Ionosonde

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The frequency Fc at which a wave just penetrates a layer of ionisation is known as the critical frequency of that layer. The critical frequency is related to the electron density in the specific layer (D, E, F1 or F2).
The frequency Fc at which a wave just penetrates a layer of ionisation is known as the critical frequency of that layer. The critical frequency is related to the electron density in the specific layer (D, E, F1 or F2).
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All transmitted frequencies above this critical frequency will penetrate the layer without being reflected. Their group velocity will however, will be slowed by any ionisation, and this will add to the time-of-flight. If such a wave encounters another layer, whose plasma frequency is higher than the frequency of the wave, it will be reflected, and the return signal will be further delayed as it travels back through the underlying ionisation. Therefore, HF frequencies between 5 and 30 MHz pass through the E layer and are reflected at the F layer. Similarly, MF frequencies (AM broadcast stations) at night pass through the D layer and are reflected by the E layer. When a ''sporadic E'' cloud passes over the ionosonde, then foEs becomes very high, often over exceeding 25 MHz. This high reflection frequency allows medium distance ionospheric communications and TV DX reception in the low VHF range (28, 50 and even 70 MHz) with very strong signal stengths.
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All transmitted frequencies above this critical frequency will penetrate the layer without being reflected. Their group velocity, however, will be slowed by any ionisation, and this will add to the time-of-flight. If such a wave encounters another layer whose plasma frequency is higher than the frequency of the wave, it will be reflected, and the return signal will be further delayed as it travels back through the underlying ionisation. Therefore, HF frequencies between 5 and 30 MHz pass through the E layer and are reflected at the F layer. Similarly, MF frequencies (AM broadcast stations) at night pass through the D layer and are reflected by the E layer. When a ''sporadic E'' cloud passes over the ionosonde, then foEs becomes very high, often over exceeding 25 MHz. This high reflection frequency allows medium distance ionospheric communications and TV DX reception in the low VHF range (28, 50 and even 70 MHz) with very strong signal stengths.
==Ionogram==
==Ionogram==

Revision as of 21:20, 13 January 2011

An IRX-58-23 ionosonde system.

An ionosonde (or chirpsounder) is a specialized radar system for the examination of the ionosphere. An ionosonde is used for finding the optimum operation frequencies for broadcasts or two-way communications in the high frequency range.

Contents

Description

Ionospheric reflection of radio waves was discovered in 1924 by Edward Victor Appleton. The basic ionosonde technology was invented in 1925 by Gregory Breit and Merle A. Tuve (1) (2) and further developed in the late 1920s by a number of prominent physicists, including E. V. Appleton. The term ionosphere and, hence, the etymology of its derivatives, was proposed by Robert Watson-Watt.

An ionosonde consists of:

  • A high frequency (HF) transmitter, automatically tunable over a wide range. Typically the frequency coverage is 0.5–23 MHz or 1–40 MHz, though normally sweeps are confined to approximately 1.6–12 MHz.
  • A tracking HF receiver which can automatically track the frequency of the transmitter.
  • An antenna with a suitable radiation pattern, which transmits well vertically upwards and is efficient over the whole frequency range used.
  • Digital control and data analysis circuits.

The transmitter sweeps all or part of the HF frequency range, transmitting short pulses. These pulses are reflected at various layers of the ionosphere, at heights of 100–400 km, and their echos are received by the receiver and analyzed by the control system. The result is displayed in the form of an ionogram.

Modern ionosondes, like the popular Lowell DIGISONDE(TM), use low power transmitters but compensate by using specialized DSP techniques in the receiver. The result is a portable instrument, which can be easily relocated to interesting HF communications sites.

Operation

As the transmitted frequency increases, the radio wave is refracted less by the ionisation in the layer, and so it penetrates further before it is eventually reflected. As the wave approaches the reflection height, its group velocity approaches zero and this increases the time-of-flight of the signal. Eventually, a frequency is reached that enables the wave to penetrate the layer without being reflected. For ordinary mode waves, this occurs when the transmitted frequency (fo) just exceeds the peak plasma frequency of the layer. These frequencies are identified by the layer where reflection takes place (foE, foF1, foF2 and foEs). In the case of the extraordinary wave, the magnetic field of the earth enhances the reflection capability of the ionosphere and reflection occurs at a frequency (fx) that is higher than the ordinary wave by half the electron gyrofrequency.

The frequency Fc at which a wave just penetrates a layer of ionisation is known as the critical frequency of that layer. The critical frequency is related to the electron density in the specific layer (D, E, F1 or F2).

All transmitted frequencies above this critical frequency will penetrate the layer without being reflected. Their group velocity, however, will be slowed by any ionisation, and this will add to the time-of-flight. If such a wave encounters another layer whose plasma frequency is higher than the frequency of the wave, it will be reflected, and the return signal will be further delayed as it travels back through the underlying ionisation. Therefore, HF frequencies between 5 and 30 MHz pass through the E layer and are reflected at the F layer. Similarly, MF frequencies (AM broadcast stations) at night pass through the D layer and are reflected by the E layer. When a sporadic E cloud passes over the ionosonde, then foEs becomes very high, often over exceeding 25 MHz. This high reflection frequency allows medium distance ionospheric communications and TV DX reception in the low VHF range (28, 50 and even 70 MHz) with very strong signal stengths.

Ionogram

An ionogram is a display of the data produced by the ionosonde. It is a graph of the virtual height of the ionosphere plotted against frequency. Ionograms are often converted into electron density profiles. Data from ionograms may be used to measure changes in the Earth's ionosphere due to space weather events.

Ionogram.png

The maximum usable frequency (MUF) for radio waves transmitted at a low angle over the horizon is approximate 3 times the highest frequency returned to the ionosonde from radio waves transmitted directly upwards (to the zenith of the location). Therefore, in the sample ionogram shown above, fxI was measured as 6.2 MHz (green dataset), and the MUF for ionospheric reflection at the area of the ionosonde was calculated as 18.7 MHz for a hop distance of 3000 km. The pink dataset, which represents the ordinary wave, shows reflections at approximately 3 MHz (E-layer) and 5.5 MHz (F-layer).

See also

References

  1. F.C. Judd, G2BCX: "Radio Wave Propagation (HF Bands)", Heinemann, London, ISBN 0-434-90926-2, 1987, pages 12–20,27–37
  2. G. Jacobs, W3ASK, T.J. Cohen, N4XX and R.B. Rose, K6GKU: "The new Shortwave Propagation Handbook", CQ Communications, Inc., ISBN 0-943016-11-8, 1995, pages 1-2 to 1-5.

Further reading


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Wikipedia article: Ionosonde
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