23/02/2013 16:39

Shortened version of article “Ionogram” (originally written in Czech).


Lots of Radio Amateurs have connection to the Internet nowadays. The Czech Institute of Atmosphere Research publishes topical Ionograms (every 2 minutes) of its Pruhonice u Prahy Observatory at IP address .  That information can be used for a basic view of the Ionosphere.

The heart of the matter of obtaining the Ionograms is transmission of tuning radio signals (frequency 1 to let say 15 MHz) vertically from the Earth surface and their reflections computer interpretation (reflections from E and F Ionosphere layers).  

For readers (who needs it): Basic information on the Inosphere can be obtained for example at /1/. Information on Ionosphere Radars can be obtained for example at /2/. You can get  ionograms from other Observatories at /3/.

Now I would like to add that the Research of the Ionosphere is a very difficult job. We (HAMs) can only try to use some outcomes of that.

An Ionogram is shown in the picture below.  There is  information on a place, date, time etc. on the highest part of the Ionogram. Red line represents ordinary wave. Green line represents extraordinary wave. Now only very briefly about that: A radio short wave is divided in the Ionosphere into an ordinary and an extraordinary wave. The ordinary wave is reflected without any change. The extraordinary wave changes its characteristics because of the Earth Magnetosphere impact.



The Height above the Earth surface (kilometers) is represented on the vertical axis. The Frequency of the transmitting radio signal (MHz) is represented on the horizontal axis.  

I add that abbreviation N/A means not available data.

The height up to about 430 km is important for us because it represents real layers of the Ionosphere. The black line (something  like ”lying mountain”) shows the concentration of Ions depending on the height above the Earth surface.

Now we will describe data in the left side of the vertical axis. We must define Critical Frequency f0 (in this case the ordinary wave – 0) at the earliest. This is the maximum frequency of vertically transmitting radio signal which still comes back to the Earth after its reflection from a respective layer of the Ionosphere. We can see following pieces of information:


f0F2   7,85       ………    Critical Frequency of the F2 layer

                                        is 7,85 MHz

f0E     2,36      ………     Critical Frequency of the E layer is 2,36 MHz

fxI      8,6        ………     Critical Frequency of the extraordinary

                                          wave (x) is 8,6 MHz

f0Es   2,9       ……….    Critical Frequency of the sporadic E layer

                                         is 2,9 MHz

fmin  1,6       ………..    Minimal Frequency of the transmitted

                                        radio signal is 1,6 MHz


Below another horizontal line is information on MUF (maximum useable frequency) for Short Wave Radio Communications between transmitter and receiver located on the Earth surface with mutual Distance D (here D=3000 km).


MUF(D)   29,16  …………..  MUF(3000) is 29,16 MHz

MUF Factor

M(D)        3,71   …………… M(3000) is 3,71


This relation is valid:

(1)     MUF = f0 x M


Remark: Distance D is mentioned as N/A there. It should be D=3000 in my opinion.

The other important part of the Ionogram is a “lying tab” on its low side. It represents relation between distance D and MUF (D=100 to 3000 km). Please see also relation (1).

The Ionograms can be useful especially for NVIS (near vertical incidence skywave) radio communications - antennas with high vertical angles of radiation. It can be used for the choice of  a Frequency Band for the Short Waves Radio Amateurs – for local or continental QSOs / Contests or for QRP HAMs. The Ionograms can be also used for the first look at which band is open (MUF).



/2/      World Data Center for Solar-Terrestrial Physic, Report UAG-          23A (sec 1 to 4),Handbook of Ionogram,NAS,WDC-USA,1978