Sporadic E:
This is a disturbance in the E layer, the lowest part of our atmosphere. It affects the entire Low VHF frequency range 30- 87.5 mhz. What happens is clouds of electrified gas enter this layer and reflect signals back to earth which would normally leave the atmosphere and head out into space!. The Sporadic E season in Europe is between Late April and September, usually peaking around June. It can occur at other times of the year as well, notably October, but this is the main period of activity. What happens is, you will start to hear short range skip on CB frequencies at 25/28 mhz, the ham 10 meter band (28mhz) will be alive and previously quiet frequencies at low VHF (30-87.5mhz) will start to become active with services from abroad. It is not unusual under these conditions to hear French PMR (35-36mhz) or Portuguese Fire brigades (40-41mhz), U.S. Forces in Europe are also often heard. As the disturbance or openings as they are known unfold, higher and higher frequencies will become effected. Hence TV broadcasts from Europe will fade up between 48 and 63 mhz and the East European FM band 66- 74 mhz will come in. Kossuth Radio in Budapest on 67.4 WFM is a regular during the summer!. Openings can last from minutes up to several days of serious activity. Reception distances for Sporadic E are typically between 500 and 1500 miles, although greater distances are possible under rare circumstances. Sporadic E can happen just about any time of day but night time is less likely, midday and about 6-8 pm I have observed to be busy times. For more info on what is possible please check out the link to the VHF Skip group at the bottom of this page.
F1 and F2:
Again here we find the atmosphere effected at the F1 and F2 levels 200 to 400 km above ground throwing signals back to earth. This is the main wellspring of major fun at Low VHF!. The cause of this mode of propagation is the same one that effects signals on HF, and that is the sunspot cycle. The sun goes through a cycle which lasts 11 years and effects radio communications on Earth. It has a peak and a trough, and at the moment, 2002, it is likely that we have just passed the second peak of the current cycle. So the likelihood of F1 and F2 skip is very good. In the Northern Hemisphere the time to watch out for this stuff is late September until April with peaks in November and January. Frequencies effected are usually 30 to 50 mhz although it can reach 70 mhz on rare occasions. You will hear public service transmissions from literally right around the Globe!. I personally have managed to hear the California Highway Patrol a mere 6000 plus miles away. The range 30- 50 mhz throws up some great reception during winter with very strong U. S. fire comms between 33.42 and 34 mhz in 20 kc steps.F2 reception starts around dawn or shortly thereafter and concludes after sunset, reception moving from east to west as the day progresses. Check out the VHF Skip group for more information.
Tropospheric enhancement:
This mode of propagation is much more commonly reported at high VHF or UHF frequencies although I have personally noticed its effects as low as 27 mhz- So It will effect Low VHF, particularly in its upper reaches above 50 mhz. Watch out for distant services which are normally not received at your location. In areas where Low VHF is in more use tropo will obviously be much more easily noticed. In Ireland and the U.K. most services are however above 68 mhz. On several occasions I have received signals around 80 mhz from distances in excess of 200 miles. Very high powered Low VHF transmitters in countries like the United States must get out very well via this mode.
Tropo occurs when cool and warm air meets about 2 km above ground. A conductive layer is formed which can act as a kind of pipe for distant signals to travel through. Distances achieved can be up to hundreds of miles. Look out for slow moving high-pressure weather systems, these often bring enhanced reception. Often the best time to check is when the air pressure has been high for a few days and then it suddenly starts to drop. Other good results are found on a cool night after a very warm day. Some of the best Tropo results are achieved near the sea and there is some evidence that sea spray getting up into the upper atmosphere can cause this mode of reception. Tropo can be forecast with a degree of certainty- the link to anticyclonic weather systems being proven. Tropo can occur any time of day but seems more likely at night and early morning after dawn. The seasons of Summer and Autumn also seem most prone to this reception. |
