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Cobra 19 CB and Larsen NMO-27B Trouble

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clanusb

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Hello everyone! Thank you for taking your time to read and help me with my antenna issue. I've read on here that the Larsen 27 is the best antenna for CB, and since all my other antenna's are Larsen as well, I was already sold.

The setup:
2005-2007 Super Duty Crew Cab Short Bed body style with a sunroof, which pretty much eliminates the roof for antenna installation. I have a headache rack for this purpose. (Yeah...I know, not the best, but it works, and I see plenty of these set ups that don't have any issues.)
I have a NMO Larsen 2/70B mounted on my headache rack via a welded L bracket, and it works flawlessly with my Yaesu 8800r.

The left front fender has a no drill (uses a preexisting bolt) NMO Z bracket on it for the Larsen 27B. It was purchased from theantennafarm. It has a star washer underneath it, and has great electrical ground. The NMO27 has a coil, no spring, and was cut per the Larsen chart to 47.5".
The coax is RG-58, continuity tests good in all aspects and is loosely coiled under the console.
I have tried everything I can think of, but still consistently get a SWR of 3.5+.

I swapped it to the headache rack mount with the same results, placed it on a Larsen magmount with the same results. I have tried a different radio with the same SWR. I have tried a different antenna (Firestik II) with the same SWR.
I am out of ideas. Besides trying a ground strap from the bolt on the mount to another good ground on the frame.

Thank you for your help.
 

mrweather

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A good electrical ground doesn't necessarily mean a good RF ground. And it's even more critical on the lower frequencies bands.

Having said that what are you using to test the SWR? The fact you've tried different antennas and different radios and obtained the same almost suggests there's something wrong with your meter.

If you can try a different SWR meter that would help rule that out. An antenna analyzer would be infinitely more useful as it would give you an idea of how the antenna behaves across a range of frequencies.
 

clanusb

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I will have to double check on the brands of the swr meter. I did use two different ones. Both gave me the same results of 3.5+.

I understand the difference with electrical and rf ground, which lead me to think of maybe needing a ground strap.
 

mmckenna

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OK…

I'm running a Larsen NMO-27 on the roof of my F150. I have access to an analyzer and was able to get the SWR to 1.03:1 on 19 and 1.3:1 or so on 1&40.
My whip is 46 inches long.

I think the 47.5 is probably too long. The thing you need to remember about those cutting charts is that they get you close. They are also for an ideal mounting location, which you don't have. The lopsided ground plane under your antenna, plus the proximity to the cab is going to throw things off.

Do not take my 46 inch length as gospel for what would be right for your installation. Yours will likely be different. You are going to have to hook up your SWR meter and do some testing/cutting to get it closer.

Some other things to look at:
DC ground and RF ground are different things. While you may have a good DC ground on your antenna base, that doesn't automatically equal a good RF ground. Make sure all the mounting bolts are tight for the fender.

Check continuity from your antenna base to the UHF connector on the end of the coax. Make sure both the center conductor and shield are making good contact. Often the UHF connectors are not installed perfectly, and the ground shield is usually where this shows up.

Check for a short circuit between the center pin and ground -WITH THE ANTENNA REMOVED- as these are DC grounded antennas. It's normal to see a direct short between the whip and outer ring of the NMO base.

If this is a new antenna, make sure the set screws are tight.
If this is a used/older antenna, take it apart and make sure there isn't any corrosion.

The base coil has a little metal tab that makes contact with the center of the NMO mount. Make sure that is making contact. You might need to bend it down slightly, but this is rarely an issue.

Make sure the NMO mount is clean and the tab on the base of the antenna is clean. A bit of burnishing with a bit of steel wool or brush might be a good idea.

Make sure your SWR meter is set correctly.

Make sure your antenna is mounted on the opposite side of the truck from the AM/FM antenna. If they are mounted on the same side, the two can interfere with each other and throw the tuning off. Any piece of metal too close to your antenna will affect it. That also means if you are testing SWR inside your garage or close to a metal garage door, lamp post, metal fence, etc, no bueno. Sometimes it is good to go out into the middle of an empty parking lot with no lamp posts to do your antenna testing.
Also, make sure your doors are closed.

If this mount has been installed for more than a few weeks, as in it's rained since it's been installed, make absolutely sure the underside of the NMO mount isn't corroded. Most of the NMO mounts are designed to be mounted through the vehicle body. The under side of the NMO mount is intended to be inside a dry vehicle.
If the underside of your NMO mount is exposed to the open air, you can easily get water, road salts, oil, etc. in the coax and under side of the base and it will make a mess.

What are you using for an SWR meter?
 
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Another thing which will help with diagnostics. Get SWR reading for CH 1, 19 and 40 as that will help inform us to whether or not the antenna is electrically long or electrically short.
 

clanusb

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The SWR meter that I am using is a Breaker 13-500. The second unit I will have to check tomorrow. Both were getting just about the same results.

Continuity between the center conductor and the ground shield was checked.

I checked for a short circuit between the center pin and to ground, and none was found.

The NMO has been on the truck, on the drivers side, for about 1.5 years. (yes that is how long I have been trying to solve this. The radio has been on for monitoring, but only transmitted when checking SWR. I have noticed that reception has gotten better and more clear since cutting the antenna down slowly.
That being said, I did use liquid electrical tape on all of the undersides of the NMO's to try and keep water out as best I could. See the attached photo for that.

SWR readings were done in an open field, doors closed, and away from anything metal. See the pictures below.


Pictures are in order of:
SWR Channel 1
SWR Channel 19
SWR Channel 40
Base of the antenna
Underside of the nmo

The little metal tab in the base of the coil, I checked and it seems to be making contact. I took the cone off and checked for continuity from the threads where the cone goes, and the center pin of the coax.
 

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mmckenna

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OK, looks good.

If that SWR meter is correct, it would indicate your antenna is too LONG (corrected in edit).

Can I ask about how you cut it to 47.5 inches? Did you remove the whip from the cone, cut the whip to 47.5 inches and reinsert it, or did you measure the entire length, whip, cone, coil as one part?

Last photo, what am I looking at? I know it's the underside of the NMO mount, but it looks like it's either loose, or the hole is much larger than 3/4 inch.
 
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clanusb

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I pulled the whip out of the cone and cut it to 47.5, cutting the base of the antenna and leaving the tip with the ball alone. I cut it with a cutting wheel with a grinder, and then ground it down to the mark with grinding stone on my bench.
When I tested the antenna from the factory prior to ever cutting it the SWR was maxed out and I could not understand any of the transmissions that I did hear.
When installed, the antenna is placed into the cone and seated as far down as it can go. Set screws are tight and held in with blue loctite.

I popped the antenna coil and ring off and loosened the NMO enough to move it around so that I could try and take a picture of the underneath for you.

Isn't the SWR on 40 greater than that of 1? Or am I reading it wrong...
I thought the SWR was getting higher/worse the further away from channel 1 I got.
 
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mmckenna

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I popped the antenna coil and ring off and loosened the NMO enough to move it around so that I could try and take a picture of the underneath for you.
OK, got it. Looked like it was loose.

So, as it goes, longer antennas are resonate at lower frequencies. Shorter antennas are resonate at higher frequencies.

And to correct my previous post, the SWR readings are indicating your antenna is too long.

Since the SWR is better on lower frequencies (channel 1) that indicates the antenna is longer than what it needs to be.
 

mmckenna

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So, you need to trim the antenna and watch the SWR meter. Start by cutting off 1/8 inch at a time. Don't worry about grinding it smooth just yet. Each time you trim off a little bit, put it all back together and check the SWR on channel 1, 19 and 40.

When you get it to the correct length, SWR should be lowest on channel 19 and roughly the same on 1 and 40.

You'll likely need to cut several times, but that's better than cutting too much.

This will be an easy fix, it's just going to take some time to do it correctly.
 

clanusb

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Copy that. I will try and get it done tomorrow and I will let you know the results.

I didn't know that I might need to go shorter than what the cut chart calls for, I just thought something was wrong, since I did what Larsen said too. I didn't want to ruin the whip.

There was a post here or on another board about a guy in a similar situation, exact same set up, except his was on a Toyota, and he had great SWR.

Thank you for your help.
 
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Copy that. I will try and get it done tomorrow and I will let you know the results.

I didn't know that I might need to go shorter than what the cut chart calls for, I just thought something was wrong, since I did what Larsen said too. I didn't want to ruin the whip.

There was a post here or on another board about a guy in a similar situation, exact same set up, except his was on a Toyota, and he had great SWR.

Thank you for your help.
I seem to remember trimming the one on the fender of my XJ to 46" or there abouts. All I had was a SWR meter when I did that one a few years ago. Like mmckenna said though, bit by bit.

So, you need to trim the antenna and watch the SWR meter. Start by cutting off 1/8 inch at a time. Don't worry about grinding it smooth just yet. Each time you trim off a little bit, put it all back together and check the SWR on channel 1, 19 and 40.

When you get it to the correct length, SWR should be lowest on channel 19 and roughly the same on 1 and 40.

You'll likely need to cut several times, but that's better than cutting too much.

This will be an easy fix, it's just going to take some time to do it correctly.

Get spoiled not having to do it with a SWR meter anymore don't we? I was questioning the too short statement but I generally submit to your more experienced judgement when the methods I don't use often come into play.





Sent from my iPhone using Tapatalk
 

mmckenna

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I seem to remember trimming the one on the fender of my XJ to 46" or there abouts. All I had was a SWR meter when I did that one a few years ago. Like mmckenna said though, bit by bit.
Yeah, the one on my F150 is happy at 46 inches. I long ago lost the cut chart for that antenna. I actually pulled the whip off my 5/8 wave VHF. The original black whip had most of the black coating missing.

This was mine:





Get spoiled not having to do it with a SWR meter anymore don't we? I was questioning the too short statement but I generally submit to your more experienced judgement when the methods I don't use often come into play.
That is true. When I tuned mine I did pull out my Bird to compare readings and check the power output of the CB. Probably the first time I've used it to check SWR in a while.

For years I've been getting the length thing wrong. I can reason it out in my head. I can draw it out, but when it comes to typing it I -always- get it backwards. Luckily I caught the mistake this time before I led clanusb on the wrong path.
 

mmckenna

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Copy that. I will try and get it done tomorrow and I will let you know the results.

I didn't know that I might need to go shorter than what the cut chart calls for, I just thought something was wrong, since I did what Larsen said too. I didn't want to ruin the whip.

There was a post here or on another board about a guy in a similar situation, exact same set up, except his was on a Toyota, and he had great SWR.

Thank you for your help.
Glad I could help.

Where are you located? I live in Watsonville, work in Santa Cruz.

I had to drive up to San Jose today, up 101. I stopped in Gilroy and put my antenna on. I heard some CB traffic, mostly in Spanish from two trucks heading north with me. When I got closer to San Jose, I heard some more. Usually I don't hear much down my way. I took the antenna off before I headed home, but was happy to give it a real world test. I haven't had a CB in my truck since the late 1990's. I'm driving out to San Antonio TX later on this month, and wanted to give it a try again. I don't expect to hear much, but having it in the truck in addition to the VHF kind of adds to the comfort level, especially since I'll be driving in unfamiliar territory.

The cutting charts usually get you close. Yours seemed pretty far out, but I wouldn't rule out some inaccuracy with the SWR meters. Unlikely, but that could be part of it.
The mounting on the side of the hood is going to throw it off from the cut chart. For the record, mine isn't perfect, either. It's mounted at the rear of the cab. The center of the roof is reserved for the VHF radio antenna.
There can really be a lot of variables. You can honestly sit there with a calculator and work it out on paper down to the fraction of an inch. You can put it all together and it'll still be off. Long ago my grandfather taught me about SWR and tuning antennas. I remember him talking about someone he worked with setting up antennas on airplanes (he worked for Lockheed). He always referred to it as a "black art". You can do all the calculations, but when it comes down to it, there is some art involved. Getting down and trimming by the fraction of an inch, watching the meter.
The calculations, cut charts, etc. don't take into account all the variables. It's always a challenge.

I think you'll have a good setup when you've got this tuned. The NMO-27 is a good solid antenna. While many favor the Firestick, Wilson, etc. (and they are good antennas) it's hard to beat using a commercial grade antenna. The NMO-27 I have on my truck is probably 20 years old, at least. It had a lot of miles on it when I put it away in 2001 or so.
Truth about antennas is that there is no magic pixy dust that any manufacturer has. All antennas are governed by the laws of physics, and there isn't any way around that. Similar sized antennas are going to work the same if they have the same install. Where it really matters is build quality. I've never had an issue with any Larsen antenna I've ever had, either personal use or at work.
 

JayMojave

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Gee Whiz!!!! With that a high SWR measurement, you have something really wrong!

You have a open or short in the coax, again something is really NOT right. Those SWR readings are NOT indicating slight SWR variations, but again something is terrible wrong.

MM SWR measurement show a slight SWR vrs the selected bandwidth, this measurement shows a typical antenna SWR that is properly working. You are now were near making any decisions as to making the antenna shorter or longer, those different SWR vrs frequency readings shown are probably showing slight differences due to coax length and different channels, with a major problem.

Jay in the Great Mojave Desert
 

clanusb

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Well I kept working my way down slowly and believe it or not its sitting at 44 inches. I think i will stop there.
1: 2
9: 1.6
19: 1.5
40: 2.5

I really only plan on using 19 and channels directly around it. Soo...
Let me know what you think.
 

clanusb

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Just double checked everything on the second swr meter. This one seems to be way more accurate.

Herald electronics model cb-21a
1: 1.6
9: 1.2
19: 1.1
40: 1.3

Probably went just a tad too short. But seeing where 19 is on this one, I'd say im happy with the SWR. now we will just have to see how it does on the road.
 
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Just double checked everything on the second swr meter. This one seems to be way more accurate.

Herald electronics model cb-21a
1: 1.6
9: 1.2
19: 1.1
40: 1.3

Probably went just a tad too short. But seeing where 19 is on this one, I'd say im happy with the SWR. now we will just have to see how it does on the road.
I'd be quite happy with that. For a mobile, I'd generally consider anything under 2.0:1 acceptable.
 
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