[time-nuts] SMA Connectors was: HP 5370B low frequency modulation

Brooke Clarke brooke at pacific.net
Sat Jul 28 20:09:22 EDT 2007


I spent a few decades in work related microwave components most of which used 
SMA connectors.  The early ones used the solid center conductor of the 0.141" 
coax as the center pin and so the male was just the nut and sleeve.

I'm not sure if it was because of that, the ground connection being the 141 
shield butting up to the reference plane of the female connector or something 
related to the female center contact that caused the spec for maximum number of 
mating cycles to be a 2 digit number.  This made a big problem for satellite 
components which must use connector savers with a log of every connection and 
of course always done with a torque wrench.

Are modern SMA connectors specified for more matings?

Have Fun,

Brooke Clarke

Didier Juges wrote:
> Hi Chuck,
> You may be correct, the deformation is not obvious when looking at it with
> the naked eye (never actually measured it, but you prompted my curiosity, so
> now I will have to check :-) That may explain the relative vulnerability of
> these connectors.
> Regardless, the manufacturer of the type of crimp-on SMA connectors we use
> does not recommend their usage above 18 GHz. They do show strange VSWR
> patterns above 18 GHz. I have a particular narrow band product operating
> just above 21 GHz where I was planning to use them (I thought it would be
> close enough), but it did not work well enough. On the other hand, the
> equivalent soldered type is specified up to 26.5 GHz. I know the standard
> SMA spec is only going to 18 GHz but a number of vendors offer 26.5 GHz
> rated SMAs. There may well be 26.5 GHz rated SMA crimp connectors, but not
> from our regular vendor and we do not use them. As a matter of fact, for
> most everything I build that operates above 18 GHz, I prefer buying pre-made
> cable assemblies. Considering the cost of connectors, cables and tooling,
> it's not that much more expensive. For reference, the 21 GHz cables we buy
> are soldered, not crimped, and they work much better than those we made.
> Didier KO4BB
> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
> Behalf Of Chuck Harris
> Sent: Saturday, July 28, 2007 1:09 PM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] HP 5370B low frequency modulation
> Hi Didier,
> I believe you are mistaken about what holds a crimp SMA connector together.
> The crimp force is entirely taken up in trying to constrict the
> circumference
> of the solid copper jacket.  Because a cylinder is one of those perfect
> structures discovered by the ancients, this crimping pressure doesn't apply
> a
> significant amount of additional pressure to the center insulator.  I have
> made
> numerous shielded opens for network analyzer use by crimping an SMA
> connector
> nut onto a piece of copper semi-rigid shield tube without using any center
> insulator, or center pin.
> The screw on BNC's that you can get at Radio Shack hardly qualify as a
> connector.
> -Chuck Harris
> Didier Juges wrote:
>>); SAEximRunCond expanded to false
> time-nuts-bounces+cfharris=erols.com+cfharris=erols.com at febo.com
>>I was referring to the crimp connectors like the SMAs where the cable is
>>press-fit in the connector. These work well at t=0 and in low stress
>>equipment, but are not too good in high vibration or thermal cycling
>>Also, I believe the deformation of the outer conductor where the 1/4"
>>semi-rigid cable is squeezed introduces impedance changes and reflections.
>>Since most of these cables have Teflon insulation, cold flow and thermal
>>expansion create additional problems. These problems are not eliminated by
>>using solder type connectors though, as I have seen a number of soldered
>>SMAs with cracked solder joint.
>>They are very easy to apply though, considerably easier than the solder
>>types, and you are less likely to let a badly crimped connector go through
>>inspection than a badly soldered one. Problems tend to be obvious. A cold
>>solder joint, or too much heat applied to the cable (melting the
> insulation)
>>while soldering are much harder to spot.
>>I have occasionally used crimp BNC connectors and they were as you
> describe
>>for the most part. Other than that, my personal experience is mostly with
>>solder-type coax connectors.
>>The cheap but not inexpensive screw-on BNC connectors you can find at
> Radio
>>Shack do not have an inner sleeve either, if I recall correctly. Not that
> I
>>would recommend using those for ANYTHING (well, I had to try them, so I
>>Didier KO4BB
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