"smf" wrote
What exactly does the /A\na on approach plates mean. I know it says
Alternate minimums not authorized. Does it mean the airport can't be used as
an alternate?

Not quite. It means that approach can't be used in determining
alternate minima for that airport.

There are three kinds of alternate minima (see 91.169c):
For a precision approach, 600-2 unless otherwise noted
For a non-precision approach, 800-2 unless otherwise noted
If no approach exists, weather conditions that allow descent from the
MEA, approach, and landing under basic VFR.

Under these rules, ANY airport (even one with no published approaches)
can be used as an alternate if the weather is good enough, and the
determination of how good the weather has to be (alternate minima for
the airport) is made based on what approaches are available.

The /A\na means that the specified approach is not to be used as a
factor in the determination. It doesn't mean the airport can't be
used as an alternate - other approaches at the same airport may not
have the same restriction, and in any case even if all approaches are
so restricted the airport can still be used as an alternate if the
weather is good enough.

I have actually seen one airport served by half a dozen approaches,
including an ILS, where all the approaches were noted /A\na except one
NDB approach. This means that rather than using the typical minima
for an ILS-equipped airport (600-2) the more restrictive non-precision
minima (800-2) must be used. Note that this does not preclude the use
of the ILS approach to get into the airport should you need to go
there.

Michael