Ionogram and frequencies

Discussion in 'Radio Propagation Questions' started by Claude Frantz, Aug 21, 2016.

  1. Claude Frantz

    Claude Frantz New Member

    Hello folk,

    The frequency scale of the Lowell Digisonde is usually starting at 1 MHz and ending between 10 and 16 MHz. The echoes of the F layer are often ending at frequencies lower than 10 MHz.

    Please explain me how such echos can serve to give us information about the propagation up to the 10 m band.

    Many thanks to you !

    Best 88 de Claude, DJ0OT
  2. Steve Nichols G0KYA

    Steve Nichols G0KYA Administrator

    Hi Claude,

    An ionosonde works by transmitting a radio signally vertically into the ionosphere and measuring what comes back. They normally sweep from about 1MHz to about 20MHz.

    At the moment, the F2 layer critical frequency (where signals just fail to be returned) is rising to about 5MHz over the UK during the day.

    Now, if we lower the angle of radiation towards the horizon we can progressively increase the frequency that will return a signal back to the ground, although that return will then be some way away from the transmitter.

    In general, the maximum usable frequency that will be returned from the ionosphere at a large distance (say 3,000km) is about 3.5-4 times the critical frequency.

    So if the critical frequency is 5MHz, you might expect a maximum usable frequency over a 3,000km path to be about 17.5-20MHz.

    The problem is that the ionosonde is measuring only what is happening above our heads and does not take into account the ionospheric differences in other parts of the world along the path of the signal, so this is only a guide. The maximum usable frequency at any time is likely to be different depending upon the direction the signal takes.

    But as a rough guide, to have an opening on 28MHz we might expect to need a F2 layer critical frequency of about 7-8MHz. Over the last few months Sporadic E has provided some 10m openings, but it may be a couple of months before we see many 10m F2 layer openings, and at this point in the sunspot cycle they are unlikely to be a) strong or b) frequent.

    I hope that helps.

    Steve G0KYA
    PSC Chairman

    PS For other readers, you can register (free) to use the RAL Chilton Digisonde at:
  3. Claude Frantz

    Claude Frantz New Member

    Many thanks for your excellent response, Steve. Yes, this help me very much.

    Just an additional question. Please explain me the extraordinary component and in which manner this information can be useful in amateur radio.

    Many thanks !

    Best 88 de Claude, DJ0OT
  4. Steve Nichols G0KYA

    Steve Nichols G0KYA Administrator

    Hi Claude,

    Any radio wave that is reflected/refracted back from the ionosphere breaks down into two separate rays due to the effects of the Earth's magnetic field. These are known as the ordinary (O) and extraordinary wave (X). You can clearly see this on an ionogram as the ordinary wave is red and the extraordinary wave is in green.

    In general, when talking about the critical frequency we tend to talk about the ordinary wave. But as a proportion of the radio wave's original energy is in the X wave part of the actual return is slightly higher in frequency. Marcus G0IJZ has written a very interesting paper that looks at this and its importance at 5MHz. I'll see if I can get a URL for it.

    Steve G0KYA

    Update: Gwyn G4FKH has a good feature on how to read ionosonde data - see
  5. Claude Frantz

    Claude Frantz New Member

    I thank you very much again, Steve.

    I would be very grateful for the URL of Marcus' paper on this matter. I will probably have further questions later. ;)

    Best 88 de Claude, DJ0OT
  6. Petr M0SIS

    Petr M0SIS New Member

    Hi Steve,

    did you found the URL for Markus, G0IJZ article please?
    Mni tks, 73 - Petr, M0SIS
  7. Steve Nichols G0KYA

    Steve Nichols G0KYA Administrator

  8. Petr M0SIS

    Petr M0SIS New Member

    Thanks Steve. Unfortunately I am no longer member of IEEE so no way to download it.
    73 - Petr, M0SIS
  9. Steve Nichols G0KYA

    Steve Nichols G0KYA Administrator

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