Parameters of Microwave Antenna - Technopediasite

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Sunday, September 20, 2020

Parameters of Microwave Antenna

Parameters of Microwave Antenna : We all know that in the telecom sector, we use three types of medium for the communication or for establishing the links. These three types of medium are- 1. Cable (optical/copper), 2. Microwave radio, 2. Satellite. Microwave antenna is a major contributor to wireless communication. 

We all also know what are the advantages of microwave antenna. In Microwave
Radio-relay systems, the most commonly used antenna is parabolic Antenna. The only purpose of this article is to tell about the parameters of the antenna, not to discuss microwave antenna in detail.
Parameters of Microwave Antenna


Parameters of Microwave Antenna

Before knowing about microwave parameters, I will mention some specific points which are important for microwave antenna like-

Multipath propagation : More than one ray reaching the receiver is called the Multipath propagation occurs. The main cause of fading in microwave antenna is Multipath transmission. Multipath can happen when dN/dh varies with height.

Elevated duct : The atmosphere has a thick layer some height above the ground. If both the transmitter and receiver are within the duct, many rays will reach the receiver. If one is inside and the other is out, almost no receiver will reach.

Formation of a duct : During the day, the sun gives a heating ground and a well mixed atmosphere. A calm night will radiate from the warm ground, causing the temperature to reverse. This in turn gives an area near the ground where dM / dh <0, resulting in a ground-based duct.

After sunrise in the morning, the ground is again heated by the sun, and convection begins near the ground. The ground-based duct is enlarged, resulting in an elevated duct. See below image for easy understanding.
Formation of Duct

Line of sight : Line of sight should be maintain between transmitter and receiver a straight line, ray banding is also found in this process and this ray bending due to K-value variation and this K-value is added to the terrain heights. Terrain heights modification is given as-
(d1.d2)/(12.74.k), K=1.33 for (5 to 15 Kms) and 0.66 for longer path lengths (15-30 Kms)

Freznel Zone : The freznel zone  is defined as the location of points with maximum energy lobe from trans.
Antenna receiving antenna
d3- (d1 + d2) = λ / 2
Where d3 = D1 + D2.
λ = wavelength

For practical application the radius F1. May be approximated by the formula:
F1= 17.3  (d1.d2/f.D)
Where f is the freq in GHz
D=d1+d2 the total path length in Km.

Antenna gain :

Gain is approximated by the formulae
Gain = 17.8 + 20 log (D.f) dBi
As used in equation D = Antenna diameter [m]   &  F = Frequency in GHz

VSWR : 

Standard type Antenna : 1.06 to 1.15 typically
High performance Antenna : 1.04 to 1.06 typically.

Cross polarization: A good cross-polarization enables full use of the frequency band using vertical and horizontal polarization.

Beam Width: Half the power beam width of the antenna is defined as the angular width of the main beam at the -3 dB point.

Space Loss : From a radiated antenna, power received at any point is inversely proportional to square of distance between them and radiating frequency. This process can be defined by a equation : Lfs= 92.45 +20 log(f.d).

Power budget : Power budget can be calculated as-

Tx power O/P = +21 dBm
Losses (feeder + branching) = 3.0 dB
Tx Antenna Gain = 36.5 dB
Free space loss = 130 dB
Rx Antenna Gain = 36.5 dB
Feeder loss Rx = 1.2 dB
Nominal input level = -40.2 dB
Receiver Threshold = -80 dBm
Fading margin  = 39.8 dB

Precipitation :  When Transmission of microwave signal above 10 GHz is vulnerable to precipitation. The transmission energy is attenuated due to radiation (scattering) and absorption(heating).

Scattering : Radio waves are electromagnetic field and its vary with time, the incident field will induce a dipole moment in the raindrop. This will be the time of the drop of rain Variations in the form of radio waves and will act as an antenna and rearrange the energy. Since the raindrops have less directivity, this will give the energy an arbitrary direction and add to the losses.

Absorption : Relative to when the wavelength becomes short (high freq. <18GHz) The size of the raindrop absorbs more energy as the raindrop heats up.

Fading : Fading events are mainly multipath fading and fading Rain. Such a large fading margin better system performance. This can be achieved by higher tx o / p, larger (gain) antennas, lower threshold level and path lengths reduced etc.

Multipath Fading : Fading due to the layering of the atmosphere is the major factor of degradation Radio-relay. The transmitted waves which are received in the receiver are refracted from the troposphere or in addition to the desired signal is reflected from the ground. Phase and the amplitude relationship determines the resulting I / P signal at the receiver.

Diversity : Common forms of variation in LOS link frequency. Place or combination of and both. Recently angle variation has also been introduced.

Space Diversity : Separating two antennas vertically on the receiver tower so that only one antenna is located in a power minimum range.

Frequency Diversity : This security technique takes advantage of the frequency. The selectivity of the multipath is fading. But as Frequency Bandwidth is expensive in some countries, this technology is being used very rarely.

Switching sections : Variation / Rx by switching or combining various channels (in frequency). When carrying the same signal (from the main antenna and space diversity antenna), it is possible to achieve correction.

Hot Standby configuration : Hot Standby configuration reduces the system outage due to equipment failures.

Hybrid Diversity : Hybrid Diversity configuration like 1+1 hot standby system having space diversity at one of the radio sites.

Combined Diversity : In Combined Diversity using the space diversity and frequency diversity at the same time.

Angle diversity : In angle diversity the two antenna feedhorns are slightly off the boresight, giving a duelbeam parabolic dish antenna. 

Last Word 

My dear telecom friends, I have tried to provide complete information about the parameters of microwave antenna in very short time. I know that there is very less time to study in the telecom sector, so I have tried to provide complete information in one page. I have tried to touch all important points related to microwave antenna. If you like this article "Parameters of Microwave Antenna", then please like and share it with your friends.

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