On 12/5/22 2:06 AM, Jim Brown wrote:
On 12/4/2022 11:07 PM, Michael Tope wrote:
I have seen it used that way in all the space hardware that I've
worked on
Mike is part of the team that put our rovers on Mars, and Jim Lux is
part of the same organization. I know he's done some pretty impressive
stuff too.
Yeah, but I've also had SMA connectors come loose (fortunately not in
space, that I'm aware of). I've had to re-run a multiday test though.
Mike's comment about big cables and tiny connectors is really
important. You'll see LMR-240 kinds of coax - stiff, heavy. I don't
think that's what SMA was designed for - it's more like 0.085 semirigid
and tenth inch coax like RG-174.
We also spent months tracking down an extra 0.1 dB change in the gain of
a radar receiver over temperature (the gain changed several dB, but
should have been a nice straight line, but wasn't, there was a step at
around 20C - and no it wasn't the PTFE step). We finally figured out
that it was a SMA that wasn't torqued correctly on the input to a
Ku-band LNA, and the tiny change in match due to CTE mismatch in the
semi rigid cables moved it just enough. It flew, since the system
calibrated itself - but the concern was "what if it's 0.1 dB today, and
10 dB in flight". The connectors were all staked with a blob of
something, and it was buried so deep the risk of breaking something else
was higher than the risk of an idiosyncracy turning into something more.
I think anyone who has done precision RF measurements has had similar
experiences. There's a reason you see people tape the cables down to the
bench. That, and keeping the big cable from pushing the tiny hardware
onto the floor. Hooking up a NanoVNA to 1/2" LMR-400 with adapters is
always sort of a tail wagging the dog exercise.
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