Some Information About HF Propagation

1. Some Information About HF Propagation

2. Electromagnetic Waves

3. Electromagnetic Waves • Self propagating electric and magnetic fields • Created by accelerating charge: speeding up, slowing down, or changing direction • Do not influence each other – add as vectors • Influence charged particles

4. HF Propagation in the Atmosphere • Reflection – wave reflected by a change in density of medium in which the wave travels • Refraction – direction of wave changes due to a change in the density of the medium in which the wave travels • Change caused by the speeding up or slowing of the wave in the medium

5. HF Propagation in the Atmosphere HF propagation is a result of the refraction (and some reflection) of the radio waves from a path out of the atmosphere to a path that returns the wave to the ground – the subject is wide and still being investigated – the refraction (and some reflection) is due to the ionization of the atmosphere since free electrons interact with radio waves more strongly than electrons attached to molecules or bound in atoms

6. HF Propagation in the Atmosphere • Index of refraction – a measure of the speed of EM waves in a medium - essentially one in the atmosphere – that is speed is essentially c for un-ionized gases - that is little refraction due to change in density • Refraction and reflection of EM waves mainly a result of the density of the free electrons (ions) in the D, E, and F layers of the atmosphere

7. HF Propagation in the Atmosphere

8. HF Propagation in the Atmosphere • Absorption: EM wave absorbed by the oscillation of an electron • Collision frequency of electrons must be close to wavelength frequency for absorption to occur • In D region, 160 meter, 80 meter, and 40 meter waves are absorbed when the region is ionized – above 15 meters the waves pass through – for 30 meters, 20 meters, and 17 meters absorption must be taken into account

9. HF Propagation in the Atmosphere

10. HF Propagation in the Atmosphere • Plasma (critical) frequency: for ionized gasses (plasmas) for normal incidence: higher – waves leave atmosphere; lower – waves returned to earth (for oblique incidence use the cosine of the angle from the vertical times the frequency for normal incidence) • Is related to the collision frequency of the electrons in the plasma – the higher the electron collision frequency, the higher the smallest EM frequency that will refract • Is important for wavelengths 30 meters through 160 meters

11. HF Propagation in the Atmosphere • D, E, F1, and F2 levels ionized during the day • Due to the lower particle density, F1 and F2 levels maintain some ionization at night and merge into the F level • D and E levels quickly loose ionization at sunset

12. RF Propagation in the Atmosphere

13. HF Propagation in the Atmosphere • Level of ionization is influenced by: • Time of day • Season • Solar rotation (28 day rotation with respect to the earth) • Cosmic radiation • Geomagnetic storms • Solar flares and sunspot numbers

14. HF Propagation in the Atmosphere Plasma Frequency

15. Sources • HF Propagation tutorial, by Bob Brown, NM7M, Ph.D. from U. C. Berkeley, http://www.astrosurf.com/luxorion/qsl-hf-tutorial-nm7m.htm • The ARRL Handbook For Radio Amateurs 2001, Chapter 21