Larsen nmo 150

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Redmaninnc

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I have been using a tram 1154 magnet mount antenna for about a year now. After much reading I purchased and installed a Larsen nmo150bk. Drilled the hole with much anxiety but it went will. Check coax with meter to make sure everything was ok. Also check to make sure mount got a good connection to the roof metal. The antenna is mounted in the dead center of the roof just as the mag mount was. After a little testing it doesn't appear to be working as well as the mag mount which is surprising considering the rave reviews the antenna gets as well as the fact that I went with the nmo mount. Anyone have any thoughts?

Thx
Randall
 

AK9R

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What do you mean by "Check coax with meter"? Is that an SWR meter? Does the antenna have low SWR at your operating frequencies? Did you cut the antenna whip to length per the included antenna cutting chart?
 

N0ZQR

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Did it come with connector attached or did you put it on?
What type of connector?
 

krokus

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What do you mean by "Check coax with meter"? Is that an SWR meter? Does the antenna have low SWR at your operating frequencies? Did you cut the antenna whip to length per the included antenna cutting chart?

Tagging in, to see the answer to this one.

Sent via Tapatalk
 

Golay

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Good chance this isn't the issue, but make sure the "tab" on the bottom of the antenna coil is actually touching the center of the NMO mount. I've had that happen where I've had to pull the tab down a bit and clean it up with some Skotch-Brite to make a good connection.
 

Redmaninnc

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I checked the coax with an ohm meter to make sure the shield and center conductor had no continuity. I put an sma connector on the end and rechecked with ohm meter. I don't have or have acces to an swr meter for the 2 meter band. I have trimmed the whip per the chart included in the package. I have check the tab on the bottom of the coil and it is making contact.
 

lmrtek

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If you aren't getting a good connection to the sheet metal, any antenna that requires a ground plane will have poor performance
.
Iv'e had to repair installs where the installer used a cheap nmo mount that didn't bite in or ones where they used flat washers on 3\8 nmo mounts that acted the same way.
.
 

mmckenna

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I checked the coax with an ohm meter to make sure the shield and center conductor had no continuity. I put an sma connector on the end and rechecked with ohm meter. I don't have or have acces to an swr meter for the 2 meter band. I have trimmed the whip per the chart included in the package. I have check the tab on the bottom of the coil and it is making contact.

5/8th's wave antennas like the NMO-150 are a DC grounded antenna. That means that there is a direct connection from the whip/center pin to ground through the coil.

That means if the antenna was connected when you did this test, you should have seen a direct short between the center pin of the coax and the outer shield. If you didn't see that, it means there is an issue.

If the NMO mount isn't installed properly, or the mounting surface is too think, the center of the mount may sit too low to make contact with the antenna. As was stated, making sure the center tab is making contact with the center button on the NMO mount is key.

Checking the coaxial connection would be important, also.
 

n0nhp

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Was the antenna trimmed to the proper length or did you use the rod as it came out of the package?
A check with a watt meter is the only answer to your problems. As stated above, you don't just plunk an antenna in a hole and call it good. That is why professional installers get paid the big bucks (I wish).


Bruce
 

Redmaninnc

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The antenna was not attached when I checked the coax. I did trim the antenna per the chart that came with the antenna. I don't have acces to any type of 2m meter. Sorry I know that would help. I see if I can find one.
 

mmckenna

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OK. The chart will get you close, following up with an SWR meter at minimum is a real good idea.

One way you can test the install is to screw the antenna on all the way and then check between the center pin and outer shield of the coaxial cable connector. You should see a direct short through the coil. That will tell you quickly if the antenna base is making proper contact.

The other variable is where it's mounted. If you can, describe the type of vehicle and exactly where the antenna is mounted on it.
 

Redmaninnc

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Did a continuity check between shield and center conductor with antenna installed. It showed a short so that's good. It's on a 2000 chevy Silverado extended cab and the antenna is mounted dead center of the roof. The nmo mount and coax are Larsen so expect both to be quality.

This is a question I think I already know the answer to but a friend is telling me adding a ground plane kit will help. Any merit to his suggestion?


BTW thanks for everyone's help. I really appreciate it.
 

KevinC

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What type of radio is this connected to? Maybe it's being overloaded now with the better antenna.
 

AK9R

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This is a question I think I already know the answer to but a friend is telling me adding a ground plane kit will help. Any merit to his suggestion?
If the antenna is installed on a metal car/truck roof, then, no, you do not need a ground plane kit.
 

mmckenna

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This is a question I think I already know the answer to but a friend is telling me adding a ground plane kit will help. Any merit to his suggestion?

No merit to it at all. The roof of your truck cab is providing a near perfect ground plane for VHF. You can't improve on that. They don't make mobile "ground plane kits" for these antennas anyway.
Sounds like your buddy is a CB'er….
 

prcguy

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In my experience there is very little difference between the various 5/8 VHF antennas of the same length. Larsen is one of the best but I've tested Maxrad, Tram and Antenna Specialists and without test equipment like a spectrum analyzer at the receiving end you can't tell the difference in performance.

Going from most mag mounts to a permanent hole mount in the same location is also a very small change and usually goes unnoticed. If the OP is seeing a drop in performance going from his Tram mag mount to a permanent Larson of similar size then something is seriously wrong. He needs to find someone with equipment to measure the SWR and properly troubleshoot the problem, otherwise we'll see this thread go on for months.
prcguy
 

majoco

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After a little testing it doesn't appear to be working as well as the mag mount

Perhaps you could repeat the test - remove the whip of the new antenna, place the mag mount close to the centre and try again - hopefully within a short space of time with the vehicle in the same place with known distant stations from different directions. Note down the reception with either antennas. For all you know you may have a totally different radiation pattern that now favours reception from another direction.

One thing bothers me - and it's not the first instance. My first bit of test gear before I even bought any transmitter was a multimeter closely followed by a VSWR meter and both got very well used, but so many new hams and other users just trust that it will work without testing.

The first thing to consider when making any test is "What am I looking at - is that the right answer".
 
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Redmaninnc

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Just to follow up on the problem, so many times people start at thread and never report back your findings after they fixed it. I know ideally you would check one thing at a time to isolate the problem but in the interest of getting the problem resolved this is what I did. Took the antenna off and checked tab. Removed nmo mount, checked everything and reinstalled. Cut sma connector off and reinstalled a new one. Bad news is im not sure which of these was the problem. Good news is it seems to be resolved. Antenna is performing equal to if not slightly better than the mag mount. Seems like the continuity checks I did both with and without antenna installed would have identified a problem with any of the items I mentioned. Who knows!

I want to thank everyone who took the time to help.
Randall
 
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