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