Hi Chaps - I saw the discussion on the modes and losses and wanted to add a bit -
When looking at WWVB 60Khz (close to 74?) contours for daytime I notice more of a Vertical peanut shape in the fiedlstrength favoring North South, with highest losses over Mountaineous terrain and likely the lowest ground conductivity, say between 0.5 and 2 milliseimens - something we see (in general) afforded by ground to the West of Ft Collins (especially NW) and to the Eastern mountain ranges, and its not too Bright on most bearings from here in Alaska as we have a lot of the pointy high snow covered thingies up here too. I see they plonked WWVB there due to having local high Alkalinity of the soil - high conductance.
Whether given the height of the mountains even at 60kHz we loose a bit more than flat Earth Id say a whopping Yes, that and some of the oddities that ground waves going over glaciers can have all add up to further attenuation.
Have a looksee at
http://www.fcc.gov/encyclopedia/m3-map-effective-ground-conductivity-united-states-wall-sized-map-am-broadcast-stations If I look at how /4 gets up here - it would in reality have to be a fairly high angle reflection, as Im close in to the Chugach and Talkeetna Mountains and those mountain ranges extends hundered of miles on the /4 bearing. - I see no signal at all (ok I wasnt using OPds) and even with a reasonable antennae and good s/n its a sky wave opening only - well, at any reasonably power that we can generate. Whereas he was strong in CA last night he was only just detectable up here - though I blame the Auroral ovaly stuff too for that.
Interestingly enough and with the limited amount of data - if Bob was to generate another 3dB he would have probably been visisble at dot 60 every night to date over the 5000Kms path or so , ie a lot less variation on a day to day than I see at 137.
On groundwave on 137 I did some reasonable tests and with my 3W ERP (ish) in the main loop on a quiet iono day in Autumn maintaining a CW just read level of some 1000Kms down to Corpus Christi from the OKie/Kansas Border and but out West was considerably poorer - we had poor conductivity to the West of us for a while....
I saw with interest on the NIST web site the scalloped nature of signal reception at night at 60KHz at some times of the night.
http://tf.nist.gov/images/radiostations/wwvb-large/0800utc.jpg Cheers from a soggy snowyish Anchorage.
