Frequency

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-

Positive Alternation

Negative Alternation

Frequency – is simply the number of times a particular phenomenon occurs in a given period of time

It may be the number of voltage polarity alternations or electromagneticField oscillations that take place in a span time

Each alternation or oscillation is called a cycle and the frequency is Measured in cycles per second (cps), or better expressed as Hertz (Hz)

Wavelength1 wavelength

t

Wavelength is the distance between two points of similar cycles of a periodic waveIt is also the distance travelled by an electromagnetic wave during the time of one cycle

Bandwith (BW)

f1 f2 f

Bandwith (BW)

Bandwith is that portion of the electromagnetic spectrum occupied by a signalIt is also the frequency range over which an information signal is transmitted over which a receiver or other electronic circuit operates

Where: f2 = upper frequency f2 = lower frequency

𝐵𝑊= 𝑓 22− 𝑓 12

Power (P) and Decibels (dB)

Power is a fundamental quantity representing the rate at which energy is used. It is more readily measurable since it can be converted to heatDecibel literally means one-tenth of a bel, a unit named after Alexander Graham Bell. It is not an absolute unit but rather, it indicates the relation between two powers.dBm is dB above or below a reference power of one milliwatt

dB tipsPower Relationship Decibel

P2 equals P1 dB = 0

P2 is larger than P1 dB is positive

P2 is smaller than P1 dB is negative

P2 is 10 times or (100 times) dB = 10 (dB = 20)

P2 is 0.1 times or (0.01 times) dB = -10 (dB = -20

P2 and P1 are related by a factor of 2 or (4)

dB = 3 (dB = 6)

P2 and P1 are related by a factor of ½ or (1/4)

dB = -3 (dB = -6)

Ionosphere Propagation Parametersa. Plasma Frequency (fn) in Hz

When the wave reaches a region of electron density (N), the relative permittivity of the ionized medium must be seen to be zero

This in turn means that the total displacement current density is zero, and hence the effective electric field is zero

𝑓 𝑛=9√𝑁Where:

N = electron density in m-3

Critical Frequency (fc) in HzIt is the highest frequency at a given ionization density that will be turned down to the earth when the beam vertically upwardThis frequency is determined by the maximum electron density

𝑓 𝑐=9√𝑁𝑚𝑎𝑥

Where:Nmax = maximum electron density

Maximum Usable Frequency (MUF) and Secant Law

It is the highest frequency that will be turned down to the earth at a given distance when beamed at a specific angle other than the normal.Normal values of MUF reach about 8 to 35 MHz, but many rise to as high as 50MHz under unusual solar activities.

𝑀𝑈𝐹=𝑓 𝑐cos𝜃

= 𝑓𝑐𝑠𝑒𝑐𝜃

Where:Ө = angle of radiation in degrees

Optimum Working FrequencyIt is the frequency that provides the most consistent communication. It is the frequency chosen to avoid the irregularities of the ionosphere.

It is chosen by practical experience about 15% lower than MUF

𝑂𝑊𝐹=0.85𝑀𝑈𝐹

Lowest Usable Frequency (LUF)It is the lower limit of the range of frequencies that provide useful communication between two given points by the way of the ionosphereAbsorption also increases at the lower frequencies. The frequency nearest the point where reception becomes unusable woul be the LUF

Gyrofrequency (fg)Electrons moving in an ionized layer take a helix pat. At this frequency, where the periodic time of the wave is equal to the time required for one complete revolution about the magnetic axis, the path of the electrons becomes a very wide single loop.

Critical Angle (Өc)

It is the highest angle of radiation that will return the wave to the earth at a given density of ionization in the layer for the frequency or wavelength under consideration.

Virtual Height (hv)

Actual Height

Virtual Height

It is the apparent height of the ionized layer and is measured by sending a wave vertically to the layer and measuring the time it takes to come back to the receiver.

h𝑣=𝑐𝑇2

Where: c = velocity of the light in m/sT = round trip propagation in sec

h𝑣=𝑑

2 𝑡𝑎𝑛Ө 1

Where: Ө1 = angle of incidences of signal

Skip Zone and Skip Distance

It is the area that lies between the outer limit of the ground-wave range and the inner edge of energy return from the ionosphereIt is the area where no signal can be heardSkip distance is the distance between the originating site and the beginning of the ionosphereic returnIt is the shortest distance at which a sky wave signal will be returned to earth measured along the surface of the earth.

Factors Affecting Optimum Operating Frequency

a. Location and Geography

Intensity of ionizing radiation varies with locations and altitudes.The intensity is the greatest in equalitorial regions, where the sun is more directly overhead than in the higher latitudes

b. Seasonal VariationsSeasonal variations are variations brought about by the revolution of the earth around the sunOur earth orbits the sun with an orbital period of 1 year bringing about our seasons: spring, summer autumn and winterThe sun is the major controlling element on the behavior of the ionosphere thus ionization is stronger in summer than winter

c. Diurnal VariationsDiurnal variations are variations brought about by the roataion of the earth arounf its axis.The earth rotates around its own axis within a 24-hr period that is broken up into three distinct time periods; day, transition, and nightTransition period occur twice: once around sunrise and once around sunsetIonization is maximum during daylight hours and minimum during the hours of darkness

d. Cyclical VariationsAre variations brought about by the solar cycle like the sunspot activities.Solar activities are characterized by sunspot numbers.Sunspot appears on the sun’s surface and are tremendous eruptions of whirling electrified gas

Ionospheric Irregularities

a. Sudden Ionosphere Disturbances (SID’s)

They are also called Dellinger Fadeouts or Mogel-Dellinger FadeoutsSID’s are caused by solar flares, which are gigantic emissions of hydrogen from the sun.

Travelling Ionospheric Disturbances (TID’s)

They are disturbances that seriously affect the accuracy of high-frequency direction finders due to irregularities of electron densities in the ionosphere.

Ionospheric StormsThey are caused by particle emissions from the sun, generally Alpha and Beta raysAt these conditions, the ionosphere behaves erratically causing signal strengths to drop and fluctuate rapidly.They take about 36 hrs to reach the earth

FadingIt is the fluctuation of signal strength at the receiver.It can occur because of interference between the lower and upper rays of a sky wave, between sky waves arriving by a different number of hops or different paths, or even a ground wave and a sky wave especially at the lower end of the HF band

I. Interference Fadingit is the most common type of fading caused by mixing of

two or more signal components propagating along different path

II. Polarization Fadingit is a type of fading caused by the so-called Faraday Effect or

Faraday Rotation

III. Focusing and Defocusing there are mainly due to atmospheric irregularities. Deformed

layers can focus or defocus a signal wave if they encounter deformities that are concave or convex respectively. The motion of these structures can cause fadeouts

IV. Absorption FadingIt is type of fading caused by solar flare activities and

particularly affects the lower frequencies

V. Selective Fading it is a type of fading having different frequency ranges. This

happen because ray paths in the ionosphere are different for different frequencies, and not all will necessarily experience a disturbance at a given region.