Which adapter/handling to choose on (Type N) rf test devices when SMA/K cables have to be connected frequently?
Now the probe vendor says that SMA cables/connectors damage the K-connector on rf-Probe.


Probes are very fragile and could be damaged by a very small error in operating them. They should practically be treated as consumables, and if you're using them you should budget replacing them occasionally.

So what I'd do is buy a set of probes with SMA connectors, and use them with your SMA cables. 

Save the expensive K-connectorized probes (and the risk of damaging them) for the times when you need the 40 GHz bandwidth and are using K-connectorized cables.


  The thing is also we have to compare measurements with same frequency response between research groups and in best case very low losses. If I have to use K-probes 1-10 GHz (they are set to 40) with max. 3 GHz test devices what do you suggest?


If you're okay with the risk your K-connector probes will be damaged, just go ahead and use them with SMA cables. 

An SMA plug with nominal geometry will not damage a K jack. The risk is that the SMA standard allows looser tolerance on the center pin length, so that an in-spec SMA plug with a maximum length center pin can damage the socket in the K jack.

So, to reduce the risk, you can use new SMA cables from a reputable vendor. These are unlikely to damage your K connectors (though I can't absolutely promise they won't). If your lab has an SMA connector gauge, you can probably check which of your cables have relatively long center pins and avoid using those with the probes.

If you can't take that risk, then all you're left with is buying some SMA-K adapters and using them to connect your cables to your probes.