I continue to receive comments and e-mail to the effect that the propagation changes are cyclic, part of the solar cycle.
Let me make it clear that I have been into radio since my elementary school days, I am 49 years old now. I’ve owned shortwave and AM receivers across that entire time frame, and AM/SW/TV DXing has been a hobby of mine also across that entire time frame.
Conditions are too noisy for effective DXing below the AM band in this area. There is a lot of power line harmonics creating interference to the long wave band. City Light, the power company here, seems to have a policy of replacing defective insulators only after someone is electrocuted or the pole catches fire. I have phoned in insulators visibly arcing over only to have the calls repeatedly ignored for months on end.
I don’t have the equipment for past the UHF television band or I’d be exploring frequencies below and above as well.
I am well aware of the solar cycle. People commonly refer to it as an 11 year cycle because peaks come, on average, every 11 years. It is really a 22-year cycle because the magnetic field reverses every peak and it takes 22 years to return to the same state and magnetic polarity.
The maximum usable frequency, that is the highest frequency which will be returned back to the Earth from the ionosphere generally follows three things; the solar cycle, the MUF is highest during solar peaks, the season, the MUF is generally highest in summer and lowest in winter, and daily, the MUF is generally highest in the daytime, particularly mid-day, and lowest at night.
So during solar minimum, during the winter, late in the night, is the time when the MUF would be the lowest, rarely above 6 Mhz or so under those conditions. I’ve been listening to a signal on 9484Khz now (9.484 Mhz) that has been coming in strong for quite some time tonight. On other nights I’ve been able to listen to signals as high as 18 Mhz. Several decades ago this would have been rare for this part of the solar cycle, this time of year, at night. Now it has become common.
When I was young, sporadic E-skip events that would bring the MUF up to television channel 2, 54-60 Mhz, was a relatively rare event except during the very peak of the solar cycle. Now the MUF reaching up to channel 10, 192-198 Mhz, during solar peaks, has become about as common as reaching channel 2-3 was several decades ago. During solar maximums, the MUF going above channels 2-3 has become so regular that it can hardly be described as “sporadic” E-skip anymore. There is nothing sporadic about it.
At the same time the MUF has increased, so has medium wave absorption, and AM stations that used to be easily and reliably receivable no longer are. AM stations in Vancouver BC, only about 100 miles from where I am, used to be easily receivable here, no longer.
Yes, these things do vary with the sunspot cycle, but superimposed on that cycle has been a steady rising of the MUF and AM absorption over the last several decades. The activity of the Sun also has increased, with the last solar peak being the most intense on record, but even during solar minimums, even during times when zero sunspots have been visible, the MUF and AM absorption have both been more than what they were when I was younger.
There are many potential factors; the weakening of the Earth’s magnetic field allowing the solar wind to interact with it more may be a factor. Changing atmospheric chemistry and resulting changes in refractive index of the ionosphere under given conditions may be a factor. Changes in the heat distribution in the atmosphere and resulting changes in refractive index might be a factor. Increasing solar activity over the longer term may be a factor. Artificially increased ionization by HAARP and similar activities may be a factor. The introduction of halides into the atmosphere, Bromine added as part of an anti-foaming agent in jet fuel, the solid rocket boosters of the shuttle having a chlorine compound, these may be factors. There are so many potential causes that I don’t know if it’s even possible to sort them out. There are probably more that I haven’t thought of.