So I decided to install some antennas on my pickup truck. I installed a NMO-150 on the roof of the cab, trimmed it a bit and have a perfect SWR reading nearly 1:1. So then I moved on to install the NMO-27 on an L bracket coming off of the front fender. (CB radio) The NMO-27 coil is brand new and so is the NMO mount with cable. The whip was also brand new so was about 48.5" long. I put the whip in and checked the SWR and it was garbage. 3+. So then I did the required cutting according to the cut chart. I cut the whip down to 47.5" But still garbage SWR! 3+. So one more time cut it down to 47". Still garbage 3+ SWR. I have stopped and given up now. I do not want to cut the whip any further for fear of ruining it. Does anyone have experience with these NMO-27 and do they get a good SWR? I am going nuts here wondering what is wrong. I have never had a NMO-27 before only the higher band models which I have been impressed with.
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Having SWR problems with Larsen NMO-27
- Thread starter pyro424
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prcguy
Member
It's common for CB and HF antennas to not have enough ground plane when mounted on the side of a vehicle. To prove this you can test the NMO-27 on your center roof mount and it should be able to tune ok there. If it does you may have to move the mount, or you could try attaching a small capacitor around 30pf at the NMO mount where the coax attaches to fool the antenna into having more capacitance coupling to the car body.
prcguy
prcguy
Well I am going to test moving it to the mount on the roof and see what happens. However I really do not see why my mounting location is an issue. I have mounted other CB antennas for friends on their pick up trucks in the past including the exact same model and year of truck and had no SWR issues. This is however the first time I have ever used a NMO-27.
mmckenna
I ♥ Ø
High SWR can mean many things, not just an antenna issue.
With the whip and coil removed, and the coaxial cable disconnected from the radio and not touching anything metal, do the following:
Check for continuity between the center pin and the outer ring. There shouldn't be any (no connection). If there is, stop and fix.
Check for continuity between the center pin of the UHF connector inside the truck and the center pin of the NMO mount, there should be a connection. If there isn't, stop and fix.
Check for continuity between the outer shield of the connector and the outer ring of the NMO mount, there should be a connection. If there isn't, stop and fix.
If you do all the above and it checks out, check the following:
Good ground connection between the NMO mount and the vehicle body. It needs a good clean path to ground.
Check the metal tab on the underside of the NMO-27 coil. Make sure that it will contact the center pin of the NMO mount and that everything is clean.
I've used a lot of the NMO-27's and never had an issue. Only difference was that I -always- mounted the dead center of the roof for the best ground plane.
With the whip and coil removed, and the coaxial cable disconnected from the radio and not touching anything metal, do the following:
Check for continuity between the center pin and the outer ring. There shouldn't be any (no connection). If there is, stop and fix.
Check for continuity between the center pin of the UHF connector inside the truck and the center pin of the NMO mount, there should be a connection. If there isn't, stop and fix.
Check for continuity between the outer shield of the connector and the outer ring of the NMO mount, there should be a connection. If there isn't, stop and fix.
If you do all the above and it checks out, check the following:
Good ground connection between the NMO mount and the vehicle body. It needs a good clean path to ground.
Check the metal tab on the underside of the NMO-27 coil. Make sure that it will contact the center pin of the NMO mount and that everything is clean.
I've used a lot of the NMO-27's and never had an issue. Only difference was that I -always- mounted the dead center of the roof for the best ground plane.
Ok so I have moved the NMO-27 to the roof and the NMO-150 to the fender. The SWR on the NMO-150 off the fender upon a quick check is good. Now the NMO-27 on the roof is still crap! garbage! 3+! so I do have a NMO mag mount base. for testing purposes I moved the NMO-27 to the mag mount and stuck it on the roof. well there it is perfect match! what the heck is going on? keep in mind that roof mount previously had a NMO-150 on it working flawlessly. And yes I have used a multi meter to check continuity from the whip to the center pin of the coax. What the heck is going on!?
prcguy
Member
I've only installed a few NMO-27s in my time (mostly in the late 70s and early 80s) and played with one on a mag mount. They are great performers for their size but I remember them being a bit finicky about having an adequate ground plane. I also remember having inconsistent results on a mag mount with mounting location changing the tuning and the coax had a lot of RF on it.
Could it be you trimmed a bit too much off the whip for it to match well on the permanent roof mount?
prcguy
Could it be you trimmed a bit too much off the whip for it to match well on the permanent roof mount?
prcguy
I don't think so. It is at 47" which is what the cut chart calls for. I have another whip sitting here that is about 48" (from an old antenna) I put that on just to be sure. No better. Same garbage SWR reading. I guess I should just give up and use the mag mount. At least that will match up. Maybe move my NMO-150 back to the roof where it works so well.
mmckenna
I ♥ Ø
Check to make sure the center tab of the coil is really making contact with the NMO mount.
I've seen certain NMO mounts that sit a bit lower than others and can sometimes keep the coil from making proper contact.
The fact it's failing on both of your permanent mount NMO's but not on the mag mount makes me think it's something simple like this.
I've seen certain NMO mounts that sit a bit lower than others and can sometimes keep the coil from making proper contact.
The fact it's failing on both of your permanent mount NMO's but not on the mag mount makes me think it's something simple like this.
Yea I did check that out. Also checked continuity from the whip itself to the center conductor coax with a multi meter and it was good.
Yes the metal contact is good there. I have a SWR of 2 now. That is lowest I can get it. I uncoiled the coax etc. and that is the best I can get. SWR of 2. That is unacceptable I think.
UPDATE***
So in one last ditch attempt I tried one more thing to get the SWR down. I found another old stainless whip in the barn (I always save this stuff). This whip is slightly thinner than the others and is 45" exactly. Since I was afraid to cut my other whip that is 47" I tried this one. Well if you would believe it the SWR is now almost 1.5 across the entire band. Bizarre. Remember the cut chart calls for 47.5 actually. So now I have pretty decent SWR. However the antenna is shorter than I expected and it is narrower than expected. (does not look as good as the brand new whip). I am accustomed to running the larger wilson whips or even the 9 FT whips on the CB band so I have to say this is rather strange to be running this 45" whip. The whole reason I wanted to get this NMO-27 was because I wanted something more discrete on this pickup. Just to accompany my VHF set up on the highway. Well it sure has been troublesome. So 45" it is. ?????
So in one last ditch attempt I tried one more thing to get the SWR down. I found another old stainless whip in the barn (I always save this stuff). This whip is slightly thinner than the others and is 45" exactly. Since I was afraid to cut my other whip that is 47" I tried this one. Well if you would believe it the SWR is now almost 1.5 across the entire band. Bizarre. Remember the cut chart calls for 47.5 actually. So now I have pretty decent SWR. However the antenna is shorter than I expected and it is narrower than expected. (does not look as good as the brand new whip). I am accustomed to running the larger wilson whips or even the 9 FT whips on the CB band so I have to say this is rather strange to be running this 45" whip. The whole reason I wanted to get this NMO-27 was because I wanted something more discrete on this pickup. Just to accompany my VHF set up on the highway. Well it sure has been troublesome. So 45" it is. ?????
prcguy
Member
Did you check the match on ch 1 and 40? Now that the antenna is in the ball park that will tell you if it could use further adjustment. If the match is better on ch 1 then the antenna needs to be a little shorter. If its better on 40 you went too far.
prcguy
prcguy
Yes the match is better on CH 1. So really what that means is the antenna needs to be slightly Longer. I cut the other 47" whip I have down to 46" and well the SWR sucks. 3+. So I am back to the slightly narrow 45" whip and the SWR seems to be pretty decent. I am thinking the width of the whip is having an effect on impedance here. Judging by the results I am getting from the thicker whip that came with the coil. I am not talking about much of a difference in width. Not even 1/8" difference. Bizarre.
prcguy
Member
If the match is better on ch 1 than 40 then the antenna needs to be a little shorter, not longer. When its close you want to make small adjustments like 1/4" or less.
prcguy
prcguy
Yea sorry what I meant is the match is worse on CH 1 but better on CH 40.
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