NMO mount antennas

[JaLion]

I recently switched to a NMO mag mount antenna on the roof of my PU truck. Installed a Larsen NMO CB27S with the 49” whip. This antenna is getting terrible SWR on 11 meters with a 49” rod length. Not like the Wilson 1000 I replaced with it. I was getting 1:0 SWR across the band with the Wilson on top of the roof. I’m thinking the Larsen needs some tuning for 11 meters cb work. Does anyone know where I can find a cutting chart for the rod length of the Larsen NMO 27CB?

 

[n9mxq]

A quick search found this

 

[mrweather]

And if you can get your hands on an antenna analyzer (as opposed to an SWR meter) that'll make a life a lot easier for trimming the whip.

 

[900mhz]

And if you can get your hands on an antenna analyzer (as opposed to an SWR meter) that'll make a life a lot easier for trimming the whip.

agreed. The antenna should be tuned in its final installation location. A cutting chart is just a generalization or rough measurement. Depending on the installation, slight nuances in the antenna environment could change the tuning. Particularly on lower frequencies. I have seen this many, many times tuning low band antennas, and 27 MHz is no exception. Make measurements and double check before trimming the whip, for obvious reasons.

 

[JaLion]

A quick search found this

Thanks n9mxq. This should get me into the ballpark with the rod length so I can fine tune it.

 

[mmckenna]

These absolutely need to be tuned after installation. 49" is the stock whip length they sell. The cut chart will get you close, but as others said, it will vary depending on the specific installation

But, once you get them tuned correctly, they work well. I've used the same Larsen NMO-27 for about 30 years now. On top of a truck, they will work very well. Here's the antenna analyzer sweep from mine:

1.35:1 or less on the two edges, 1.07:1 at channel 19.

 

[JaLion]

Thanks. Unfortunately I don’t have an antenna tuner. That’s going to be next on my list. Any suggestions for a good but inexpensive tuner?

Thanks mmckenna. It looks like the Larsen has low loss across the band. Maybe not as low as the Wilson 1000, but I’ll take it for the versatility of the NMO. I plan to go either 10 meters or maybe 144 MHZ after I get my licenses. Plus I think the Larsen just looks better on the roof.

 

[mmckenna]

Thanks. Unfortunately I don’t have an antenna tuner. That’s going to be next on my list. Any suggestions for a good but inexpensive tuner?

I think your terminology might be a bit different that what we are using.

An antenna tuner will help match the antenna to the transmitter. It essentially hides any SWR issues from the radio. Not necessary for what you are doing, and really not what you want to do.

An antenna analyzer will look at the antenna performance by generating a signal and looking at what gets reflected back. A good antenna analyzer will give you a display like what I posted above.
A pretty good little inexpensive unit is the nanoVNA. You can find those on Amazon for less than $100. While not on par with the professional grade stuff, it does work surprisingly well considering the much lower price. With something like a nanoVNA, you can take a look at all your antennas and get a pretty good idea where they are resonate and if you have them tuned well.

I have one at home that I use when I forget, or don't want, to bring the work unit home. Nice little unit for the price and great for spot checking things.

Thanks mmckenna. It looks like the Larsen has low loss across the band. Maybe not as low as the Wilson 1000, but I’ll take it for the versatility of the NMO. I plan to go either 10 meters or maybe 144 MHZ after I get my licenses. Plus I think the Larsen just looks better on the roof.

I think some of the larger Wilson and other brand antennas might give you a bit more usable bandwidth, which can be good if you are operating outside the legal limits of the CB band. But for regular CB use, the minimal performance impact between a 1.35:1 SWR and a 1.07:1 SWR will not be noticed.

The big benefit to the Larsen antennas is that they've been around for a long time. They are a very popular brand in the commercial/public safety radio sector. They make great antennas that will outlast your vehicle (and probably several after that). Like I said, I've been using the same one for 30 years, and probably installed a hundred of them over my career. Easy to get replacement parts if something gets damaged, and they really stand behind their stuff. I've talked to them at trade shows and they know what they are doing.

And as you pointed out, the versatility of the NMO mount is worth it alone if you ever plan on expanding beyond CB.

They do look better. They look even better with a permanent mount. If I'm going to do a permanent install on my own truck, I always go with the professional antennas.

 

[mmckenna]

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www.amazon.com

There are some good tutorials on the Youtubes on how to use them. Unfortunately there are some bad tutorials on how to use them. If you watch a video and it doesn't make sense, find another one. Some people are better at explaining complex test equipment than others.

You'll also want some adapters to fit the UHF/PL259 connector on the antenna:
Something like this would be a good start as it takes the strain off the little SMA connectors on the nanoVNA.

https://www.amazon.com/Superbat-Coaxial-6inches-Applications-Handheld/dp/B086JK4KM7/ref=sr_1_17?dchild=1&keywords=sma+male+to+PL259&qid=1627500350&sr=8-17

 

[JaLion]

Thanks for the info mmckenna. … Looks like I’ve got some homework to do. … Already dropped the nanoVNA into the shopping cart. I figure at that low price I don’t have to worry about sneaking it past my wife lol 😆 … I appreciate the info.

 

[tvengr]

How high is the SWR? If antenna has a set screw holding the rod, loosen the set screw, pull the rod out some, and retighten the screw. Does the SWR go up or down? Are you using the same mount with the Wilson? Extremely high SWR is usually caused by a cable or connector problem. Is the center tab on the antenna pushed in and not contacting the center of the NMO connector?

 

[JaLion]

How high is the SWR? If antenna has a set screw holding the rod, loosen the set screw, pull the rod out some, and retighten the screw. Does the SWR go up or down? Are you using the same mount with the Wilson? Extremely high SWR is usually caused by a cable or connector problem. Is the center tab on the antenna pushed in and not contacting the center of the NMO connector?

It’s high. At 48 3/8” rod length SWR is 3.5, 3.7, 4.0 (ch 1, 20, 40).
I need to cut some more out of the rod length. The cutting chart should get me in the ballpark. I plan to use the analyzer for the fine tuning.

 

[mrweather]

A 49" whip is too long for the NMO27 on 11m. Even Larsen's own cutting chart says to start at 47" at 27 MHz (roughly CB channel 4).

And unless the Wilson comes in NMO I don't think the OP is using the same mount.

As was mentioned above, tuning an antenna at 27 MHz is going to be highly dependent on where it's located on the vehicle. My car has NMO mounts on the trunk and roof and I've experimented with an NMO27 at both spots. What worked on the roof didn't work on the trunk, and vice versa.

 

[mmckenna]

I figure at that low price I don’t have to worry about sneaking it past my wife lol 😆 … I appreciate the info.

Understood. I'm sure she'll agree.

They are handy little tools, though.

 

[rescue161]

I recently added a CB back to my truck and tried an NMO27 with the 47" whip and it sucked. After some research, I decided to buy a 65" whip and an NMO30 coil and was able to get the antenna tuned very well for the entire band at less than 1.3:1. Most of which being around 1.1:1.

I used a NanoVNA and the sweep on the normal NMO27 was horrible. Using the other parts, the sweep looked great. I also bought other coils to test other bands and found for 10 meters, the NMO34 and the 65" whip worked great with different length whips for the SSB and FM portions.

 

[mmckenna]

I recently added a CB back to my truck and tried an NMO27 with the 47" whip and it sucked. After some research, I decided to buy a 65" whip and an NMO30 coil and was able to get the antenna tuned very well for the entire band at less than 1.3:1. Most of which being around 1.1:1.

I used a NanoVNA and the sweep on the normal NMO27 was horrible. Using the other parts, the sweep looked great. I also bought other coils to test other bands and found for 10 meters, the NMO34 and the 65" whip worked great with different length whips for the SSB and FM portions.

Well, see my sweep above. I was showing 1.35:1 or less across the CB band with 1.07:1 at channel 19, tested on a $20K calibrated machine. I've set the NMO27 up for others and they've all tested the same. Might have to do with your ground plane.

But the NMO-34 coil and a longer whip does sound interesting. Maybe some day I'll give that a try.

 

[rescue161]

Yeah, probably to ground plane, but the NMO30 coil and 65" whip gave a much better sweep and was a lot wider bandwidth than the NMO27 with the short whip.

 

[mmckenna]

Here's the NMO-27 looking at 26 - 28MHz. I suspect you setup would look a bit better:

 

[hypersight]

Yeah, probably to ground plane, but the NMO30 coil and 65" whip gave a much better sweep and was a lot wider bandwidth than the NMO27 with the short whip.

I've been running this setup for several years now and I haven't looked back. I have mine mounted right in the middle of the roof on my car. I added a spring on mine. ( Of course I had to cut a few inches off from the whip due to adding the spring.)

 

[Project25_MASTR]

On my Jeep I believe I had to go trim down to 45" to get the NMO-27 to match correctly. That antenna has been on there now for 8 years and works for the purpose.

Being such a broad purpose antenna, you really have to tune them.