To SMA or not.. and 50ohm traces


#1

I’m getting to the point on my board I need to choose antenna connector. (UFL, SMA)…
A few things going thru my mind and going to ask what would you guys think !

I have this board that I’ll be placing in a Hammond enclosure.

I need to make 50ohm traces for my antennas

Trying making the BOM smaller, I would use a SMA connector directly on the edge of the board.
good thing is that I do not have to worry about small UFL connector and coax cable inside my case(reducing the BOM).

but I’ll have to manage the hole on the case and it will have to be larger than the SMA connector and I don’t ever saw a model like it that will be able to add a nut on the outside to make it sturdy and prevent dust to enter the case.
image
also, all pressure applied to the antenna will be directly to the board…

easy route would be to use
image
but I feel like that connector(UFL bord side) is flimsy and I will never remove the cable from the board… ever.
so I searched and found this; LINK
image
I’d use a SMA footprint and place it a bit more off the edge of my board to solder directly the cable to the board and have a big SMA connector on the case strongly attached with two nuts and locktite.

this way most (if not all) my issues are fixed, smaller BOM(not that much), stronger connector, no dust infiltration, no possible damage to board by antenna…

what do you guys think ?

looking at the application note of my chip, it seams that they are adding capacitors ‘PI Matching’ if using external SMA…and it looks like making 50ohm trace is a bit complex. but my board is very small and the antenna connector should be a few mm away from the chip output… is this so complex ? -where can I start with simple info on making my +/- 10mm trace 50ohm… ( I do more search and look into PCB calculator ) but I feel like it should be simple ?!


#2

You could go for SMA bulkhead connectors like these:

https://www.mouser.de/ProductDetail/Johnson-Cinch-Connectivity-Solutions/142-0701-411?qs=sGAEpiMZZMuLQf%2bEuFsOrowCuZJUT8YlZuzfigQdRUA%3D

https://www.mouser.de/ProductDetail/Amphenol-RF/901-9841?qs=sGAEpiMZZMuLQf%2bEuFsOrh35k6jkGdFnTVjqMOFpRQE%3D

Creating the hole in the case at the exact location is still somewhat tricky …


#3

I have struggled with this decision myself… currently I have moved away from the hammond cases, I shrunk my board design and put the sensitive bits under a can shield. Now I can use a 3d printed plastic clamshell style case which locks tightly around SMA board edge connectors.


#4

Last hammond Radio I made was done with a right angle through hole SMA.
Combine it with Stepup and you are good to go :slight_smile:


#5

You could also look at right angle MMCX connectors. They are a good bit smaller than SMA, and snap in pretty tight. After that, you are down into the UFL or solder to the board range as you have found.


#6

just found this; perfect…

image

soldered to the board, scewed to the panel and water tight…

if they dont provide 3D, that will be a heavy stepUp work… :slight_smile:


#7

Hi,

you didn’t told us the frequency. U.FL ist OK for 2.4GHz and might be acceptable for 5GHz. But above you need better connectors like good edge mount SMA/2.92mm or SMPM. However I’ve seen broken solder joints with the edge mount version - especially when the cable is removed often which is common in instrumentation.
Direct soldering to the PCB is bad in terms of RF performance since there is a relative huge gap and the reproducibility is limited. I wouldn’t use it above 1.5GHz unless verified by measurement.
This also leads to broken wires since the solder make the first part of the inner and outer conductor stiff and the wire break easily on the transition between solder/no solder. This is especially critical when the assembly is exposed to vibrations. (e.g. automotive, aviation or space applications)
I would recommend you the U.FL version with pigtail or utilizing a screwed connector like the 32K243-40ML5.

Best regards,
Martin Laabs


#8

I’m working on 915mhz… sorry I didn’t specify…

since I will have a hard time inserting the board into my case (got already 4 other signal connector on my board edge) I was thinking to go back to coax cable in a through hole custom pad on one side and a SMA panel mount on the other side.


#9

Can someone please help figure out that ‘‘Pi Matching’’ circuit my manufacturer suggest?

I’ve made it like this for now;


in case I want to use a real SMA connector, I can use it on ‘edge’
and probably end up using 'bare coax cable to panel mount SMA.

but the C17 and C19 value are not specified in the spec, I guess it will all depend on the ‘pi matching’ and 50 ohm impendance calculation. witch I have no idea who to do… and if I trace my board right… I plan to use fr4, 2layer, 1,6mm