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Almost all of the microwave test equipment I've ever seen and used has N connectors, with exception of some scopes that use either custom fancy BNCs (keysight does/used to do this)) or SMA.

Exceptions that I know of are VNAs which seem to use special connectors (but the ones I've used are also rated up to 110 GHz and used 1mm-based connectors).

Of course, there is nothing wrong with the N connector - in fact I like it, it feels rugged and like it can take a good bit of use (is this the reason?).

However, every single occasion I've worked with the instruments, or seen someone work with them, the first thing they do is put on a N-to-SMA (or 3.4mm or 2.4mm) adapter. So why not just put those connectors on the equipment in the first place? Is it really just the ruggedness, some other reason I'm missing, or is this some silly "because it used to be that way and nobody likes change"?

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  • \$\begingroup\$ Isn't it better to have a standard and compatible connection on a similar equipment? \$\endgroup\$ – Eugene Sh. Apr 12 '17 at 17:13
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    \$\begingroup\$ Ruggedness is probably much of the reason: if you break an N-SMA adapter, you just need to replace the adapter. If you broke an SMA connector on the test equipment, replacement would be more painful. \$\endgroup\$ – uint128_t Apr 12 '17 at 17:17
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Two Reasons: Pout and S11.

N connectors have a large surface contact area for >1A power levels or >10dB and large diameter means better "potential return loss over a wide GHz range is possible with machining tolerances of 0.1% affecting this.

However SMA is more common for low power apps but quality varies with undocumented suppliers. (greatly and not gauranteed)

But never try to pump >>1 Amp thru 1u" flash gold plated SMA connector (crap...

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  • \$\begingroup\$ From looking inside a "N", there is a lot more metal devoted to precise positioning, to maintain a clean interface and a consistent mechanical ratio of inner/outer diameters, keeping Zo better controlled. \$\endgroup\$ – analogsystemsrf Apr 12 '17 at 17:47
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    \$\begingroup\$ True: skin effects and log Radius ratios affect return loss w.r.t. Zo \$\endgroup\$ – Sunnyskyguy EE75 Apr 12 '17 at 17:54
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N is simply bigger - as you said, ruggedness might be an important factor!

Now, most lab usage will happen at low power, but I've seen Spectrum analyzers that have inputs rated for peak voltages >= 100V– you wouldn't want that to happen across the air gap between center and outer conductor in SMA. So, in that aspect, I fully agree with Tony's answer.

Now, for things like network analyzers, where calibration is a time-intense, and often very costly, procedure, having a connector that you can very reliably fasten with a high, easy-to-measure torque, and that has large internal contacts leading to reproducible, and low, resistance, is relevant.

Having one common connector to "go smaller" from is probably also a good thing – screwing a SMA adapter onto a N jack is much less likely to damage the N equipment than attaching a half-rigid BNC or a heavy-duty N connector to a cute SMA connector.

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Ruggedness is huge (You would much rather break the adaptor then the connector on the instrument, as others have said), and actually a little imperfection makes very little different to transmitted power (Run the numbers if you don't believe me), so for a scalar instrument N is good enough to at least 8GHz or so.

Now a vector instrument (that is sensitive to phase and reflected power) needs something all together better altogether sooner, hence the wide assortment of APC and friends that you see (These are smaller so they avoid waveguide modes to a higher frequency, but also often less robust).

I would note that good quality adaptors are worth the (not inconsiderable) money even for lowish frequency work, and that fitting M/F 'connector savers' is usually worth doing for the the smaller instrument connectors (And I usually keep something even on N types).

Finally, watch out for the 50/75 ohm trap, SOME connectors are intermatable, some are NOT (Including N type, which can take damage if you get this wrong!).

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