antenna lab report 天线实验报告

1. Abstract

In this experiment, we compare the properties of several types of antennas at microwave frequencies. The antennas include dipole antenna and helix antennas which are left-hand antenna and right-hand antenna and tube antenna. We find that the polarisation on dipole antenna is in one orientation while it on helix antennas are omnidirectional.

2. Introduction

Antenna is an important front-end device for any radio communication system.

Although the mission of the equipment is not the same, but the antenna in which the role of is basically the same. For instance, some antenna's task is to transmitter output energy of high frequency current (guided wave) to convert into electromagnetic radiation, or transform space radio signals into high frequency current energy to

receiver. In order to achieve these purposes and satisfy the system working frequency bandwidth, features for antenna has a certain direction, high conversion efficiency, As a radio antenna system is indispensable and important component, its quality directly affects the overall performance of the radio system.

3. Theory

Antenna polarization feature based on antenna radiation of electromagnetic wave on the maximum radiation direction of electric field intensity vector to define the space orientation, is to describe the parameters of the antenna radiation of electromagnetic wave vector space. Due to electric field and magnetic field is constant, hence generally with the electric field space vector as antenna polarization direction of the radiation of electromagnetic wave. The antenna polarization are divided into linear polarization, circular and elliptical polarization. Linear polarization is divided into horizontal polarization and vertical polarization; Circular polarization is divided into right-hand and left-hand circle circular polarization.

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In this lab, we come into contact with the five kinds of things, are respectively rectangular waveguide antenna, dipole antenna, circular tube waveguide antenna, left hand circular (LHC) helix and right hand circular (RHC) helix. And we don’t touch with the elliptical polarization in this experiment.

Electromagnetic wave is a three-dimensional shear wave, it is because of its character of vector to produce polarization in this phenomenon. Here is the figure about linear polarization, circular and elliptical polarization (from left to right).

Figure 1

1 Linear Polarization ○

The electric field vector in space fixed direction called linear polarization. Sometimes as ground as a reference, the electric field vector called horizontal polarization direction parallel to the ground, called vertical polarization of vertical to the ground. The electric field vector and the direction of propagation of plane is called polarization plane. Electric field of the horizontal component and vertical component of the same phase, or vary l80 ° sine waves. The electric field component can be written as

After combination:

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(3.1)

(3.2)

Although the value of the combination electric field changes over time, but the direction remains same, that is, the end of the E vector trajectory is a straight line, as shown in figure 2, the so called linear polarized wave.

Figure 2

2 Circular polarization ○

When the angle between the normal to the surface and the ground plane of polarization of the radio waves from the changes between 0 ~ 360 ° cycle, namely the electric field the constant size, direction change over time, the end of the trajectory of the electric field vector perpendicular to the direction of propagation of the plane projection is a circle, called circular polarization. In the horizontal component and the vertical component of the electric field of the same amplitude, a phase difference in 90 ° or 270 °, the circular polarization can be obtained. Circular polarization, if the plane of polarization and propagation direction of rotation over time, the right relationship with the electromagnetic spiral, said right circular polarization; on the contrary, if a left spiral relations, said the left circular polarization.

Hence, sine electric fields of two components can be written as:

𝐸𝑥=𝐸𝑚𝑐𝑜𝑠𝜔𝑡 (3.3) 𝐸𝑦=𝐸𝑚𝑠𝑖𝑛𝜔𝑡 (3.4)

Then we get the value of the synthesis of the electric field:

2+𝐸2=𝐸=𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 (3.5) E=√𝐸𝑥𝑦𝑚

Its direction is changed over time, this can be made of tgθ=𝐸𝑌=𝐸𝑚=

𝑋

𝐸

𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 (3.6), as shown in figure 3. When E𝑦 lag the E𝑥 90 degrees, the electric

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field vector counter clockwise; E𝑦 ahead of the E𝑥 90 degrees, the electric field vector rotate clockwise.

3○

Figure 3

Also in the works is explained with left or right circularly polarized waves in the direction of rotation of the electric field vector. If the face of the electromagnetic wave

propagation direction open to observation, the electric field vector is rotated clockwise, the wave is called right-hand circularly polarized waves. If the electric field vector is rotated counter clockwise, the left-handed circularly polarized waves is called.

4. Results and analysis

In the experiment, we set record different voltage levels. Then, find the maximum voltage linear waveguide antenna. Normalization of the maximum voltage receiver. According to Equation (4.1)

1 Use the rectangular waveguide as a receiver, and the transmitting antenna are ○

diploe antenna, LHC, RHC, tube. We got some data of rectangular is under below for figure 4

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Figure 4

Where, a=6.3cm.b=2.6cm.c=12.3cm.d=16.5cm

Also for the tube, the diameter of the round aperture is 85.53mm. The whole length of tube is 25.4 cm and the distance from the backing plate is 20.4 cm. Here is the result and the figure of power versus angle.

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From the four group data, we can get the largest normalized power is when we use diploe antenna as transmitter. When we use tube as transmitter, normalized power is from the highest to a minimum and then rise to the highest from the lowest, the lowest point in 90 degrees, the highest point at 0 degrees and 180 degrees. It is similar to a sine curve. Three others on the contrary, is similar to a cosine curve. 2receiver: tube ○

Transmitter: dipole, LHC, RHC.

The results and figure are under below:

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In this time we can compare with figure from○1

LHC as a transmitter, tube and rectangular waveguide as a receiver, the images are similar sinusoid. The highest normalized power are both around 0.6 w. RHC as a transmitter, rectangular waveguide as a receiver, the image is similar to a sine curve highest normalized power is 0.5 w; but when tube as a receiver, the image is similar to a cosine curve, highest normalized power is 1 w. Diploe as a transmitter, tube and rectangular waveguide as a receiver, the images are similar sinusoid, but when

rectangular waveguide as a receiver, highest normalized power is 1 w; when tube as a receiver, the highest normalized power is 0.0016 w

3 use RHC as a receiver, LHC as a transmitter, compare with LHC as a receiver, RHC as ○

a transmitter. The results and figure are under below:

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Then we find , They look similar images.

5. Discussion

Question 1. How does power relate to the measured voltage?

The power is proportional to quadratic of voltage and Z can be considered as constant number. We assume z=1

𝐏=

(𝑼⁄𝑼

𝒎𝒂𝒙

)

𝟐

𝒁

(4.1)

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Question 2. What is the frequency of the transmitted signal, and hence its wavelength? How does this relate to the physical dimensions of the antenna elements?

Fellow the equation: (4.2)

We set the frequency of transmitted signal is 3GHz, hence the wavelength of transmitted signal is 0.1m.

In the theory, the length of antenna in the vertical component should be 0.25λ, Antenna corresponding electromagnetic energy generated large impedance, then induced electromagnetic voltage signal can be easily passed instrumentation or other devices, received the strongest signal. The length in fact is around 2.5 cm which is Corresponds to the theoretical value.

Question 3. How does the geometry of the antenna elements affect the polarization properties?

The geometry of the antenna elements affect the polarization properties through the propagation direction of Electric and magnetic fields

In have mention in “Theory” part that, the propagation direction of the linear polarized antenna on a plane and Propagation direction is circularly polarized antenna clockwise or counterclockwise rotation.

Question 4 what is the function of the backing plate on the antennas?

One is Ensure that the circuit connections are intact, the other is the wave orientation, to ensure that only the specified direction of flow wave

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Question5. How does the waveguide antenna cavity affect the waves that enter the aperture? What happens to the electric field of the incoming wave at the walls of the waveguides cavity? What is the largest wavelength that a waveguide will admit? According to the theory of waveguide, and waveguide tube has a cut-off frequency, when the signal frequency is higher than the cut-off frequency, the waveguide is in “break over” condition, electromagnetic wave is cut-off or attenuation and cannot transport. The electromagnetic wave propagation in the waveguide is limited by the waveguide wall and reflection.

Waveguide is used to transmit high frequency electromagnetic wave, when the electromagnetic wave propagation in metal waveguide, the metal wall will produce high frequency current by induction, through which the pulse signal can be sent to the destination with minimum loss; The size of the waveguide inner diameter because of transmission signal wavelength.

Figure 5

In the experiment, we use 𝑻𝑬𝟏𝟏 mode, which is a circular waveguide. And here is the structure of 𝑻𝑬𝟏𝟏 mode (The arrows indicate the direction of propagation)

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Figure 6

The largest wavelength id cut-off wavelength. For rectangular receiver, λc = 2a=2×𝟔.𝟑𝐜𝐦=𝟏𝟐,𝟔𝐜𝐦. For tube receiver, use the equation:

𝛌𝒄=𝟏.𝟖𝟒𝟏=14.59cm.where, R=42.76mm is the radius of the cross section of tube and 1.841 is constant of 𝑻𝑬𝟏𝟏 mode.

Question 6. Which of the antennas are the most efficient at transferring the power delivered by the signal generator?

Dipole antenna is the most efficient at transferring the power delivered by the signal generator. Dipole antennas are generally omnidirectional antenna, omnidirectional antenna 360-degree horizontal coverage, let your horizontal and vertical polarization can achieve better results. And our experiment also show that.

Question 7. Why do terrestrial antennas at low frequencies generally use vertical antennas for transmission and reception? How are higher frequency transmissions, like FM and digital, polarized?

Due to the nature of radio waves, determine the level of polarization signal propagation when close to the ground will produce polarization current in the earth surface, the polarization current due to impact of heat leaving the earth impedance field signal decay rapidly, while the vertical polarization is not easy to produce the polarization

𝟐𝝅𝑹

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current, thus avoiding the significant attenuation of energy, to ensure the effective dissemination of signals.

Horizontal antenna, high transmission frequency, short wavelength, work less susceptible to interference.

Question 8. Why do satellites and airplanes use circularly polarized antennas? Do the results of your experiment show why?

Circular-polarization antenna has an important role in the radio field. Especially in spacecraft, due to the fixed position and attitude of the aircraft, their communication and control equipment are required to be conformal, light weight, small size and low cost of a circularly polarized antenna. Circularly polarized micro strip antenna is ideal to meet these requirements antenna.

When the polarization direction of the polarization direction of the incoming wave and the receiving antenna is inconsistent, during reception usually have to produce polarization loss, but a circularly polarized antenna can be transmitted in either direction, so reducing the losses.

6. Conclusion

Through the antenna measurement and analysis, so I understand the principle and the role of antenna. Although the common antenna in life, but I never know what the significance of it. The experiment, let me take the initiative to find information, and constantly and classmates. Through the study and analysis of the experimental results on the measurement principle, let me know diploe antenna is the most efficient antenna, etc., so I was looking at five antennas have a deeper understanding. However, in the trial, due to the high degree of sensitive equipment, data changes quickly, we

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sometimes cannot be accurately measured values, so we averaging multiple measurements to reduce measurement error.

7. Reference

[1] Figure 1. Linear polarization, circular and elliptical polarization, http://baike.baidu.com/view/270536.htm

[2] Linear Polarization and figure 2. http://baike.baidu.com/view/3310805.htm

[3] Circular polarization and figure 3, http://baike.baidu.com/view/141252.htm [4] figure5 lecture note on KNE374, Peter Watt, https://mylo.utas.edu.au/d2l/le/content/90665/viewContent/1568835/View [5] 𝑇𝐸11 Mode and figure6, http://wenku.baidu.com/link?url=XMdZOVgDVU80rRtFCWscnbyirqpsNcePPZGizMkkGX06tK80ev56OwTr3jwX2jCIol4q58XFAPo-gVo2xdOEGKpKw9uns1fbLTWKAZgs25_

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