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[N1AUP]

I installed two NMO mounts with Pl259s, RG58 off of a 500 foot reel, and a Larsen Ford F150 taillight mount.  One works, one does not.  About 18 feet of coax between the mount and the radio.  No coils, no kinks.

I disconnected the NMO antenna on the bad mount, and checked for any shorts between the center pin of the PL and the ground.  None found.  Infinite resistance.

I shorted the center on the PL to the outer barrel, and checked resistance between the center pin on the NMO and the ground.  0 ohms.

Yet, I moved the functioning NMO Mount 2 M / 440 antenna from the good mount to the bad one, and found 440 SWR to be great (2:1 or less across the 5 mhz 445 - 450 range) while 2 meters sits around 4:1.  (I used an Antenna analyzer).   The 2 meter swr on the good mount is 1.5 to 2 across the 4 mhz range.  On 10 Meter FM (which is what I want to run on this mount), I have 10:1 SWR.

Aside from ripping the entire thing out, and replacing with new (which is a major league PITA), is there a way to figure out where the problem lies?  I'm leaning toward NMO mount as I had to connect the cable to the mount, as opposed to having it come installed from the factory.

Any thoughts?

Thanks

[K0UA]

There is one more check you didn't make. Lets run thru this. You checked on the PL259 with NO antenna mounted on the NMO mount from center pin to the connector body and found and open. That's all well and good, and then you put a clip lead between the center pin and shield body on the PL259 and then went to the other end of the vehicle and measured a short between the center pin and the shield body of the of the NMO mount.  All fine and the dandy. BUT your did not check the actual resistance of the NMO mount to the vehicles chassis ground. and you did not check between the PL259 shield body and the vehicles actual chassis ground.  It is possible that your NMO is not properly grounded to the vehicles body. Quite likely a high resistance joint between the NMO and the actual vehicle.

[K5LXP]

It seems you don't have a mount problem, but maybe a chassis grounding problem.  If continuity is good at DC but SWR is bad then I'd be looking at how the mount is connected to the chassis/groundplane.

Mark K5LXP
Albuquerque , NM