Saturday, October 25, 2008

Daylight DX Time Limits

Some time limits should be applied when searching for daylight DX in the AM broadcast band. In my experiments, 2.5 hours is about as close as you can get to sunrise and sunset times before the band starts to change dramatically. The sun is setting here currently at 5:43 PM. I find that by 3:30PM the characteristics of the band have shifted somewhat toward longer range DX as late afternoon approaches. Morning sunrise here is 6:42 AM, and by 9:30 AM we are back to daytime conditions again. So, for purposes of tallying daytime DX on the AM band, and adding in an extra 30 minutes for safety padding, 3 hours after sunrise and 3 hours before sunset will be the start and stop limits for my DX tests.

Several stations have been received during daylight hours in excess of 500 miles. Stay tuned.


Stephen said...

I realize I'm a bit late commenting on this (maybe you get notified when someone comments?), but I'm finding that a 2.5 or 3 hour guard zone after sunrise / before sunset often isn't nearly enough. For example, the upper few hundred kHz or so of the AM band has been active in the noon hour the past couple months. Signals I've received in broad daylight using the Tecsun PL-606 and Select-A-Tenna include:

1680 KGED Fresno, CA (also possible, but unconfirmed reception of KNTS Seattle, WA)
1670 KHPY Moreno Valley - this one's fairly strong even using the radio's built-in ferrite, but does have the periodic fades associated with skywave, like the others do too. Normally this is not usually receivable in the daytime in summer without using the SAT, or is extremely weak.
1580 KMIK Tempe, AZ is often audible when I null KBLA Santa Monica, CA. (At night, KMIK is usually the strongest skywave signal, sometimes hitting 70dBu on the PL606's display using its internal ferrite, 80+dBu with the SAT, and 91dBu when adding a utility pole grounding wire.
1570 KPRO Riverside, CA - yes, this is audible here too at noon, even though their pattern would seem to indicate otherwise. ;)
1530 KFBK Sacramento, CA, has even made a couple noontime appearances, but this one's more rare for some reason.
Also stronger than normal (or being received when they shouldn't be) are 1650 KFOX Torrance, CA, 1600 KAHZ Pomona, CA, 1560 KNZR Bakersfield, CA, 1540 KMPC Los Angeles, CA, and most of the graveyards except 1450 KFSD and 1240 KNSN have multiple stations on their frequencies.
Also there've been unID'd stations on 1520 (not KVTA), 1510 (not KSPA), just to name a couple.
All this has been at around the noon hour.
I've even noticed 1290 KKDD on top of the frequency around noon, whereas normally on summer days KZSB owns the frequency here thanks to a saltwater path, in spite of being only 500 watts at 195 miles.

Considering the midday skywave I seem to be experiencing even as far south as 32°45'40"N latitude, do you have any suggestions on how to ensure, when I'm doing midday DX tests, that I'm only receiving groundwave? Should I wait until closer to summer, maybe?

Radio-Timetraveller said...

Hi Stephen,

Thanks for your comments and considerable report.

2.5 hours I find is bare minimum, and moreso during the winter DX season. I find the band changes really all day long. Some DX heard at 11 AM, for instance, is almost inaudible at 1PM. The deadest time I find to be between noon and about 1:30PM. I try to do all my DX testing between noon and 2 o'clock.

Groundwave can be detected fairly well this way. Tilt your receiver to vertical and see if the station's signal almost totally disappears (won't work for a powerhouse station too close by). If you still have an appreciable signal, you have some skywave mixed in there.