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Cutting advice for Larsen NMO

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I have a roof mounted Larsen NMO27. Unfortunately I live in the city and would have to drive for hours to get far enough out into the open to do a proper SWR test.

I lost the cutting chart that came with the antenna. I know I could find it online, but I want to add a 2 1/4” long mini spring off of a NMo800 Ham antenna so I need to know how to factor my cut. Do you think the cut chart will be reliable if I add this spring and compensate by cutting 2 1/4” additional off the whip? Also in that cut chart do they compensate for the little metal piece with the set screws in it, or do I need to minus that plus the spring out of the cut? Thanks. Radio is working OK but I’m staying off it until I get this cut down right. Will be out in the country sometime in January so I can test SWR when I’m down there.
 

mmckenna

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It would be the entire length of the whip and the spring all the way down to the top of the base coil. While the spring will change things slightly, it's CB and it's not going to make a huge difference. The cutting charts get you close. You need to use an SWR meter or an antenna analyzer to get it exact. I wouldn't get too hung up on it. Even if it's off by a bit, your radio will not explode into a mushroom cloud of burnt electronics. Considering that most CB'ers don't even know how to spell SWR, never mind actually tune their antennas, you'll probably be way ahead of the curve.

The cut charts are on line.
 

cmdrwill

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That is pretty much correct. The cutting chart will get fairly close, and I usually cut a little long, 50+ years of experience.

Most open areas would be fine to check the SWR, no need to drive to the boondocks or the mountain top. You need about 30 feet clear of any objects.

Hopefully someone close by will help the OP check the antenna with a good antenna analyzer or SWR meter..

If the OP was close by, I would be glad to help test.

Side story, we were testing a new CB antenna on my 55 Ford using a RF Field Strength meter I had built. Then we decided to go to the mountain top to do more testing. We soon found out that all the television transmitters were pegging the field strength meter as we neared the top of Mt Wilson. 1962.
 
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arlo

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My chart says NMO27 for 27.0 MHz is 47.5 inches without spring. 45 inches with Larsen spring.
 

RFI-EMI-GUY

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Always start with the antenna a bit long. Check VSWR on channels 1, 19 and 40. If it is better (lowest) on 1 than 19 or 40, it is too long. Better on 40 and you cut too short. Try to get it lowest on 19 and with 1 and 40 about same as each other.



Be sure the doors, trunk and good are all closed when measuring as they will affect the measurement. Also be sure to set the forward power reference each time you check reflected as it will wander greatly while making changes.

Sent from my SM-T350 using Tapatalk
 

spongella

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Best advice as previously stated by posters, a CB SWR meter is your best antenna cutting tool guide. With that you can determine what the SWR is on the low (Ch 1) and high (Ch 40) ends of the band. I cut for center of the band.

Some CB radios have an SWR meter built in and can be read using the S meter. If not get a meter or borrow one hi hi.

Have you checked on line for your particular antenna model's specs?

Yeah, and no need to drive out into the country just to check SWR

Have fun on CB.
 
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Thanks for the responses so far. The Larsen cutting chart says for 28.5Mhz (middle between 27 and 30)
For a roof mount NMO, 42 3/4” “without spring” and 40 7/8” “with spring”.

I’m not sure if they are referencing a specific Larsen spring, but the spring I want to use is 2” total additional length, meaning from bottom to top of the spring (not including the 1/4” hole at the bottom or the 1/4” nub at the top which would screw into the antenna base with the set screws). That said, if I go by this chart I would say I should cut the whip 1/4” long so say 41” to start? And then get it all assembled and tested with a SWR meter. Thoughts?
 

prcguy

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28.5Mhz will leave you with a shorter whip than for 27MHz. I would find the closest stated length for 27MHz then cut at least an inch longer then trim with an SWR meter or antenna analyzer.
prcguy

Thanks for the responses so far. The Larsen cutting chart says for 28.5Mhz (middle between 27 and 30)
For a roof mount NMO, 42 3/4” “without spring” and 40 7/8” “with spring”.

I’m not sure if they are referencing a specific Larsen spring, but the spring I want to use is 2” total additional length, meaning from bottom to top of the spring (not including the 1/4” hole at the bottom or the 1/4” nub at the top which would screw into the antenna base with the set screws). That said, if I go by this chart I would say I should cut the whip 1/4” long so say 41” to start? And then get it all assembled and tested with a SWR meter. Thoughts?
 
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Oh ok so for domestic US use the frequency is 27mhz? I thought domestic CB was graduations of channels 27-30mhz in 40 channel increments. Learn something new each day...

So best bet is to minus maybe 1 1/2” off the 27mhz line (“47”) so cut it to maybe 45 1/2”?
 

mmckenna

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26.965MHz is Channel 1
27.405MHz is Channel 40

Frequencies are not equally/incrementally spaced, so Channel 19 is actually the center of the band. 26.185 would be the ideal point to tune the antenna for.

I've got an NMO-27 I occasionally use on my truck. Using the antenna analyzer I have at work, I was able to nail it at 1.1:1 on channel 19 and 1.3:1 on 1 and 40. The SWR curve across the band is really nice and smooth.

I don't recall the exact length of mine, but it wasn't 47 inches. Closer to 45 or so sounds right. But trim in small increments and watch your SWR meter.

As you are trimming it, watch where the SWR is lowest. Check Channels 1, 19 and 40 and you should start to see the SWR getting better on Channel 1 as you get close. Go slow.

And close by metallic objects will throw the SWR off, even your doors open on the car. Ideally you want to test out in the open, like a empty parking lot with no nearby lamp posts. Close all the doors on your car and keep your hands/body away from the antenna as you are doing the readings.

This is doable with a basic SWR meter, but I will say it's a lot more fun when you have an antenna analyzer at your disposal. Watching the low SWR on the screen come closer and closer to your frequency makes it easy. If you were local, I'd happily assist you, but unfortunately so many of you guys are on the wrong side of the country. I will say the weather out here in California is nice in the winter, so if you felt like a long road trip…..
 

RFI-EMI-GUY

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Reminds me ot the time I was cutting some lumber. I cut a 2 x 4 six times and it was STILL TOO SHORT!
 

mmckenna

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You are correct. Typing way too fast again. Thanks for catching it.
27.185 is correct.

As for cutting it too short, yeah, once you've gone to far, you're stuck. You can purchase replacement whips for these antennas. Hopefully you won't need that.
 
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Ok, somehow I’m still confused: the whip itself out of the box and uncut is 49” from end to end including the ball at the top. When I assemble the whip to the part with set screws and then add my spring I am coming in at about 51 1/2”.

The cutting chart lists whip only on 27Mhz to be 47” if used without a spring and 46 1/4” if used with spring. I don’t know what spring they are referring to. The spring I want to use adds 2.5” to the whole assembly.

How long should the whip only be trimmed to? I’m thinking 44.5”, add the spring and the whole assembly above the base coil is 47”. Or I could cut the whip to 45”, and have a 1/2” left to fine tune with on the SWR meter?
 

mmckenna

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Ok, somehow I’m still confused: the whip itself out of the box and uncut is 49” from end to end including the ball at the top. When I assemble the whip to the part with set screws and then add my spring I am coming in at about 51 1/2”.

The cutting chart lists whip only on 27Mhz to be 47” if used without a spring and 46 1/4” if used with spring. I don’t know what spring they are referring to. The spring I want to use adds 2.5” to the whole assembly.

How long should the whip only be trimmed to? I’m thinking 44.5”, add the spring and the whole assembly above the base coil is 47”. Or I could cut the whip to 45”, and have a 1/2” left to fine tune with on the SWR meter?

OK, not a direct answer to your question, but I'm at home and pulled the antenna out of the back of my truck and measured it.
I don't use the spring….
46.5 inches from the top of the whip to the base of the cone. That got me 1.1:1 on 19 and 1.3:1 on Channels 1 and 40.
Of course each install will vary a bit, but that length was right for my particular install.



Cutting to 45 inches and adding the spring would be a good place to start. Since each install is different, it really does come down to your own individual testing with an SWR meter. Really does sound like you are on the right track.
 
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arlo

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A Larsen brand spring measures 2 & 5/16 inches.
 

RFI-EMI-GUY

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A Larsen brand spring measures 2 & 5/16 inches.


I would suspect the spring has some additional series inductance and stray capacitance which affect the overall rod length somewhat. If this were less an art, and more a science, Larsen would simply ship a "CB" rod cut to channel 19 and there would be no market for VSWR meters.

Cutting tip:

1) Use a Dremel tool with cutoff wheel to scribe around the circumference of the whip where you want to cut.

2) Use a pair of vice grips or electricians pliers to "snap" the rod on the scribe line.

Make cutting easy so you can make many small iterations without frustration.
 

mmckenna

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While not useful for what you are doing, here's the trace from my NMO-27. It'll show you what the SWR would look like if you plotted it out using the readings from your meter.

Your specific install will be different, even if I flew out there and hooked up my analyzer, but you get the idea

5ik0x17.jpg


Marker 1 shows ~about~ channel 1
Marker 2 shows channel 19
Marker 3 shows channel 40
On the bottom pane:
X value = frequency
Y value = SWR
 

mmckenna

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So, here's another view of the same antenna. This time it's stretch out to show 26MHz to 28MHz. The 3 markers are in the same place, channels 1, 19 and 40.

rwzmbSX.jpg
 
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