Mating 2.92mm with 3.5mm RF connectors - mismatch issue? - Page 1
Forum section: electronics / rf-microwave

author: wb0gaz
author_rank: Frequent Contributor
timestamp: December 16, 2019, 09:37:05 pm
content: I understand 2.92mm and 3.5mm connectors mate non-destructively, however, I want to know if they are electrically compatible, or is there a mismatch/reflections introduced due to the differing diameters (OD of the center pin and ID of the shield)?Any references appreciated!Thank you

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: December 17, 2019, 07:38:38 am
content: I am not aware of any significant mismatch. We do this all the time in our labs, having a lot of things like DC block, attenuators, etc only in 3.5 mm but all our cables are 2.92 mm.I imagine there is a very small amount of fringe inductance/capacitance but it should be negligible.

author: wb0gaz
author_rank: Frequent Contributor
timestamp: December 17, 2019, 02:22:17 pm
content: Thanks --- I'd been puzzling over the effect that the physical step in diameters (and dielectric material step in the case of SMA vs 3.5 or 2.92) at the interface point would have, even with constant impedance. This must be why SMA, 3.5 and 2.92 are electrically interoperable, although I'd be fascinated to see the math worked out (or at least hear about same) --- I'd imagine that requires some kind of finite element E/M modeling.Dave

author: German_EE
author_rank: Super Contributor
timestamp: December 19, 2019, 07:15:18 pm
content: Where it becomes a problem is fitting a 3.5mm plug into a 2.92mm socket on a regular basis. The center connector on the 2.92mm socket will be over stressed so that the next time you try fitting a 2.92mm plug the contact will be loose.

author: wb0gaz
author_rank: Frequent Contributor
timestamp: December 19, 2019, 07:31:29 pm
content: Thank you, German_EE.I have a new question about mating 3.5mm male to SMA female - it appears that the inserted portion of the 3.5mm male is somewhat larger diameter than SMA female receptacle, and I found damage to SMA female when I tried this (fortunately, damaged SMA female component is easy to replace this time, but now I am concerned about future interoperability).Is 3.5mm male to SMA female mechanical dimensions a case where I should avoid this connection type? If so, should I use 3.5mm (f) to SMA (m) adapter? I have not found such an adapter.Thank you again, this is very helpful detail advice for me!

author: KrudyZ
author_rank: Frequent Contributor
timestamp: December 19, 2019, 07:40:09 pm
content: This should answer any question you have on the matter:https://www.researchgate.net/publication/295282915_Intermateability_of_SMA_35_MM_and_292_MM_connectors

author: wb0gaz
author_rank: Frequent Contributor
timestamp: December 19, 2019, 07:48:47 pm
content: Thank you!That really helps understanding the electrical properties, very helpful indeed!I am still puzzling why 3.5mm male has tight/difficult clearance into SMA female in my experiments, but that is more mechanical issue, and the GORE paper you directed me to indicates this interconnection should work if the participating connectors are in good condition, which I cannot guarantee (I am not working in professional lab and have limited mechanical measurement tools.)

author: German_EE
author_rank: Super Contributor
timestamp: December 19, 2019, 08:28:29 pm
content: Given that adapters between series (male and female) are widely available why not just mate like with like? You only need to destroy one connector on a piece of expensive calibrated test equipment, after that you learn your lesson and stop messing around.Bonus points if you color code equipment connectors and cables to stop this happening.

author: wb0gaz
author_rank: Frequent Contributor
timestamp: December 19, 2019, 08:53:01 pm
content: Hello German_EE -Yes, I cannot easily recover from expensive test equipment connector damage!Do you know who makes SMA/3.5MM between-series adapters? I did not expect problems with this pairing (mainly, main 3.5mm to female SMA), but I also have not ever seen adapters for this combination.Thank you again

author: pigrew
author_rank: Frequent Contributor
timestamp: December 19, 2019, 09:05:13 pm
content: I usually blindly mate 3.5mm to SMA or 2.92mm to 3.5mm, though I normally gauge the connectors to verify pin recession, and quickly inspect under a microscope first.... Cables in my shared university lab are not well taken care of (and most of the cal kits are damaged...).I usually spend my time at <2 GHz, so it's not so bad. I have my own sets of cables for higher frequencies.Quote from: wb0gaz on December 19, 2019, 08:53:01 pmFairview,MiniCircuits,Pasternack, for example.

author: chris_leyson
author_rank: Super Contributor
timestamp: December 19, 2019, 09:08:46 pm
content: Shahriar from The Signal Path did a tutorial on various pieces of RF hardware. SMA and K connectors are discussed about 5 minutes in.

author: tkamiya
author_rank: Super Contributor
timestamp: December 30, 2019, 12:22:14 am
content: I heard this both ways....3.5mm and SMA are inter-connectable in every direction.3.5mm male PINS can go into SMA female socket but not the other way around.For the second, an explanation was given that:SMA is non precision where as 3.5mm is. Inserting non precision SMA center pin into precision 3.5mm socket can cause socket to expand/deform. Next time this 3.5mm is mated with proper 3.5mm, it may not make a proper contact. 3.5mm male pin can go into SMA female side without damage.To me, this kind of make sense. Non precision connector can have center pins slightly larger (as well as smaller) center pins than spec calls. Since 3.5mm connectors cost SO much more, it warrants not doing this. In my personal lab, I observe this rule.Anyone have comment?

author: rf-messkopf
author_rank: Regular Contributor
timestamp: January 01, 2020, 03:30:22 am
content: Quote from: tkamiya on December 30, 2019, 12:22:14 amTrue. Mechanically, 3.5mm and SMA are, in theory, compatible. But, as you say:Quote from: tkamiya on December 30, 2019, 12:22:14 amThis may indeed be an issue, especially with the very cheap SMA connectors which are not manufactured with much precision. Specifically, in cheap male SMA connectors the pin depth may be out of tolerance (too high is dangerous for 3.5mm female connectors), there may be burrs on the pin, and it may be off-center.But also cheap SMA female connectors can be dangerous for precision 3.5mm male connectors. The cheap ones often have a single-slotted center pin contact that has sharp edges. These can damage the surface of the male center pin when mating. Always check if the edges of the center pin sleeve can touch the male pin before mating cheap SMA female connectors with expensive precision ones.And there are the slotless PC3.5mm female connectors from Keysight, which are used in VNA test port cables. In these the center pin receptacle is solid, i.e., without slots, but it has small and very thin metal fingers in the pin receptacle bore, which are easily destroyed. See here for pictures:http://na.support.keysight.com/pna/connectorcare/Inspection.htm. These connectors are very fragile. I would not mate them even with good quality non-precision connectors. And never turn their connector body while mating or while tightening the nut, this will instantly wreck the tiny metal fingers.

author: tkamiya
author_rank: Super Contributor
timestamp: January 01, 2020, 05:34:58 am
content: Someone asked if there are properly made 3.5mm and SMA adapters.Mini circuits has a fewPasternak has manyThey are quite expensive though.I have just one equipment that uses 3.5mm. I have two known good adapters, one to N and another to BNC that SMA side that was measured by a lab. I treat them as a connector saver. I also have SMA to SMA that came out of HP gear that was used for such purpose.