Properties of Atmosphere and Ionosphere

The typical electron distribution in the ionosphere E F F1 D E F2 N (1/m 3 ) The ionosphere can be modeled as a lossy dielectric whose relative permittivity varies with height (electron density) and with the frequency of wave

N is a function of height When a wave penetrates into ionosphere, it is refracted continuously and follows a curved path, finally it will be returned to earth from a level where refractive index For a given frequency f, the wave will return back if or d, skip distance h´, virtual height: apparent height of reflection at this point

If the maximum electron density in ionosphere is N max, then with vertical incidence (  i =0), the frequency that will be reflected back is The frequencies above f c will not be returned back to earth with vertical incidence. Critical Frequency (f c ) and Maximum Usable Frequency (MUF) h N N max f=fcf<fcf>fc escapes f>MUF f=MUF f<MUF

Maximum Usable Frequency (MUF) N=N max f=MUF Note that there is an upper limit to  i due to curvature of earth

h´ aeae a e =4a/3  8497km d1d1 d s,max Radiation leaving the antenna in horizontal direction (at grazing angle) For the F2 layer, h´=300 km  i,max  75 

Maximum skip distance, d s,max =2d 1, d 1 =a e ,  in radians,  =  -  /2-  i,max For the F2 layer, h´=300 km,  =90  - 75  = 15   rad d s,max =2 (8497) 0.262=4449km Following approximate formula can also be used: If the desired range is less than the maximum skip distance, the transmitter beam must be elevated above the horizon, resulting in a lower value for MUF.

h´h´ O T R dsds Ionosphere F Layer aeae aeae    ii Local horizon aeae Using the law of sines for the triangle TRO, it can be shown that where  =180  -  -  i  90  is used, The law of sines for the triangle TRO For a given skip distance what is  or  i ?

If the desired range is greater than the maximum skip distance, a multi-hop link must be used.

Ionospheric Measurements A sample ionogram Plot of virtual height as a function of frequency Normal incidence the virtual heights increase steeply as the critical frequency is reached. there are double reflections from the F1 and F2 layers.

Oblique incidence ionogram showing reflections from different heights Oblique incidence sounding stations: transmitter & receiver are located at the end points of the path Drawbacks: difficulty in syncronization and fixed locations Oblique incidence backscatter sounding stations: transmitter & receiver are located at the same site

In order to establish an ionospheric propagation link between two stations on earth, one needs to know the MUF for that path. Practically, daily MUF charts are prepared for different locations and propagation distances and these charts are used to determine the frequency of operation Typical MUF Chart for propagation paths of different heights

Critical frequency: the maximum frequency that can be reflected by a layer for vertical incidence There is a frequency below which radio communications between two stations will be lost due to reduced SNR. Decrease in frequency multiple hops increase in the losses increase in the losses due to the D layer

The ITU-R Recommendation P definitions Operational MUF: The highest frequency that would permit acceptable performance of a radio circuit by signal propagation via the ionosphere between given terminals at a given time under specified working conditions, (antennas, power, emission type, required SNR, and so forth). Basic MUF: The highest frequency by which a radio wave can propagate between given terminals, on a specified occasion, by ionospheric refraction alone. Optimum working frequency (OWF): The lower decile of the daily values of operational MUF at a given time over a specified period, usually a month. The frequency that is exceeded by the operational MUF during 90% of the specified period. Highest probable frequency (HPF): the upper decile of the daily values of operational MUF at a given time over a specified period, usually a month. The frequency that is exceeded by the operational MUF during 10% of the specified period. Lowest usable frequency (LUF): The lowest frequency that would permit acceptable performance of a radio circuit by signal propagation via the ionosphere between given terminals at a given time under specified working conditions.

Attenuation of Waves in Ionosphere

Monthly average of diurnal variations of critical frequency and virtual height of regular ionosphere layers for summer

Monthly average of diurnal variations of critical frequency and virtual height of regular ionosphere layers for winter