Propagation Modes

This page is a discussion around the different propagation modes that effect VHF radio signals

  • Tropospheric Ducting


    This propagation produces strong constant signals with minimal fading. These signals can remain present for hours.


    This mode is caused by a temperature inversion. A temperature inversion is when the air above is hotter in temperature than the air below

    Scenario 1 Scenario 2
    Upper Troposphere Upper Troposphere
    Warmer & Reduced Humidity Normal Temp
    Colder & Increased Humidity Warmer & Reduced Humidity
    Warmer & Reduced Humidity Colder & Increased Humidity (Fog)
    Ground Level Ground Level

    It has been noted that relative barometric pressure needs to be above or equal to 1020 hPa for tropo to appear

  • Sporadic E


    The signals reflected via the E layer often are over in a matter of minutes. These E signals build into a strong signal and then fade back as quickly as they arrived.

    The science in the section is based on observations of patterns of external influences which might be triggering sporadic E


    Noteable Sporadic E times during May to Sep in the day are:

    • 08:00 - 10:00
    • 12:00 - 14:00
    • 17:00 - 19:00

    Can be triggered by intense thunder storms. The mid point between 2 DX stations is often where the thunder storm is. In the image below X's marks the thunder storm activity


    Can be triggered by a sudden rising of barometric pressure. Sudden rising such as a difference of around 20 hPa in 2 days.


    Can be triggered 24 hours later by an Earth facing C class solar flare

    Seems to also be better when there are no Sun spots

    Seems to happen within a couple of hours of a solar flare

    Seems to happen when there has been several days of no solar activity and suddenly a sun spot with crackling magnetic fields appear over the edge of the sun

    Waves of ionisation across Europe caused by solar flares seems to result in Sporadic E

  • Aurora E


    Signals are similar in characteristics to Sporadic E except the signals flutter in strength as apposed to fading in/out


    Solar flares from the sun! energising the E layer