ANTENNA AND WAVE PROPAGATION: IONOSPHERIC FADING EFFECT, CAUSES OF FADING, INTERFERENCE FADING, POLARIZATION FADING AND ABSORPTION FADING, SKIP FADING, SELECTIVE FADING, CHARACTERISTICS OF AMPLITUDE FADING, REGIONAL ANOMALIES

1. IONOSPHERIC
FADING EFFECT
PREPARED BY:- SHIVANGI SINGH
E.C DEPT.
SIXTH SEMESTER

2. IONOSPHERIC FADING EFFECT
 Experience has shown that information concerning the mean value of the received
signal is not sufficient for planning radiocommunication systems.
 The variations in time, space and frequency, collectively described as fading, also
have to be taken into consideration.
 Fading has a decisive influence on the performance of radiocommunication systems
and on the type of modulation that may be used effectively.
 It is essential to know the severity and rapidity of fading to be able to specify the
power required for transmitters, the necessary protection ratio to guard against
interference and, with additional knowledge of the correlation of signals at separate
antennas or frequencies, to be able to determine the most efficient and economical
diversity or coding systems.
 Similar considerations may also apply to the noise and interference at the receiving
site.

3. CAUSES OF FADING
Fading may be caused by several different effects, such as:
– movement of the ionosphere, and multipath changes causing interference fading;
– rotation of the axes of the polarization ellipses;
– variations of the ionospheric absorption with time;
– focusing and temporary disappearance of the signal due to MUF failure.
Fading may appear either as amplitude fading or as a Doppler-frequency shift.
Amplitude fading may cause dispersion in both time and frequency. Motion of the
transmitter, receiver or ionospheric reflector causes Doppler shifts. In general
multipath signals have differing amplitudes and frequency shifts.

4. INTERFERENCE FADING
 Interference fading results from interference between two or more waves which travel
by different paths to arrive at the receiving point.
 This type of fading may be caused by interference between: sky-wave and ground-
wave, multiple reflected sky-waves, the ordinary (O) and extraordinary (X) waves,
and various scattered signals from irregularities.
 Interference fading may last for a period of a fraction of a second to a few seconds,
during which time the resultant field intensity may vary over wide limits.

5. POLARIZATION FADING AND
ABSORPTION FADING
Polarization fading occurs as a result of changes in the direction of polarization of the
downcoming wave, relative to the orientation of the receiving antenna, due to random
fluctuations in the electron density along the path of propagation. Polarization fading
also lasts for a period of a fraction of a second to a few seconds.
Absorption fading is caused by variation in the absorption due to changes in the
densities of ionization and it may sometimes last longer than one hour.

6. SKIP FADING
 Skip fading may be observed at receiving locations near the skip distance (the
minimum range from the transmitter before the ray passes through the ionospheric
layer rather than being refracted down from it) at about sunrise and sunset, when the
basic MUF for the path may oscillate around the operating frequency.
 The signal may decrease abruptly when the skip distance increases past the receiving
point or increase with a decrease in the skip distance.

7. SELECTIVE FADING
 Fading tends to be faster at high frequencies than at low frequencies, because for a
given movement in the ionosphere, there is a greater phase-shift on the shorter
wavelengths.
 Motion of the ionized regions causes selective fading (frequency-dependent fading)
when, on a modulated carrier, the frequency components in the sidebands fade
independently, giving rise to distortion of the modulation envelope.
 The motion produces changes in path length, and Doppler shifts of frequency on each
of the individual contributing signal components. Selective fading may also be caused
by multipath propagation at HF.

8. CHARACTERISTICS OF AMPLITUDE
FADING
I. Depth of fading
 Depth of fading is measured by the amplitude distribution, or probability density
function of the amplitude of the down coming wave.
 The amplitude distributions normally conform to one of three standard statistical
curves or distributions: Rayleigh, normal or Gaussian distribution, and log-normal.
II. Rapidity of fading
 The rapidity or speed of fading can be characterized in different ways. One statistical
property of the instant-to-instant variation of amplitude is given by the
autocorrelation function.
 The rapidity or the speed of fading can be described in terms of the time auto-
correlation function of the amplitude or in terms of the power spectrum of fading,
which is the Fourier transform of the auto-correlation function.

9.  The frequency spectrum of the fading signal may be obtained with the aid of the auto-
correlation function. The width of the fading power spectrum is related to the speed of
fading.
 Another definition of fading rate is the number of positive crossings per unit time
through any specified level, or the number of maxima N of the amplitude of the signal
envelope per unit time. It has been shown that if σ is the standard deviation of the
power spectrum of the received signal, N = 2.52 σ.

10. REGIONALANOMALIES
Features of fading encountered in the Tropical Zone
Fading of signals in the tropics at low geomagnetic latitudes has special characteristics
due to the regular daytime occurrence of sporadic E, and to irregularities in the night-
time F layer (spread F).
I. Fading due to sporadic E :Fading observed during daytime in the equatorial zone is
often attributed to sporadic E. In a narrow zone near the magnetic equator (±6°
magnetic dip), a special type of highly transparent sporadic E called equatorial
sporadic E or Es-q appears regularly during daytime.
II. Surge fading :Another type of fading in the Tropical Zone is surge fading, which is
slower than flutter fading, but is deeper and accompanied by severe distortion. It is
worst after sunset and more pronounced during autumn and winter. The recurrence
rate is a few surges per minute.

11. III. Flutter fading :
• Very rapid fading has been observed in the equatorial region after sunset, where it
represents one of the most important factors in the degradation of communications,
particularly for broadcast services.
• This flutter fading is caused by F-region irregularities, known as spread F.
• In the equatorial zone after local sunset, such irregularities develop in the F-region
ionization between 30°N and 30°S geomagnetic latitude, their occurrence being
particularly frequent within ±20° geomagnetic latitude.
• Under these conditions, the F region increases markedly in height and breaks up into
patchy irregular plumes associated with plasma instabilities.

12. • The spread-F irregularities are seen normally after sunset and before midnight. The
seasonal variations show maxima at the equinoxes near sunspot maximum.
• Both north-south circuits and east-west circuits are affected. The time of start is well
defined and sudden, but the time of disappearance is gradual.
• The fades are deep and the signal is almost completely drowned in noise, even though
the mean signal strength remains high.
• The flutter fading rate is proportional to the wave frequency, ten fades per second
being typical at 15 MHz.

13. IV. Fading allowances :
A discussion of the fading allowances required for planning broadcasting services in the
Tropical Zone is given in ITU-R Report BS.304. Recommendation BS.411 proposes
values of fadingallowance to ensure that the steady-state ratio is attained for 90% of the
time, under three conditionsof reception, i.e., where the RF signal-to-noise ratio is as
follows:
– ratio of wanted signal to interference: 16 dB;
– ratio of wanted signal to atmospheric noise: 17 dB;
– ratio of wanted signal to man-made noise: 12 dB.