Critical frequency

sky wave propagation

1. Critical Frequency

2. Definition:- The maximum frequency which can be reflected by a particular layer of ionosphere at vertical incidence. This highest frequency is called critical frequency for that particular layer. Critical frequency is different for different layers of ionosphere. 2

3. Critical frequency for particular layer is proportional to square root of the maximum electron density in the layer. Where f c is critical frequency (in MHz) and N m is maximum electron density (in per cubic meter)   3

4. 4 Critical Frequency changes as the day passes and also due to atmospheric conditions. Because of the change in Ionosphere during day and night, during daytime higher frequency bands under critical Frequency work best, but during nighttime the lower frequency bands work best

5. Derivation:- from Snell’s Law   angle of incidence= ∠ i = 0 , N= N m , f=f c = 0     Refractive index of ionized medium is given by   5

6. What is the critical frequency for reflection at vertical incidence if the maximum value of electron density is 1.24 * 10 6 cm – 3 . Solution: N m = 1.24 * 10 6 cm – 3 = 1.24 * 10 6 * 10 - 6 m – 3 MHz 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: or For E layers, f c = 2.5MHz = 0.07716 * 10 12 m -3 (ii) For F layers, f c = 8.4 MHz = 0.8711 * 10 12 m -3  

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

9. Thank YOU 9