There have been many theories put forth as to what causes this effect, and I am not sure if anyone actually has it nailed down yet. What appears to happen is that clouds of high ionization in the ionosphere's E layer form and allow signals of much higher frequency than would normally be reflected by the F layer to be propagated. Sometimes it is less important to know the scientific or physical reason something happens than it is to just know its there and use it!
Early in the month of June I had begun noticing some of the 6-meter amateur beacons from the Central US and Northwest starting to show up. I find most of them in the 50025-50075 khz range. They are often very weak and one must tune carefully to find them. However, once the band fully opens, they can be quite strong, often S-9 or higher, despite their power being relatively low, often in the 1-5 watt range.
That is one thing I have noticed about Sporadic E skip: power levels can be quite low and still a strong signal be delivered. Back in the 1960's when most 6 meter radios were low powered, there was considerable DX worked. I myself used an AM transceiver that only put out about 6 watts and worked several states and Caribbean countries with a simple antenna in 1965-67.
There were also many adventures with TV DX over the years. Now it is somewhat more difficult because of the advent of digital television. One must have a TV or converter box that can be force tuned to a given RF frequency rather than just scanning for signals to program in. There is also the difficulty in noticing if there is even a weak signal to be worked with unless the TV or converter box has a feature for seeing signal strength either on the box itself or displayed on the tv screen.
There are times frequencies reflected in Sporadic-E prop reach even higher frequencies. There are times FM broadcast stations will skip halfway across the country. Since the prop is frequency dependent, this will show up first on the lower frequency end of the band, in the United States, this will mean the non commercial or educational stations will be affected first. Listen for unusual heterodyne beats down in this part of the band and if heard, tune carefully. I remember once hearing a station from Minnesota coming into Texas beautifully in the car.
Sometimes Sporadic E will appear to “ bend” or come from a direction other than that from which you know the station is broadcasting. Sometimes this can occur if there is more than one “ hop” or the signal is reflected from more than one cloud of high ionization. Sometimes, if the effect is noted fairly early in the morning, it might mean a combination of things are happening. Perhaps the signal has hit the Sporadic E cloud after already having an extended trip as a result of a temperature inversion.
This time of year when there might be a cool night and the temperature rises rapidly in the morning, these inversions can themselves bring extended signal range, though not as far as Sporadic E. The air is not warmed by the sun, but rather by the earth which is first warmed by the sun. The boundary layer between the warm air at the surface and the cooler air above results in a bending effect. That boundary will rise as the morning wears on, stretching the range farther and farther until the critical angle for refraction back to the surface is reached and the band appears to “suddenly” go dead. However, if this refraction is still present, that signal might continue up above the surface and if it runs into a Sporadic E cloud along the way, it will be reflected back down with interesting results. I have seen interesting things happen when the temperature inversion is to the west of the sporadic E opening. Right in the middle of stations coming in from the Midwest into Texas by Sporadic E there would suddenly appear one or maybe two stations from the West or Midwest.
The other thing that is noticeable about Sporadic E prop is that beam headings often don't seem as sharply defined as during tropo or regular ground or space wave prop. Sometimes this is more a product of the antenna's vertical pickup pattern at various angles of arrival. Often Sporadic E signals come down at a rather steep angle, depending on the distance away and the height of the cloud. The signals are striking more the top of the antenna than the front.
One other thing about DX-ing Sporadic E: high antennas are not necessary. For weak signal extended ground wave and tropo work, high antennas are needed because of the near line of sight effects of the signals in the high VHF and UHF ranges. Height is distance. But since the Sporadic E signal is coming down from high in the atmosphere, the height of the receiving antenna above ground is less important. I have had good results with antennas no more than fifteen or twenty feet high! As long as the antenna is fairly well in the clear and does not have any major obstructions directly in front of it, you will find that height is not critical at all!
In a similar vein, lower gain antennas are sometimes an advantage in that they might have a broader vertical angle response. I once many years ago went to a lot of trouble to wide space stack two five-element yagis for the lower television channels hoping to increase my DX catches. While the effort did result in great increases in signal strength and day to day picture quality on TV stations at extended range during normal conditions, I found things were actually diminished when it came to Sporadic E because the received angle response was too low and too narrow! Sometimes less is more.
How far can Sporadic E signals go? I really don't know. There are cases of Transatlantic reception. This can be the result of double hop prop, but then again, it might be the result of some really unusual F layer skip.
How do you know when to look for DX via this mode? Sometimes its a matter of just listening. There might be some indicators. If you have a receiver capable of receiving the six meter amateur band, the best way is to look for the beacons. The internet has several listings of beacon frequencies, callsigns and locations. Checking the low TV channels for stations on channels normally blank is another, though this is more difficult in the digital age. ( perhaps that would be a good subject for another article, how to DX and search for digital TV signals...I will need some help from others on that one...if you have experience, drop me a note through “ comments”). Listening in the 30-50 mhz public service band for unusual signals is another good way, as is tuning through the lower end of the FM broadcast band looking for “interlopers”. Another way is to look at DX Summit on the internet and select “ 50 mHz” and see if there is any activity being reported.
I have been logging stations via Sporadic E for over 50 years and there are many aspects of it that just seem to have to be learned with experience. Sometimes its a matter of getting a feel for how things are working. All of my experience has been in North America. I would love to hear from any readers with experiences to share from other parts of the world, either SWL or ham. If you have such please drop a note to comments with your email address and I will respond with an address to which to send them.
This year, during the Amateur Radio Field Day event, I decided to do an all VHF operation. I will be posting the results of that shortly. Good DX!