Despite forty years working for radio stations, admittedly mostly FM, I never heard of “critical hours” until messing with the Part 15 hobby. Those are two hours after sunrise and two hours before sundown when the sun angle is relaxing and the ionosphere begins to lift, causing certain stations to skywave across the horizon only to vanish. Critical hours seem to be especially effective on the upper AM band. My main frequency of 1550 KHz is clear daytime and after dark, but during the critical hours no less than three stations come to life at various power levels depending on the day and weather. They all have daytime power of 5 kw, and come from Cape Girardeau, Springfield and Kansas City, all in Missouri. Perhaps they only vanish at night because they drop to a few hundred watts. So I am planning to add a timer which will switch up to 1640 KHz during the critical hours.
At 1640 KHz there’s also somebody two states away that creeps in during the critical hours, but the higher the frequency the more brief is the appearance and disappearance of the distant signal. Sometimes a signal will jump from total static and become as loud as a local, only to vanish within another fifteen seconds.
Dusk/Dawn Propagation
Most times it is hard to predict what exactly will happen on a given frequency at at given time regarding “skip”.
Though this doesn’t pertain to MW AM broadcasting, I had an experience on the ham band a while back which was, to me, unexpected. After dark, I like to tune around the 15 and 10 meter bands since sometimes locals get on to ragchew. This silliness resulted in the creation of an informal net in the Cincy area on 15 m / LSB just to be different. Using LSB on 15 attracted a lot of local attention. One night while snooping, after sundown, I heard a CQ on 15 meters and answered. It was a lady who had drawn night watch on a small vessel in the Atlantic Ocean and who was just passing time on the radio. We QSOed for about 45 minutes Q5. Then, suddenly as if a plug were pulled, the QSO was over. That was a great experience for me. For those who don’t know, 15 and 10 meters are “useless” for DX after dark because the skip is not available so this was a rare contact.
When I was a kid, just after radio was invented, I used to broadcast band DX with my trusty White’s radio log as a guide and an “all American five” receiver as equipment. It was amazing what could be heard then. Now I DX here on the internet. Though not as much fun, it is nice to meet and converse with some great folks.
Neil
Greyline propagation
Thats called greyline propagation,neat stuff! I had some very rare coditions one winter evening while also talking on 10 meters to the locals,a Long Delayed Echo. Not all of us were hearing it,those with vertical antennas heard it best,dipoles didnt hear it at all,beam antennas could not determine which direction it came from,it seemed like it was everywhere,but it was very short-lived,gone in about 20 minutes. This was several hours after sunset and so very rare.
I also get amazing ducting and inversion here on lake Erie when you can work a station hundreds of miles away with one watt on 6,2,or 70cm. One morning I awoke to hear a qso on my handheld I keep on my nightstand,it was Detroit a repeater on my 2 meter repeater frequency,capturing my repeaters transmitted signal of 100watts RMS,500wattsERP in the next room! And yes I talked to them as well,from my bedside one watt handheld inside the house! Many local FM AND TV stations also get captured during a good inversion,even the 50,000 watt flamethrower!
Temper all that with our cold winter when we cant get a 2meter signal 28 miles across Lake Erie to Canada…