> but I think that skip is significant at the edges of the > coverage area. THIS is the area that the old-timers call the 'fading-wall', where ground wave and sky-wave meet and alternately interfere (cancel) or reinforce each other. This has also got to be a factor in experiences by some people some distance away from Colorado (note: those charts ALSO show a fairly strong level of field stength where the 'red' ends!). If you look at those 'prop' charts - ALMOST the entire continental US is covered during the day (again, to some arbitrary level) - with considerable extension over water at night (along with a small portion into the NE). But again, NOTE the signal level at which those charts are plotted at too (life, and signal strengh, does NOT simply end when the 'red' runs out!). Compare the coverage for any AM broadcast station - or even longwave station (200-400 KHz) and the usable coverage area comes nowhere as close as the propagation for 60 KHz does (during the day esp.) and that's because of the **excellant** ground-wave propagation that 60 HKz enjoys. Ground wave is *the* name of the game at 60 KHz - otherwise you would have *no* usable signal strength roughly 5% beyond line-of-sight ... this is also the reason why a 5,000 watt AM radio station 150 miles away is easily receivable on a 2" long ferrite loopstick YET a 100,000 watt FM station is not - ground wave is the answer ... RF Jim ----- Original Message ----- From: "Sean H. Breheny" To: Sent: Tuesday, December 17, 2002 4:24 PM Subject: Re: [PIC]: How to obtain the official US time clock? > Hi Jim, > > Your explanation makes sense and perhaps I am wrong, but I think that skip > is significant at the edges of the coverage area. If you look at the plots > of signal strength vs. location on WWVB's site you will see some ridges > (parallel regions of high strength with regions of low strength between > them) that look awfully characteristic of skip near the edges of the > coverage area. In addition, the area changes shape and size significantly > throughout the day. Have a look at > http://www.boulder.nist.gov/timefreq/stations/wwvbcoverage.htm > > Sean > > At 12:28 PM 12/17/2002 -0600, you wrote: > > >Signal propagation at these frequencies is primarily via > >"ground wave" - with little propagation via 'sky wave' as > >one is accustomed to experiencing at night on the AM broadcast > >band ... > > > >More likely your clock sets itself at this time (night) versus > >continuously running the VLF receiver (and thereby draining > >the battery) OR there is less interference from man-made > >devices at this time (TV sets, standard video display monitors, > >etc). > > > >The over-earth path loss at 60 KHz is around 120 dB. > > > >Given WWVB's approximate 50 KW transmit power (and assuming > >through antenna efficiency values for simplicity at 50%) the > >resultant received values are [1]: > > > > Field strength at receiving site 1.56 micro-volts per metre > > 50-ohm matched receiver input 981.52 micro-volts > > Receiver S-meter reading S9 + 26 decibels > > > >Real-world receive power-value figures will be much lower due > >the ferrite rod antenna normally used to receive the 60 KHz > >WWVB signal. > > > >RF Jim > > > >[1] Program name: GRNDWAV3.exe > > Author: R.J.Edwards G4FGQ > > (C) 4th July 2001 > > > > -- -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu