lower end of 2m, it went up but still was acceptable, which I expected for a dual-band antenna. I'm going to call that good enough.

 

[rf_patriot200]

Okay...get ready to laugh...

After I changed the coax connector and tested the SWR...I forgot to put the antenna on. I replaced the antenna and tested again and got SWRs mostly in the 1.5 +/- range all the way up and down the 70c and 2m bands. As I got to the upper end of 70cm and the lower end of 2m, it went up but still was acceptable, which I expected for a dual-band antenna. I'm going to call that good enough.

Oops ...

 

[K4EET]

Okay...get ready to laugh... <snip>

😂😂😂

 

[mmckenna]

Okay...get ready to laugh...

After I changed the coax connector and tested the SWR...I forgot to put the antenna on. I replaced the antenna and tested again and got SWRs mostly in the 1.5 +/- range all the way up and down the 70c and 2m bands. As I got to the upper end of 70cm and the lower end of 2m, it went up but still was acceptable, which I expected for a dual-band antenna. I'm going to call that good enough.

Yeah, that happens.

So now you learned something about the install guy. And you learned how to do some basic trouble shooting. And you learned that sometimes we forget to reinstall the antenna. And you learned to laugh at yourself when it happens.

Glad to hear it's working. 1.5:1 probably isn't bad for that antenna. Switch to a standard 1/4 wave VHF whip and you'll see it drop down more. Or, upgrade to a dual band whip from Larsen and you'll see it improve.

 

[mrweather]

In a mobile environment, I'd accept that 1.5:1 SWR and call it a day.

 

[OkieBoyKJ5JFG]

Yeah, that happens.

So now you learned something about the install guy. And you learned how to do some basic trouble shooting. And you learned that sometimes we forget to reinstall the antenna. And you learned to laugh at yourself when it happens.

Glad to hear it's working. 1.5:1 probably isn't bad for that antenna. Switch to a standard 1/4 wave VHF whip and you'll see it drop down more. Or, upgrade to a dual band whip from Larsen and you'll see it improve.

I did indeed learn some things which are good to know. Thanks to all you Elmers who replied!