Larson NMO-27 Question

K6GBW · Nov 25, 2017

I have two NMO mounts in the roof of my 2016 Chevy Colorado. One directly in the center of the roof and one about six inches forward of the rear edge on the drivers side. Originally I wanted to run a Larson NMO-27 in the center of the roof for a CB and a Diamond dual band antenna on the left rear for my ham radio.

When I mounted the Larson I noticed, that as the antenna swayed, the roof seemed to flex quite a bit. I don't know if these newer cars have thinner roofs or what, but this seemed like it would eventually damage the roof. Has anyone run this or a similar antenna on a thin roof roofed vehicle? If so, has there been any problems with it? I see the CHP using a similar antenna on ALL of their vehicles and as far as I know they have no problems. Am I just being overly paranoid?

I'm now thinking about putting the CB antenna on the headache rack. I know this isn't ideal but I don't want to damage the roof.

Any comments would be appreciated.

FiveFilter · Nov 25, 2017

This is an interesting question. It is always pointed-out that a hole through the roof is the best way to mount an antenna. However, if the metal is too thin, perhaps a magnet mount would treat the roof better?

In my long-ago youth I was driving a 1956 Chevy when a friend in the passenger seat loaded a semi-automatic 12 gauge shotgun to get ready to jump out and shoot some doves in the field up ahead. When the round slammed home, the gun went off right behind my head and into the Chevy's roof. The metal in that roof was thick enough to not be breached by the shot even from a foot away, and only a big bulge resulted from the shot. I wonder how today's cars would fare in such a test.

twjr80 · Nov 25, 2017

I know someone who had the same issue and what they did was mount a metal plate on the underside to stiffen up the mount area.

There are discussions about using a metal plate on here and places such as eham.

mmckenna · Nov 25, 2017

KE6BAY said:
I have two NMO mounts in the roof of my 2016 Chevy Colorado. One directly in the center of the roof and one about six inches forward of the rear edge on the drivers side. Originally I wanted to run a Larson NMO-27 in the center of the roof for a CB and a Diamond dual band antenna on the left rear for my ham radio.

While less than ideal, you could put the taller antenna towards the rear. It won't have all the ground plane it desires, but it will work. In reality on a truck that size, the CB antenna will never have all the ground plane it needs.
You may find it doesn't flex so much at the rear of the cab.
All roof tops are going to flex a bit, and it's OK. But having a really tall antenna like that in the middle with no bracing under it might be a problem.

As for the CHP whips, you might find that there are some cross members or stiffening braces under the roof near that antenna.

KE6BAY said:
I'm now thinking about putting the CB antenna on the headache rack. I know this isn't ideal but I don't want to damage the roof.

Any comments would be appreciated.

It'll "work", but you will be reducing the ground plane even farther. I'd try the back of the cab first.

I had a 2005 GMC Canyon and still have a 2007 Chevy Colorado at work. Different body than the 2016's, but both those had antennas on the roof tops without issue, but those were all VHF quarter wave or smaller antennas. Even a few times hit by low tree branches without issue. Not sure about the new body styl, though.

K6GBW · Nov 25, 2017

Ah, I love a challenge! I'm going to try the rear NMO first and see if that matches up. I put the antenna on there earlier today and it still flexes a little bit, but not nearly as much as the one in the center. If it still ends up being a problem then I'll start looking at the headache rack. I'll save that as a last resort.

Thanks for all the feedback!

mmckenna · Nov 25, 2017

I rarely use a CB, only on long road trips. When I do, I put the Larsen NMO-27 that I've had for going on 30 years now, on.

I've got a crew cab F150. The prime spot, center of the cab, is for the VHF radio. I've got an extra NMO mount near the back of the cab (centered in the right/left plane) that I used for an 800MHz radio for year. I use that mount for the CB antenna when needed.

Even at the rear of the cab, about 6" forward from the back edge, the antenna still tunes up just fine. 1.1:1 on channel 19, and 1.3:1 on 1 and 40. I tuned it using my work analyzer, and it's got a nice smooth curve across the band.
While I rarely use it, it does work well. Again, not the "ideal" location, but for my intended usage, it works just fine.

K6GBW · Dec 4, 2017

Well I tried to use the NMO-27 but the roof of these new Chevy's is just way too thin. Even sitting in the driveway you could see the sheet metal warping just from the afternoon breeze! After I gave up on that I tried mounting it on my backrack on a standoff. That also failed miserably. The SWR was just way way out of whack and no amount of cutting would bring it down. I'm pretty much resigned to just throwing a magmount up there and forgetting about it.

Thanks for all the ideas though. I appreciate it.

FiveFilter · Dec 7, 2017

That's what I use, magmounted antennas, Wilsons and K40s from three feet to five feet long. The relatively large magnet bases not only hold on tight, but also spread the load over several inches to make it easier on the paper-thin metals used on vehicles today.

However, there is a benefit to these thin vehicle shells: the reduced weight which, combined with sleek aerodynamic shapes, are great for fuel mileage. My '15 Mazda3 is averaging 44 mpg on highway trips. That makes me smile.

K6GBW · Dec 12, 2017

Finally a fix

So I keep looking for answers for my antenna problem. I really wanted to run this base loaded antenna on the roof but the flex was making me very concerned. Fortunately, I found a solution using a seriously over-built NMO mount made by Breedlove mounts (www.breedlovemounts.com/home.html).

I removed the old mount and installed the Breedlove and it was much more stable. I had to re-configure the coax for an old style ring connectors but that was not big deal.

I really didn't like the idea of a mag-mount and this seemed much "cleaner" than having cables visible on the roof of the truck. I found its much more sensitive as well. I'm hearing guys at the Port of Los Angeles over ten miles away loud and clear.

I still haven't matched it up yet. I'll do that later tonight when I have more time. I'm pretty sure it's going to match up well though.

K6GBW · Dec 12, 2017

SWR Update

So it only took about fifteen minutes to match the antenna up. I ended up taking 2 1/4" off the whip to get it down to 1.1 on channel 19 and 1.3 on channel 1 and 1.4 on channel 40.

Loving this antenna! Looks nice and professional and works really well.

mmckenna · Dec 12, 2017

KE6BAY said:
Loving this antenna! Looks nice and professional and works really well.

I've been trying to tell people that for almost 30 years. But, people love their truck stop mag mounts with secret-golden-screwdriver-CB-sauce that makes it "swing and talk farther than all their friends".

K6GBW · Dec 12, 2017

I hear what you're saying. Too much hype out there. I've got this thing hooked up to an absolutely stock Uniden Pro-505XL. It actually works really pretty well. I live about 6 miles east of Downtown Los Angeles. When I did a radio check from the driveway I got a guy down at the port of Los Angeles! Better than I expected.

I appreciate all the comments. They helped me in making my decision on this.

mmckenna · Dec 12, 2017

Glad I could help.

Like I said above, the NMO-27 I have is pushing 30 years old. It's been on a few trucks, even a friend had it for a while. It's been beat hard off road, been on numerous long road trips, and it's always performed well.

The antenna "hype" I read on here sometimes is discouraging. People actually believe that some companies have figured out how to cheat the laws of physics. The more outlandish the claims, the more some will buy into them.

I don't use it very often now, usually just long road trips when I'm on my own. I've been all over the western US with it, and it's worked well. Even with bone-stock CB's.

Pretty sure you'll be using that antenna for decades to come.

prcguy · Dec 12, 2017

I wonder if that was me you got a radio check from, I was playing on the band a few days ago in that area.
prcguy

KE6BAY said:
I hear what you're saying. Too much hype out there. I've got this thing hooked up to an absolutely stock Uniden Pro-505XL. It actually works really pretty well. I live about 6 miles east of Downtown Los Angeles. When I did a radio check from the driveway I got a guy down at the port of Los Angeles! Better than I expected.

I appreciate all the comments. They helped me in making my decision on this.

K6GBW · Dec 12, 2017

Possible

It might have been. A couple of days ago I was doing radio checks on this same radio but with a Wilson Lil' Wil magnetic mount. I just got the Breedlove mount yesterday and installed it this morning. The radio is a lot quieter than it used to be, which is nice...but also not so nice since there are fewer people to talk to. At least the people I've got on the air over the last few days have all been really nice. Only heard a guy screaming AUDIOOO, once in the last few days!

FiveFilter · Dec 13, 2017

Yep, that big base should spread the load like a mag mount.

That'll keep from flexing the thin skin too much.

Congratulations!

prcguy · Dec 13, 2017

Pick a day, time and channel and we can try to test your antenna over the air again.
prcguy

KE6BAY said:
It might have been. A couple of days ago I was doing radio checks on this same radio but with a Wilson Lil' Wil magnetic mount. I just got the Breedlove mount yesterday and installed it this morning. The radio is a lot quieter than it used to be, which is nice...but also not so nice since there are fewer people to talk to. At least the people I've got on the air over the last few days have all been really nice. Only heard a guy screaming AUDIOOO, once in the last few days!

K6GBW · Dec 14, 2017

Cool

Outstanding! I've got one hell of a work week coming. But I should be on on Monday around 10:00am. If you're around I'll call out and see if I can make the trip!

mmckenna · Dec 14, 2017

Thought I posted this hear, but I guess I didn't. Here's a Larsen NMO-27 on my analyzer. 1.31:1 SWR on channel 1, 1.07:1 SWR on channel 19 and 1.35:1 on channel 40. Nice smooth curve like you'd expect:

Here's a wider view, from 26MHz to 28MHz:

K6GBW · Dec 15, 2017

Smilar

That looks very similar to my readings. I don't have a cool graph display like that, just an old analog meter, but that antenna is one of the easiest and best matching I've every done. I had Firestik's on my jeep before getting the pickup. I got acceptable matches but nothing as good as I'm getting now.

I'm definitely a Larson convert!

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