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How to read an ionogram - PA9X
How to read an ionogram
How to read an ionogram
Posted on November 9, 2024 by Jean-Paul PA9X
When you first see an ionogram, it can look quite difficult to read. With all those numbers,
lines, and colored dots, you may not know where to start. So here is my guide to understand
what an ionogram tells you and how to use it for HF ham radio communication. It covers the
key components, the different ionospheric layers, and terms like critical frequency and
maximum usable frequency (MUF).
Listen to this article as a podcast (AI generated):
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What is an ionogram?
An ionogram is a graphical representation of the ionosphere above a specific location, obtained using
ionospheric probes (digisonde). It shows the heights at which various frequencies of radio waves
reflected in the ionosphere. By interpreting an ionogram, you’ll be able to find out which frequencies
are best for short and long-distance (DX) radio communication.
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How to read an ionogram - PA9X
How an ionosonde works. Image form ampledata,org.
The layers of the ionosphere
The ionosphere consists of different layers, each with its own properties. They are determined by the
density of ionized particles and vary in altitude depending on the time of day and llocation, the sun’s
position in the sky, solar activity (solar flux, X-ray flux, etc.) and geomagnetic activity.
1. D Layer (50 – 90 km)
The D layer is closest to the Earth and is present during the day. It absorbs lower HF frequencies,
which means radio signals are not well-reflected but dampened instead. By sunset, this layer
disappears, making lower frequencies work better for DX.
2. E Layer (90 – 120 km)
The E layer is located above the D layer and reflects lower HF frequencies, generally in the
range of 2-4 MHz. It’s mainly active during the day and largely disappears after sunset. The E
layer is useful for short- and medium-distance communication (up to several hundred kilometers).
3. Es Layer (Sporadic E Layer)
The sporadic E layer (Es layer) is an unpredictable layer that can appear randomly and often
reflects higher frequencies (up to 150 MHz). This phenomenon is seasonal and most common in
summer and mid-winter. The Es layer can be very useful for bridging (intercontinental) distances
that are normally unreachable on VHF frequencies.
4. F Layer (Above 150 km)
The F layer is the main layer for long-distance radio communication. During the day, it splits into
the F1 and F2 layers, both of which reflect frequencies in the HF band.
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F1 Layer (150 – 250 km):
The F1 layer is active during the day and reflects lower HF frequencies. At night, it merges
with the F2 layer into a single F layer.
F2 Layer (250 – 400 km):
The F2 layer is the highest and most important layer for long-distance radio communication.
IIt reflects frequencies up to 30 MHz (even up to 50 MHz in years of very high solar activity).
At night, it merges with the F1 layer into the F layer.
Critical frequency (foF2) and maximum usable
frequency (MUF)
When reading an ionogram, you’ll often encounter the terms critical frequency (foF2) and maximum
usable frequency (MUF). These two frequencies are needed to understand which radio waves are
reflected by the ionosphere.
Critical frequency (foF2):
The critical frequency (foF2) is the highest frequency vertically reflected by a specific layer. For
the F2 layer, this means that radio waves with a frequency below foF2 are reflected by the F2
llayer when sent straight up. Waves with a higher frequency pass through the ionosphere and
disappear into space. The foF2 is essential as it gives an indication of the maximum frequency
that can be used for vertical propagation.
Maximum usable frequency (MUF):
The maximum usable frequency (MUF) is the highest frequency that can be reflected by the
ionosphere over maximum distance. The MUF depends on the angle at which the signal hits the
ionosphere. At a lower angle, the MUF is much higher than the foF2, allowing higher frequencies
to reflect over longer distances. The MUF is calculated using the critical frequency and the angle
of the radio signal.
Next up, reading an ionogram
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How to read an ionogram - PA9X
Let’s take a look at how to read an ionogram.
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Ionogram from the Sopron ionosonde in northwest Hungary at November 9th 2024, 1355 UTC.
IIn an ionogram, you usually see height (y-axis) and frequency (x-axis). The dots on the graph
indicate the heights at which radio waves with different frequencies are reflected.
1. Identify the critical frequencies | foF2, foE, etc.:
Find the foF2 value in the data block on the left side of the graph. This is the highest frequency
that the F2 layer can reflect. For other layers, like the E layer, the critical frequency is indicated
as foE.
2. Look for the MUF value | MUF(D):
The MUF indicates the highest frequency reflected by the ionosphere over a specific distance.
This value is useful for selecting frequencies for DX. On the ionogram, this is displayed as
MUF(D).
3. Check the height information | (hmF2, hmE, etc.):
The height data, such as hmF2 and hmE, indicates the peak height of the F2 and E layers. This
information shows how far the ionospheric layers are from the Earth. The higher the layer, the
greater the distance the radio signal travels after reflection.
4. Interpret the lines on the graph:
The y-axis of the graph shows height in kilometers, while the x-axis shows frequency in MHz. The
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How to read an ionogram - PA9X
colored dots on the ionogram represent reflection points of radio waves at different heights and
frequencies.
The black dashed line on some ionograms shows a ‘theoretical boundary’ for frequencies that are
not reflected by the ionosphere. It helps to determine the MUF for higher frequencies.
5. Using the data:
The foF2 and MUF are frequencies best for which type of communication. Frequencies below
foF2 are ideal for short-distance or NVIS propagation, while frequencies just below the MUF are
suitable for DX.
Browse ionograms worldwide
I am currently creating pages that display latest ionograms from different areas around the world. Not
all are ionograms as displayed above. Some only show foF2.
Africa
Asia
Europe
North America (soon available)
Oceania (soon available)
South America (soon available)
Finally
Reading an ionogram seriously helps your ham radio communication. By understanding how the
ionosphere reflects radio waves and which frequencies are most useful, you can make the most of
the ionosphere’s natural properties. Experiment with different frequencies and times to see how
ionospheric conditions vary and what works best for your setup. With a bit of practice, an ionogram
will tell you more about the fascinating dynamics of the ionosphere!
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