Stopping vibration from unscrewing this antenna on a truck

[ivan747]

Hi guys,

So I need to mount a wireless product to a fleet of vehicles (trucks). This wireless product has a couple of SMA connectors for antennas. For cost reasons I will be using the included antennas. They are rather firm but I am worried that it will vibrate lose over time, specially since it's an SMA connector and the antennas are rather large. I have attached pictures of a similar antenna below.

I was thinking I could use Loctite, at least partially (putting a drop of it at the end of the connector only, so that I don't insulate the SMA connectors from one another).  Another thing I thought was mounting the device so that the SMA connectors are vertical (instead of being parallel to the ground) and the antennas are also vertical. Having both antennas vertical may impact MIMO performance, though. I'm not sure by how much.

Please let me know your thoughts, I want to know if this is okay or if it's a no-go.


Thanks!

[james_s]

I would think a dab of loctite would work well, just don't use very much, you don't want it all over the threads.

[Benta]

SMA is for consumer electronics - sat dishes etc.
You want BNC or better for this application.

[hendorog]

SMA > BNC

Edit: Looks like a frequency where BNC is not normally used and the bnc quick disconnect advantage is not useful for a permanent install

I'd run a cable to the antennas and mount them seperately if you can.

[DaJMasta]

There are plenty of SMA/3.5mm connectors used in professional equipment, and it's often because of frequency - BNCs aren't a good choice up at 2.4GHz, for example.

Could you apply the loctite or a dab of electronics silicone on the connector after assembled instead of in the threads?  I don't know if it would hold up as strong, but it seems to be common in a lot of applications to just put a dab of something on it after its assembled, and that would avoid any grounding problems.

[innkeeper]

sma is not the right way for an external antenna to be held in place, its a great termination for the coax, but for anything exposed to vibration, external wind forces etc, its not the right way for the antenna itself to be held in place.

look at adapting a commercial antenna for the purpose. likely you will need to trim the antenna for the frequency you wish to operate at in the worst case.
you will find that no commercial antenna uses an SMA connector to mount the antenna element. to the mount.

[ivan747]

It's 2.4GHz or thereabouts and the connector type is SMA. Sadly, I have no choice over the connector used, it's an existing product and it's one of the very few we have found suitable for the requirements.

There's also WiFi (at 2.4/5GHz), but I am using a different type of WiFi antenna, and that one comes with a coax cable and is screwed into a panel, so vibration is not a big concern there.

[ivan747]

There are plenty of SMA/3.5mm connectors used in professional equipment, and it's often because of frequency - BNCs aren't a good choice up at 2.4GHz, for example.

Could you apply the loctite or a dab of electronics silicone on the connector after assembled instead of in the threads?  I don't know if it would hold up as strong, but it seems to be common in a lot of applications to just put a dab of something on it after its assembled, and that would avoid any grounding problems.

Good idea, I'll test that as well and see how it holds up after it sets.

[ivan747]

sma is not the right way for an external antenna to be held in place, its a great termination for the coax, but for anything exposed to vibration, external wind forces etc, its not the right way for the antenna itself to be held in place.

look at adapting a commercial antenna for the purpose. likely you will need to trim the antenna for the frequency you wish to operate at in the worst case.
you will find that no commercial antenna uses an SMA connector to mount the antenna element. to the mount.

It's not the right connector for these applications. I am aware of that, I work a lot with products like this and kind of hate the fact that the antennas can become unscrewed just from adjusting them. We evaluated 9 different product and only one had a robust connector that didn't come lose by moving the antenna around (the connector was a TNC, by the way). The rest was SMA, kind of annoying.

[hendorog]

sma is not the right way for an external antenna to be held in place, its a great termination for the coax, but for anything exposed to vibration, external wind forces etc, its not the right way for the antenna itself to be held in place.

look at adapting a commercial antenna for the purpose. likely you will need to trim the antenna for the frequency you wish to operate at in the worst case.
you will find that no commercial antenna uses an SMA connector to mount the antenna element. to the mount.

It's not the right connector for these applications. I am aware of that, I work a lot with products like this and kind of hate the fact that the antennas can become unscrewed just from adjusting them. We evaluated 9 different product and only one had a robust connector that didn't come lose by moving the antenna around (the connector was a TNC, by the way). The rest was SMA, kind of annoying.

A short SMA cable between device and antenna would prevent them from rotating off, particularly if you also loctite them.

[D3f1ant]

Have done the same in the past on a whole fleet of buses, loctite works fine, use the medium or low strength stuff. Don't use the wicking variety, it sets so hard it's difficult to remove unless you apply a lot heat.

[jbb]

In addition to the SMA unscrewing, I’d be a little worried about the effects of vibration on the transciever (e.g. solder joints on SMA jack cracking).

Maybe you should get a unit vibration tested by a lab?

[mikerj]

In addition to the SMA unscrewing, I’d be a little worried about the effects of vibration on the transciever (e.g. solder joints on SMA jack cracking).

That would be my biggest concern for this application, especially as this is a very bulky antenna compared to the usual 1/4 wave stubs.  Also the "hinge" near the SMA is likely to become loose.

[ivan747]

Have done the same in the past on a whole fleet of buses, loctite works fine, use the medium or low strength stuff. Don't use the wicking variety, it sets so hard it's difficult to remove unless you apply a lot heat.

I am considering Loctite 222 for this.

In addition to the SMA unscrewing, I’d be a little worried about the effects of vibration on the transciever (e.g. solder joints on SMA jack cracking).

That would be my biggest concern for this application, especially as this is a very bulky antenna compared to the usual 1/4 wave stubs.  Also the "hinge" near the SMA is likely to become loose.

I'm also worried about the hinge, so I think I won't place the antenna on a 45 degree angle. I'll either have it parallel or perpendicular to the SMA connector.

In addition to the SMA unscrewing, I’d be a little worried about the effects of vibration on the transciever (e.g. solder joints on SMA jack cracking).

Maybe you should get a unit vibration tested by a lab?

The unit itself is certified for vehicular applications, including shock and vibrations. Of course, I didn't do the certification, so I took it apart 

I took it apart and the SMA connectors are screwed to the chassis using star locking washers. The enclosue is a extruded metal enclosure and the PCB is sloted into the enclosure, but between the PCB and the enclosure goes rubber piece that dampens vibration. The thing looks pretty solidly build. There is a massive chunk of metal that mounts on the PCB that serves as a heatsink and I imagine it also serves some type of mechanical function (it's really massive).

The WiFi SMA connectors are soldered to the board, but luckily I'll be using cables for those antennas.


I can post some pictures after the project is approved.

[Phoenix]

Is this the right model? https://fccid.io/UXX-S5A643A/Internal-Photos/Internal-Photos-3170889 The exterior is as your picture, but the inside isn't as described?

Die-cast metal enclosure. All SMA use a uFL/IPEX pig tail, none are soldered to PCB.