Is this considered professional and is it hindering performance?

[kayn1n32008]

So the radio is a Motorola XTL2500 450-512 UHF, it is currently programmed to GMRS frequencies, so the antenna that I was using initially on the mag mount was a Laird B4502N which is a 450-470MHz NGP 1/2 wave, which seemed to work really well. However, being that it is a unity gain antenna, I figured I could increase my transmission and reception by changed to a 3DB 5/8, specifically a Laird B4503 450-470MHz. The antenna came with a cut chart so I cut the antenna for 465MHz on the chart since that's in between the simplex and GMRS repeater frequencies. The antenna also came with a ground plane chart that said for UHF it needed 13 inches of ground plane, which none of NMO mounts are able to provide due to their location, so I was hesitant to even put it on the mount being afraid of a super high SWR. That's when I noticed the range was noticeably less, so I put the NGP antenna on the NMO mount and it seemed to be the same. So then I just put the 5/8 B4503 on the mag mount which should give it the proper ground plane it needs. I was just trying to get rid of the mag mount since I have the privilege of buying a truck with 3 existing NMO mounts.

There is little performance difference between a 1/2 wave and a 5/8 wave, sure the 5/8wave has 3dB of gain, but it's half a S-unit difference. You are over thinking it, I would suspect either a mount and feedline issue, or an antenna issue.

Not having a perfect 1/4wave ground plane around your mount isn't going to be a big issue, especially for a drilled NMO mount, the antenna will be slightly directional, nothing more.

 

[kayn1n32008]

So I purchased a truck that came with 3 NMO mounts in the roof. I wanted to install a Motorola UHF radio but all the connectors on the end of the coax were BNC not mini UHF. Instead of having to buy speciality tools and probably messing up the first time I tried, I decided to take it to a local shop who I guess is a Motorola service center. I showed them what I wanted to do and they said they could replace the connector and that it would be $60 in parts and $100 in labor. That sounded kind of high so I remembered I had an extra connector that came with a Motorola antenna I had ordered and they said I could bring it in and they wouldn’t charge me the $60. Fast forward a week to when they could get me in I also asked if they could check the SWR of the antenna and tune it if necessary to reduce reflected power to the radio and they looked at me like they had no idea what I was talking about and just told me if it works it works… I pressed a little harder and they said they could put a meter on it and I said okay then told me it would be $160 which I figured the extra was for the meter they were going to put on it. I paid then realized they still charged me $60 for the connector but at this point I was like whatever I guess I’ll just hold onto mine for the future. So they change the connector and the lady brings out the meter which I don’t believe she had ever seen till that day and was just keying up the mic and watching the needle go up and said you’re good. Well my transmission and reception distance has been terrible vs the mag mount I had been using. I thought it was due to the antenna no longer being dead center on the roof since the NMO mount is towards the rear driver corner of the roof. So I was going to put the mag mount back on and test with my buddy then noticed their installed connector. So is this fine or could it be causing my issues? I feel like they really don’t know what they’re doing and prefer not to go back.

Going from dead centre to back corner won't have the effect you are describing. Something else is going on.

I had a VHF 1/4 antenna where the GPS/SiriusXM antenna goes on the high end Jetta, on my VW Jetta, and it worked just fine.

 

[kayn1n32008]

And truthfully, I chose that corner because it allowed me to chop off several feet of excess coax cable since I had read that the shorter the cable, the less loss there would be.

The amount of loss your eliminating isn't all that significant.

 

[Huntercw]

Is there decent continuity between the mount threads/coax screen and the vehicle bodywork? A common cause of performance issues with a through-body mount is the lack of decent continuity there leading to poor antenna performance.

I tested the continuity again with my buddies better Blue Point multimeter, and it showed zeros for the positive and ground between the connector and the mount.

 

[Huntercw]

I think you are getting way too hung up on this coax loss thing.

It's not as big an issue as you are thinking. I'd move the Motorola radio onto the center NMO, that would help your radiation pattern. Chopping 5 feet off your RG-58 is going to have a few tenths of a dB improvement, and again, that's not going to be noticeable unless you are on the very fringes of coverage.

Same with adapters. Yes, not using adapters is better than using one, but it's really somewhere around a tenth of a dB for a good quality adapter. The Lands Precision you mentioned at Antenna Farm should be good quality. I think I've used that brand before and I don't recall any issues.

Got it. I ordered a couple of the adapters, the SWR meter should be here in a couple of days as well. I will move the antenna to the center mount and use the adapter and check SWR once everything arrives.

 

[merlin]

I don't know where the grease is supposed to be applied?

I apply a generous coating to all the rubber and fill any gaps. It is a dilectric, same as Dow #4 compound
Mainly, a sealant. It displaces water, protectes rubber from dry rot.

 

[G7RUX]

I tested the continuity again with my buddies better Blue Point multimeter, and it showed zeros for the positive and ground between the connector and the mount.

I was meaning specifically from the ground threads of the mount directly to the vehicle bodywork, without either the antenna fitted or the coax plugged in to the antenna.

 

[Huntercw]

I was meaning specifically from the ground threads of the mount directly to the vehicle bodywork, without either the antenna fitted or the coax plugged in to the antenna.

Got it, I will test again for that, what part of the vehicle should I go to for the ground? Just a non painted surface?

 

[mmckenna]

Just a non painted surface?

Yes. Or a bolt head. Something that is connected to the body steel.

 

[G7RUX]

Got it, I will test again for that, what part of the vehicle should I go to for the ground? Just a non painted surface?

As mmckenna said, something unpainted like a bolt or similar.
A poor connection to the bodywork for the mount screen will give pretty much the symptoms you are talking about and will make it a very poor performer.

 

[mastr]

...I've seen this kind of stuff enough to think that there is someone out there actively teaching radio shops to do piss poor work like this, and then charge top dollar...

If not that, there is certainly a notable deficiency in after installation inspection and quality control. It seems to get worse as time goes on.

 

[Huntercw]

Okay so I tested for continuity between the threads of the mount and the door latch bar bolted to the cab and it tested good at 0 ohms. I also received my meter today and got some interesting results. I am still waiting on my BNC to Mini UHF adapters to come in so I can position the antenna in my center rear NMO mount but for now I just wanted to see if I was getting a major difference between the two mounts and the two antennas, I'm not quite sure what to make of this, perhaps its inaccuracy of the meter but the 1/2 NGP wave seemed to perform much better on the permanent mount and the 5/8 3Db favored the mag mount which I had placed dead center in the roof. I checked SWR on channels 1 and 22 which only had a .01 difference so I just included results from channel 1, and then I checked it on a common repeater channel. The truck was in the same position for all test and all doors were closed. So I guess this means everything is okay? Perhaps the difference I was noticing in performance was just that particular location we tested when I first used the NMO mount, maybe a bunch of interference in that area or something. I guess the thing to do once I get my adapters so that I can utilize the center rear mount is try both antennas on there and just stick to the one that has the lowest SWR or would "performance" still be better with the 5/8 wave even if the SWR is still a little higher since is would have slightly more gain?

 

[mmckenna]

None, not one, of those SWR readings are an issue.

But, keep in mind that SWR is not the be all/end all number that indicates that an antenna, mount, coaxial cable, connector, or location is good.

All low SWR means is that the energy from the radio is getting absorbed somewhere and not reflected back to the radio.

A 50Ω dummy load will show a perfect 1.0:1 SWR, but not radiate RF worth crap.

But, what you have done is ~likely~ proved that your antenna/mount is good.

Radiation pattern is another thing, all together.

I think you next step is to wait for that adapter and try using the center NMO mount. Make sure you test away from other structures, preferably out in the open.
And, consider that the issue -may- very well be on the far end. This is TWO way radio, and it takes two radios to communicate. Unless you know for 100% certain that the other radio/user is good, take any radio check reports with a grain of salt.

 

[dave3825]

If it were me, I would name the shop that did the crappy crimping so others don’t have a bad experience there.

Then again, if I asked them to check swr, and they gave me the look they gave you, I’d have been out of there.

 

[Huntercw]

None, not one, of those SWR readings are an issue.

But, keep in mind that SWR is not the be all/end all number that indicates that an antenna, mount, coaxial cable, connector, or location is good.

All low SWR means is that the energy from the radio is getting absorbed somewhere and not reflected back to the radio.

A 50Ω dummy load will show a perfect 1.0:1 SWR, but not radiate RF worth crap.

But, what you have done is ~likely~ proved that your antenna/mount is good.

Radiation pattern is another thing, all together.

I think you next step is to wait for that adapter and try using the center NMO mount. Make sure you test away from other structures, preferably out in the open.
And, consider that the issue -may- very well be on the far end. This is TWO way radio, and it takes two radios to communicate. Unless you know for 100% certain that the other radio/user is good, take any radio check reports with a grain of salt.

Yes that's also a good possibility, I've learned a lot from this thread. When the adapters come in I will test the antennas and go with the one with the lowest SWR, thanks for your help!

 

[fontanei]

Yes that's also a good possibility, I've learned a lot from this thread. When the adapters come in I will test the antennas and go with the one with the lowest SWR, thanks for your help!

Huntercw,

From what I gather you have better performance using the mag mounted antenna on the center of the roof…

From a radiation pattern perspective:
You have 3 nmo mounted antennas near the back of the truck cab. Keep in mind that metallic structures near the radiating antenna WILL alter that antennas radiation pattern (both elevation and azimuth)…that radiation pattern is the same for tx and rx (on the same frequency) by how much? I can’t tell you unless your setup is tested in an anechoic chamber.

Also, keep in mind that. 1/4, 1/2, 5/8 wavelength radiators (antennas) have different radiation patterns that change with frequency.

5/8 wavelength radiator does have a higher gain than a 1/4 wavelength radiator; however, their respective peak ‘gain’ is achieved at different elevation angles. Hence, this is why you’ve probably heard people say ‘my 1/4 wavelength antenna is performing better than my 5/8 wavelength antenna’

Adapters:
Try not using adapters. Adapters add insertion loss (S21) and might not offer the best match (S11 and S22). This gets worse as the operating frequency increases.

Recommendation:
1. Cap those 3 nmo mounts near the back of the cab.
2. Drill a new hole in the center of the cab and install a new nmo mount. Get one with the right connector and if not the right connector install one yourself. YouTube is a great resource to learn how to install connectors.