Wireless Diagnostics: what does "_handleLinkEvent: Got an error trying to queyer WiFi for power. Resetting state variables" mean?
Example on my MBA 10.9.3

6/18/14 01:48:14.000 kernel[0]: AirPort_Brcm43xx::powerChange: System Sleep 
6/18/14 01:48:16.000 kernel[0]: AirPort: Link Down on en0. Reason 8 (Disassociated because station leaving).
6/18/14 01:48:16.579 airportd[80]: _handleLinkEvent: WiFi is not powered. Resetting state variables.


So that is a normal message following a drooped connection or closing the lid.

Open your console and report the lines before that message to see who is shutting your airport down.

Run netstat -i to get a overview of all ongoing activity

sample

Name  Mtu   Network       Address            Ipkts Ierrs    Opkts Oerrs  Coll
lo0   16384 &lt;Link#1&gt;                          7925     0     7925     0     0
lo0   16384 localhost   ::1                   7925     -     7925     -     -
lo0   16384 127           localhost           7925     -     7925     -     -
lo0   16384 localhost   fe80:1::1             7925     -     7925     -     -
gif0* 1280  &lt;Link#2&gt;                             0     0        0     0     0
stf0* 1280  &lt;Link#3&gt;                             0     0        0     0     0
en0   1500  &lt;Link#4&gt;    14:10:9f:f0:29:8c   830672     0   665716     0     0
en0   1500  mba-host.lo fe80:4::1610:9fff   830672     -   665716     -     -
en0   1500  10.5.50/24    10.5.50.95        830672     -   665716     -     -
en3   1500  &lt;Link#5&gt;    32:00:1a:e9:a9:a0        0     0        0     0     0
bridg 1500  &lt;Link#6&gt;    16:10:9f:0f:1e:00        0     0        0     0     0
p2p0  2304  &lt;Link#7&gt;    06:10:9f:f0:29:8c        0     0        0     0     0


Run Airport -I to get more information on the current connection.

sample

agrCtlRSSI: -33
     agrExtRSSI: 0
    agrCtlNoise: -86
    agrExtNoise: 0
          state: running
        op mode: station 
     lastTxRate: 130
        maxRate: 144
lastAssocStatus: 0
    802.11 auth: open
      link auth: none
          BSSID: 9c:d3:6d:b3:75:f0
           SSID: Mijas900850808-80_EXT
            MCS: 15
        channel: 8


For debugging and monitoring you can use the build in tool.

Hold the alt key and click on the WiFi icon

at the bottom of the pop up you will find the Open Wireless Diagnostics

In the new window that opens select Utilities in the Window tab.

Run test like Performances to see the signal stability