work on docs

docs/c-netz.html

@@ -373,6 +373,13 @@ When we make a call and another phone is regisered, the phone will loose coverag
 After the call, the other phone re-registers again.
 </p>
 
+<p>
+You should tune the receiver to 465.090 first, to check if you can hear and decode the idle signal from the base station.
+Then tune to actually uplink frequency 455.090 MHz.
+The actual level is not yet relevant.
+(You may check the quality using '-L 2' command line option and build a radio loop by tuning the receiver to the transmitter.)
+</p>
+
 <p>
 Now power up a phone.
 Depening on your card reader, insert chip or magnetic card.