When I purchased my Inverto Black Eco LNBs, they came with a rubber tube cover for the f-connector port, are these just simply to protect this during transport? or are they meant to be on all the time as there seems no way for this to happen when in active use.
The vast majority of such caps are merely for protection during transport as, apart from being "blind", they are not long enough to be fitted over the F-connector on the cable and still cover the exposed threads on the connector on the LNB itself.
However, you can buy various types of F-connector boots which are designed for that purpose (
such as these), but be aware that not all of them will actually fit "all" the various types of F-connector, and moisture can thus still seep in.
Personally, I've become a bit of a fan of
this this sort of stuff from Plasticote as you just tighten the connector on the cable on that on the LNB, and then spray this stuff over it - also get's right into the spaces between LNB's on twin/quad etc., LNBs and DiSEqC switches, and it is easy to remove when needed.
BTW: I also have some similar "brush-on" stuff but found that far less easy to ensure that everything well-sealed.