[Elecraft] Critical frequency

Sat Nov 23 19:38:00 2002

Being a curious sort I looked up critical frequency.  This is what I found:

Near-Real-Time F2-Layer Critical Frequency Map

The following image is a recent high-resolution global map of F2-layer critical frequencies. This corresponds to the maximum radio frequency 
that can be reflected by the F2-region of the ionosphere at vertical incidence (that is, when the signal is transmitted straight up into the 
ionosphere). It is also a map showing the current location of the auroral ovals, the sunrise/sunset terminator and the regions of the world 
where the sun is 12 degrees below the horizon (which estimates the gray-line corridor where HF propagation is usually enhanced). This is 
one of a plethora of constructable maps that is produced by PROPLAB- PRO Version 2.0 (formerly known as SKYCOM PRO), a very 
powerful radio propagation software package for IBM or compatible computers, ideal for amateur or professional radio communicators. 
Instructions on how to use this map follow below.

This was found on : http://www.spacew.com/www/fof2.html

   73,
        Kevin.   KD5ONS

11/23/2002 5:25:35 PM, "Ron D'Eau Claire" <[email protected]> wrote:

>"Critical frequency" is defined as the lowest frequency at which a
>signal beamed straight up will still be refracted straight down by the
>ionosphere.  Signals beamed straight up which are at a
>higher-than-crticial-frequency will penetrate the ionosphere.
>
>The critical frequency varies with time of day and solar activity, of
>course.
>
>73, de Earl, K6SE
>
>That's a new one on me, Earl, since "refraction" is bending, not
>reflection. The less refraction, the further away the signal returns to
>earth. At some point between the amount of refraction that occurs and
>the angle at which the signal interacts with the ionosphere the
>refraction still bends the signal, but not enough for a return to the
>surface. That is what I have always understood is the "critical
>frequency". Signals at higher angles (including 90 degrees or 'straight
>up') will pass on through as well. 
>
>Ron AC7AC
>
>_______________________________________________
>Elecraft mailing list: [email protected]
>You must be a list member to post to the list. 
>Postings must be plain text (no HTML or attachments). 
>See: http://mailman.qth.net/mailman/listinfo/elecraft
>Elecraft Web Page: http://www.elecraft.com
>