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

[WRTC928]

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?

[SteveShannon]

2 minutes ago, WRTC928 said:

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?

Look for a damaged coax. 

[WSEZ864]

Sounds like the new mount is defective, maybe a bad connection at one end or the other. Can you check it with an ohm meter and ensure it is not shorted (check resistance between the center conductor and shield) or open (check continuity from end to end on the center conductor and then on the shield).

You might also inspect the NMO mount and make sure it is clean and making good contact.

[WRTC928]

9 minutes ago, SteveShannon said:

Look for a damaged coax. 

That was my first thought. I really don't want to remove the headliner, but I don't know any other way to find the problem.

[SteveShannon]

9 minutes ago, WRTC928 said:

That was my first thought. I really don't want to remove the headliner, but I don't know any other way to find the problem.

Test at the connections. You don’t need to remove the headliner if everything rings out well. Look for no continuity between the center and the shield without the antenna. It should be the same on both ends. 
If you put a dummy load on the end you should see 50 ohms between center and shield. 

[WRYZ926]

14 minutes ago, SteveShannon said:

Test at the connections. You don’t need to remove the headliner if everything rings out well. Look for no continuity between the center and the shield without the antenna. It should be the same on both ends. 
If you put a dummy load on the end you should see 50 ohms between center and shield. 

This would be the fastest and easiest way to test both the coax and the mount itself.  If you get any resistance between  the center conductor and shield or do not see 50 ohms when a dummy load is connected then you can start to look at the coax and the mount itself.

I would check the dummy load with a multimeter to see what it reads before attaching it to the mount so that you have a good baseline measurement.

[nokones]

Bad connector installation on the coax. You are most likely shortening/grounding out the cable with a little itty bitty braided wire strand with the connector.

[SteveShannon]

2 hours ago, nokones said:

Bad connector installation on the coax. You are most likely shortening/grounding out the cable with a little itty bitty braided wire strand with the connector.

Yup, it is surprisingly common. It only takes one. 

[WRTC928]

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.

[WRTC928]

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.

[SteveShannon]

39 minutes ago, WRTC928 said:

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.

Good job!  Trust me when I say any one of us could do the same thing. 

[nokones]

I make it a constant habit to continue checking for shorts along each step of the connector installation before crimping the connector than after crimping I check again. It saves a lot of money and time. If I don't see "0L" on my volt meter than I look for that lowly wire strand.

[SteveShannon]

3 hours ago, nokones said:

I make it a constant habit to continue checking for shorts along each step of the connector installation before crimping the connector than after crimping I check again. It saves a lot of money and time. If I don't see "0L" on my volt meter than I look for that lowly wire strand.

I had one on a four foot RG-8x jumper I built just the other day.  I didn’t do the repetitive checking you correctly recommend.  I knew it was almost certainly just a single strand, and I was even tempted to connect the shield and center to the two posts on a battery to see if I could just zap it like a bug zapper. Instead I cut off the end and replaced it, being more careful that time. I had checked the other end before crimping so I was certain which end it was in.