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Old 07-17-2018, 10:51 AM   #1
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Default I have a question


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.

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Old 08-15-2018, 10:19 AM   #2
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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.
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