FC

1. Critical Frequency

2.  The maximum frequency which can b
e reflected by a
particular layerofionosphereatvertical incidence.This
highest frequency is called critical frequency for that
particularlayer
.
 Critical frequency is different for different layers of
ionosphere.
2 Definition:-

3.  Criticalfrequencyforparticularlayerisproportionalto
squarerootofthemaxim
u
melectrondensityinthelayer
.
𝒇𝒄 = 𝟗 𝑵𝒎
Where fc is critical frequency (in MHz) and
Nm is maximum electron density (in per cubic meter)
3

4. 4
CriticalFrequencychangesasthedaypassesandalsodueto
atmosphericconditions.
Becauseo
f thechangeinIonosphereduringdayandnight,
during daytime higher frequency bands under critical
Frequency work best, but during nighttime the lower
frequencybandsworkbest

5. Derivation:-
from Snell’s Law
𝜇 =
sin i
sin r
angle of incidence= ∠ i = 0 , N=Nm , f=fc
sin r
𝜇 = sin i
= 1 − 81𝑁𝑚
𝑐
𝑓2 = 0
𝑓𝑐 = 9 𝑁𝑚
𝜇 = 1 −
Refractive index of ionized medium is given by
81𝑁
𝑓2
5

6. What is the critical frequency for reflection at vertical
incidence if the maximum value of electron density is
1.24 * 106 cm – 3.
Solution:
m – 3
𝑓𝑐 = 9 𝑁𝑚
Nm= 1.24 * 106 cm – 3 = 1.24 * 106 * 10 - 6
𝑓𝑐 = 9 1.24 MHz
𝑓𝑐= 10.026 MHz
6

7. 7
The observed critical frequencies of E and F layers at Guwahati at a
particular times are 2.5MHz and 8.4 MHz respectively. Calculate the
maximum electron concentrations of layers.
Solution:
𝑚
𝑓2
𝑐
81
𝑓𝑐 = 9 𝑁𝑚 or 𝑁 =
(i) For E layers, fc= 2.5MHz
𝑁𝑚 =
(2.5∗106)2
81
= 0.07716 * 10 m
12 -3
(ii) For F layers, fc= 8.4 MHz
𝑁𝑚 =
(8.4∗106)2
81
= 0.8711 * 10 m
12 -3

8. Reference:
1. K.D. Prasad, Antenna & wave propagation,
Satya Prakashan, 3/e (2010-11)
8

9. Thank YOU
9