antenna design course work

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Antennas Principles And Practices 1

1.0 INTRODUCTION

1.1 What are antennas?

Basically, antennas are devices that carry out the conversion of guided waves in

a waveguide, micro strip or transmission line into radiating waves travelling in free

space at the transmitting end. At the receiving end, antennas do the reverse of this

process. There are many types of antennas that are put into different uses and are

designed according to the applications they are to be used in. In short, antennas can be

seen as an interface to couple RF power into free space at the transmitting end and

retrieve it back at the receiving end with as minimum power loss as possible. ()

1.2 Types of antennas

There are numerous types of antennas available for the transmission and

reception of different signals. The antenna type for any application is chosen

according to the operating frequency, gain, size and most importantly directivity. All

these parameters involved in designing antennas are interrelated mathematically and

should be put into close monitoring in the designing process. The types of antennas

are;

Element antennas

Aperture antennas

Printed/patch antennas

Reflector antennas

Array antennas

Leaky wave antennas

Lens antennas

In this practical assignment, I will choose two antennas to be used in two

different applications having different specifications. My choice will be based on the

frequency range involved for each application and the gain magnitude.

1.3 Antenna parameters

In order to carry out effective antenna design, it is mandatory to know the

important antenna parameters and how they relate to each other. The following is a

list of some important basic antenna parameters.

Antenna height

Antenna gain

Operating frequency

Input impedance/feed impedance

Directivity

Other parameters are introduced depending on the physical shape of the

antennas and the intended application. For example in helix antennas we have

parameters like pitch angle and number of turns which are very crucial in determining

the performance of the antenna.

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2.0 ANTENNA CHOICES

2.1 Task 1: Axial mode helix antenna

The first task is to design an antenna to be used in receiving signals for meteor

burst packet radio. The feed point of this antenna should be approximately 71 ohms at

a selected resonant frequency. For this application, an axial helical antenna is the most

suitable one. Meteor burst packet radio operates at frequency range of 30-100 MHz.

the axial mode helix antenna is suitable for this application as it possesses the

following characteristics:

Specified according to the spacing between turns(S) and number of turns N

S= C sinα

Circumference C limited Such that ¾<C/λ<4/3

Input impedance (resistive) R= 140(C/λ)

Axial mode helix antenna is used because packet radio operates in the VHF

range

Diameter of each turn is greater than 1/3 λ

Circular polarization

Directivity increased by increasing number of turns.

Helical antennas are very space effective and easy to setup. Defining a theoretical

model of this type of antenna is relatively difficult. Due to this fact, tuning is mainly

based on empirical modifications. The fact that this type of antenna possesses

narrowband characteristics, it is very difficult to fix the resonant point of this antenna.

[4]

To design the axial mode helix antenna, first we have to look into the parameters

involved and then key in the parameters in the NECWIN95 software to carry out the

simulation and observe the radiation patterns. The parameters involved are shown in

the figure 1.

The first important parameter to look into is the circumference of the helix. If this

circumference is of the order of one wavelength, maximum power is radiated in the

direction of its axis and it is said to be in axial mode. This phenomenon is shown in

figure 2.

Figure 1. Radiation pattern of axial mode helix at C≈λ

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Figure 2. (a) Helical antenna geometry (b) unrolled turn of the helix

The geometry and dimensions of the helix shown in figure 1 are related as follows.

D diameter of helix

S spacing between turns

N number of turns

C circumference of helix pD

A total axial length NS

α pitch angle

In axial mode, the radiation field is almost circularly polarized around the

axis. The polarization is related in the helix winding in a way. This mode has been

used in a wide range of frequencies. The circumference(C) and the pitch angle (α) are

restricted as follows. [3]

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2.1.1 Parameter Calculations

For this task we only know the feed impedance which is 71 Ω and the operating

frequency of meteor burst radio is around 70 MHz. these two parameters are used to

calculate all the required dimensions and geometry for the helix antenna.

First we find the wavelength (λ).

λ=300/f (in MHz)

Having f=70MHz, then

λ= 300/70=4.28 m

The next parameter we find is the circumference;

Using the relationship: R= 140(C/ λ)

Having R=71 Ω and λ=4.28m then:

71=140(C/4.28), hence,

C=71(4.28) ÷ 140 = 2.17 m

But for an axial mode helix antenna, the circumference should practically be almost

one wavelength (λ). Therefore;

We take 0.95(λ),

0.98x4.28 = 4.19 m

Then we find the diameter

Having C= π D, Therefore:

D= C/ π,

D= 4.19/ π = 1.33 m

Now we find the spacing S between turns

Knowing that, C= √2S λ, we have C=4.19, λ=4.28, we can find S.

S=C2

/ 2 λ, hence,

S= (4.19)2/2(4.28) = 2.06 m

The wire length can be calculated as;

C*N, where n is the number of turns, therefore; wire length L=4.19*8=33.5 m

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Now we determine the feed length by using,

Feed length=N*ℓ Where,

ℓ=√ (C2 +S

2), hence,

ℓ=√ (2.172+2.06

2) =2.99 m, therefore

Feed length= 2.99*8(N) =23.9 m

2.2. Task 2: Yagi Uda antenna

This task requires the design of a high gain antenna that operates in the lowest of

the UHF amateur radio bands (430-440MHz). The feed point impedance should be 50

Ω, with a gain of 8 dBi and a front to back ratio of 15dB. The side lobe suppression

should be 15dB. The most suitable antenna for this application is the Yagi Uda as it

possesses the following characteristics. [1][2]

Linear polarization

High gain of 5 to 18dBi

Narrow bandwidth of 5 to 10%

It is non-dispersive

Has impedance Zin of 50 to 300 Ω

High efficiency of above 90%

Figure 3. Structure of a seven element Yagi Uda

As shown in figure 3, a yagi uda antenna is composed of three types of elements

(rods), the reflector, the driven dipole and the directors. These rods are connected by a longitudinal rod.

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The lengths of the rods in a Yagi uda are all in the range of about half

wavelength (1/2 λ) and the space between the elements is in range of about a third

wavelength (1/3 λ). The only rode that is connected directly to the feeder is the

driving element. The other elements’ main purpose is to couple to the transmitter

power through the locally available electromagnetic fields which induce the current.

A folded dipole is often used as the driving element. [3][1]

A folded dipoles standing alone will possess a driving impedance of around 300

ohms to the feeder. Due to the presence of the other elements, a shunting effect occurs

that will reduce the driving point impedance to the range of about 20-90 ohms. The

restriction of the maximum gain in a yagi uda antenna is defined by the gain of a

dipole and is about 1.66 times the number of elements used. The yagi uda takes the

form of a travelling wave structure and increasing the number of elements normally

improves the directivity, gain and front to back ration. However there is a trade-off

where increasing the number of elements increases the number of side lobes. [3]

2.2.1 Dimensions and Geometry Calculations.

For this task we are given the feed point impedance= 50ohms, gain of 8 dBi, front to

back ratio of 15Db and side lobe suppression of 15dB. This antenna is to operate in

the lowest of the UHF amateur radio, this is about 430 MHz. therefore,

λ=300/f (in MHz)

λ =300/430 = 0.69 m

As mentioned earlier, the length rods should be in the range of about 1/2 λ and the

space between the elements is in range of about 1/3 λ. Therefore the dimensions

should be approximately;

Rod length= 1/2*0.69= 0.345 m

Space between antennas= 1/3*0.69= 0.23 m

The total length of antenna=1.5 λ= 1.035 m

The height of antenna= 10 λ= 10*0.69= 6.9m

I will design a 7 element yagi uda with the range of dimensions values around

the ones calculated above. The table 2.2.1 below shows the antenna elements and the

dimensions. (Taking spacing of 0.17 m between elements which is about 0.25 λ

between the reflector and the driven element and 0.2m(0.29 λ) between directors.

Element Length in λ Length in m

Reflector 0.50 0.34

Driven 0.47 0.33

Director 1 0.43 0.30





Table 2.2.1 seven element yagi uda antenna dimensions

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3.0 SIMULATION RESULTS

Using the NECWIN95 software for antenna design, the following results in form

of diagrams and plots were obtained.

3.1 Axial mode helical antenna.

Figure 3.1 (a) helix generator with calculated parameters

Figure 3.1(b) 3D image of the generated helix

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Figure 3.1(c) the generated helix’s current flow and pulses

Figure 3.1 (d) the helix antenna’s 3D radiation pattern

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Figure 3.1 (e) the antenna’s impedance plots

Figure 3.1 (f) the radiation pattern of the designed axial mode helical antenna.

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3.2 The 7 element Yagi Uda antenna

Figure 3.2 (a) the geometry of the seven element yagi uda (the 8th

wire is the middle

backbone of the antenna)

Figure 3.2(b) the 3D image of the antenna generated by NECWIN98 Software

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Figure 3.2(c) the pulses of the antenna and the pulse numbers

Figure 3.2(d) the 2D image of the seven segment yagi uda

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Figure 3.2 (e) the current flow of the antenna

Figure 3.2(f) the 3D radiation pattern of the designed antenna

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Figure 3.2 (g) the impedance plot of the designed Yagi Uda.

Figure 3.2(h) 2D radiation pattern at zenith

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Figure 3.2 (i) 2D radiation pattern at azimuth

3.2 (j) the optimizer interfaces (optimising the resonance)

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3.2 (j) the optimizer interfaces (optimising the front to back gain ratio)

Figure 3.2(k) geometry of antenna after optimisation

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4. CONCLUSION

In any communication system, the antenna is the most crucial component and if

the antenna does not perform well, the whole system is a failure. Antenna are

classified or grouped according to the way they function rather than the applications

they are used in.

In this experimental procedure of designing two antennas according to given

specifications, it is learnt that this designing process considers a lot of factors ranging

from the operating frequency to the input impedance and the gain of antennas.

In helix antennas it is observed that the number of turns plays a major role in

determining the directivity of the antenna. The spacing between the turns and the

pitch angle are also of considerable effect to the antennas performance. The helix

generator available in the NECWIN95 software simplifies the work of determining

the geometry of the antenna for the designer.

The antenna choice for the second task was the yagi uda because it is the most

appropriate to the given specification like high gain and frequency range of UHF. The

number of directors and how they are spaced from each other affects the performance

of this kind of antenna. It is observed that the parameters of this antenna are entirely

dependant on the wavelength (λ). The length of the elements and the distance between

the reflector and the driven element are also of considerable effect on the antenna

performance.

To obtain an optimum antenna performance, there must be a trade-off between

certain characteristics like number of side lobes and the number of element in the

antenna. More elements might increase the gain but at the same time increase he

number of side lobes which in most cases is undesired. To achieve the best optimal

antenna performance, the parameters must be well balance to get the best of each

desired characteristics without losing another one. Antenna optimization can be done

easily using computer programmes like NECWIN95.

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References:

[1] John D. Kraus and Ronald J.Marhefka, Antennas for All Applications, third

edition, McGraw Hill, 2002.

[2] Constantine A.Balanis Antenna Theory, Analyis and Design, second edition, John

Wiley and Sons, 1997.

[3] Yagi uda antennas, available at:

http://personal.ee.surrey.ac.uk/Personal/D.Jefferies/yagiuda.html (accessed February

2008)

[4] ‘Application note 1, V/UHF Antenna Design, available at:

http://www.numatechnologies.com/pdf/an_antenna.pdf (Accessed march 2008)

antenna design course work

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