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Author Topic: Properly assembly for a Larsen 270NMO antenna  (Read 5857 times)
KC3AYG
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Posts: 27




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« on: January 24, 2017, 11:32:50 AM »

Hi all,

I just did my first install of a mobile VHF/UHF rig. The car is a Honda Fit; the rig is a BTech UV-2501 with a Larsen NMO and a Larsen 270 half-wave whip antenna.

The roof does not appear to leak, nor has the car caught on fire. This is the good news.

The bad news is that receive is horrible; while, oddly, transmit seems OK. I can get in to repeaters but can't clearly copy the response. The BTech also has a commercial broadcast FM receive capability, and this is largely uncopyable as well, even while I can tune into the same station with the car's built-in FM radio and get perfectly clear audio.

Then the whip part of the antenna fell off while I was driving through a parking garage, when the whip hit an exposed beam. After the whip came out, I noticed that there was *no change* to the receive quality, which made me think that I had perhaps improperly assembled the antenna, as it came from the vendor in separate parts - specifically, I thought perhaps I had inadvertently failed to insert the whip far enough and thus failed to make a solid contact between the whip and the base.

The antenna has three parts: the whip, which is just a length of wire with a twist in the middle to make it dual band; a base, which screws into the NMO fitting; and a small conical part the whip goes into and that then screws into the base. The conical fitting has two set screws that hold the whip in place. I had assumed that there would be some sort of female receptacle in there that ensured electrical conductivity from the base to the whip. But there is no such thing, so I'm left wondering how exactly the whip is electrically connected to the base - through the set screws? But that doesn't seem right.

There are zero instructions for how to assemble the antenna that I can find, leading me to believe that it can't be that difficult, but somehow I've still managed to screw it up. Does anyone have any insight into this problem?
« Last Edit: January 24, 2017, 11:46:08 AM by KC3AYG » Logged
KD5BVX
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Posts: 112




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« Reply #1 on: January 24, 2017, 12:20:42 PM »

I'd try two things to test it -

1)  Place the antenna whip all the way down into the base and tighten both set screws.  Check rx on the radio.  If no change,

2)  Try a different radio.  It could be the rx of your radio is bad.  If you don't have another radio yourself, ask a fellow ham to bring theirs over to your vehicle for a temporary install/try.  It could even be an h/t if they or you have the adapter to connect the antenna coax.

That antenna is regarded as one of the best in many circles (it receives praise on here, QRZ forums, RadioReference.com, etc.).  Since tx is OK (per your post) that would lead me to believe it is not the coax or the mount, though, as we know in radio, anything is possible.  Try those two troubleshooting steps and let us know.  
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Mark
W9IQ
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Posts: 3244




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« Reply #2 on: January 24, 2017, 01:01:57 PM »

Carl,

It isn't all that unusual that an antenna problem will allow transmissions to reach their destination but be deaf on receive.

It is standard that the whip gets its connectivity with the base assembly via the set screws.

I would start by getting out your digital volt meter and doing resistance checks on your wiring.

1.) Disconnect the antenna from the NMO and the radio from the coax. Set the meter to measure low resistance (e.g. 200 ohms).
2.) On the radio end of the coax, measure between the center pin of the connector and the outer part of the connector and confirm that it is open (infinite resistance - same as when test leads are touching nothing).
3.) Measure from the outer part of the connector and a ground point on your vehicle (e.g. the lighter/accessory outer metal shell). This should show a short (low resistance).

You may need some additional wire to extend your meter test lead for the following steps:

4.) Measure from the center conductor of the connector at the radio end to the center pin on the NMO. This should show a short (low resistance).
5.) Measure from the center conductor of the connector at the radio end to the outer rim of the NMO connector. This should show an open (infinite resistance).
6.) Measure from the outer part of the connector at the radio end to the outer rim of the NMO connector. This should show a short (low resistance).

If any of your measurements do not agree with the above, you have a wiring, connector, or NMO mount problem.

Otherwise you can start seriously considering that there is a problem with the antenna. A common problem there is that the flexible center pin on the base of the antenna is not making connection with the NMO center pin.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
KC3AYG
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Posts: 27




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« Reply #3 on: January 24, 2017, 01:11:50 PM »

"Otherwise you can start seriously considering that there is a problem with the antenna. A common problem there is that the flexible center pin on the base of the antenna is not making connection with the NMO center pin."

I.e., the strip of spring steel inside the bottom of the base isn't connecting to the metal disk at the center of the NMO? That's possible...

Would an SWR reading help me diagnose this? I don't have an SWR meter but it's on my list of things to buy.

On a related note - I've seen comments here and there about the need to properly ground the antenna and/or the radio chassis to the body of the car. On my install, the only ground is from the ground power terminal of the radio to the negative side of the battery. Is there supposed to be more than that, and could it explain my problem?
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W9IQ
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« Reply #4 on: January 24, 2017, 01:47:13 PM »

Carl,

An SWR meter is a great accessory to have but in this case, it isn't likely to give you any additional information for troubleshooting. The resistance readings I suggested will probably yield more usable results. Once you have the antenna basically working, the SWR meter may help you to fine tune the length of the antenna.

The antenna gets grounded via the outer shell of the NMO connector. The outer braid of the coax should also be connected to the outside of the NMO when the NMO is installed.

On the radio side, the outer braid of the coax connects to the outer conductor of the connector.

Other than your power ground, no other grounds should be needed.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
KC3AYG
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Posts: 27




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« Reply #5 on: January 24, 2017, 01:53:42 PM »

OK, thanks. This particular NMO came with the coax on that side already installed; all I needed to do was affix a PL-259 to the radio side. So I'm going to assume that my grounding is good and that the NMO-side coax is good. I'll do the continuity checks and report back.
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W9IQ
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Posts: 3244




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« Reply #6 on: January 24, 2017, 01:58:12 PM »

Carl,

That sounds good. If you have never successfully installed a PL-259 connector before, I would bias my thinking to look for the problem there. But the resistance checks will help zero in on the underlying cause so proceed with those.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
KD5BVX
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Posts: 112




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« Reply #7 on: January 24, 2017, 02:49:59 PM »

W9IQ has offered more detailed help than I did, but I think I'd still consider that the radio has a problem as I mentioned in my first reply...unless you've tested it before and know it has great rx in another environment/antenna setup.  It would be simple to connect another radio to the antenna and see how it performs...
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Mark
KC3AYG
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Posts: 27




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« Reply #8 on: January 24, 2017, 04:55:36 PM »

I do have another radio, an ancient Icom IC-27H. But it works fine, and it's an easy check. Good suggestion.

The more I think about it, the more convinced I am that I didn't do a good enough job soldering through those holes in the PL-259...
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KT4NR
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Posts: 571




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« Reply #9 on: January 25, 2017, 09:43:29 AM »

Whats the vehicle body made of in the area you mounted the NMO?
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KC3AYG
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Posts: 27




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« Reply #10 on: January 25, 2017, 10:29:24 AM »

I mounted it in the center of the roof. It appeared to be sheet steel. I cut the hole with a hole saw, and made no special effort to remove paint in the area surrounding the hole. I guess it's possible that the NMO isn't properly grounded to the body, but if so I don't know how to go about fixing that...
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W9IQ
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Posts: 3244




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« Reply #11 on: January 25, 2017, 10:34:04 AM »

There is no need to speculate about the grounding condition of the NMO. Take the resistance measurements that I previously described and you will know.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
KA3NXN
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« Reply #12 on: January 25, 2017, 01:22:54 PM »

I can't believe that no one has suggested putting a SWR meter or better yet an antenna analyzer on your installation. This will tell you if the problem is radio or antenna related.  Before I connect any radio to a newly installed antenna I break out the test equipment to make sure I didn't screw anything up because when the radio blows its finals that will tell you something is wrong with your antenna or it's installation

Jaime-KA3NXN
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KC3AYG
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Posts: 27




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« Reply #13 on: January 25, 2017, 01:37:57 PM »

"Take the resistance measurements that I previously described and you will know."

Yeah. Gotta wait until Saturday, though. Puttering around out of doors with a multimeter while the wife is making dinner is frowned upon.

I will post results when I get 'em.

"I can't believe that no one has suggested putting a SWR meter or better yet an antenna analyzer on your installation. "

As a generally Good Thing To Have, I'm in the market. Can you recommend one?


Glen
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KC9CFM
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« Reply #14 on: January 26, 2017, 10:39:15 AM »

I'm surprised you actually drilled a hole in your roof, I thought I was the only one that ever does that  Grin  But that is real touchy, even making sure you don't under or over torque the mount.  I never bothered to scrape any additional paint off for the ground, and it always worked fine.  I'm sure you either did not have the set screws tight enough, or did not insert the whip far enough into the base and that caused it to separate.  

But, soldering connectors sucks.  Very well could be your problem.  I got so tired of trying to find the good silver plated connectors and making sure they were not nickel, then trying to get the connector and braid to both get hot enough for the solder to stick, but not so hot it melted the dielectric.  Blech.  Then the heavens opened, and down came a Paladin CrimpAll 8000, and some die inserts for various sized coax.  Love it, works the first time, every time.  
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