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ANTENNA FOR RFI MONITORING.

AJAY TIWARI Department of Electronic Science, UoP.

ASHISH Kr. ROY

Department of Electronic Science, UoP.

SUMMER TRAINING PROGRAM 28th May 2009 – 28th July 2009

Under the guidance of :

S. SURESHKUMAR, Engineer – E.

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ACKNOWLEDGEMENTACKNOWLEDGEMENTACKNOWLEDGEMENTACKNOWLEDGEMENT

We take this opportunity to express our sense of profound gratitude to all the people

who have been instrumental in making our training a great learning and rich experience.

At the outset we would like to thank Prof. Yashwant Gupta, Chief Scientist, GMRT

observatory, Khodad, Shri. A.Praveen Kumar, SF, Group Co-ordinator Front-end and

Fiber Optics and Prof. S. Ananthkrishnan, Raja Ramanna Fellow for providing us this

opportunity of summer training. . We would like to extend my gratitude towards S.

Sureshkumar, our guide for the project for extending his full support to us.

Also we offer heartly thanks to Mr. Amit Sawant, Mr. Hanumanth Rao, Mr. Pravin

Raybole, Mr. S. Ramesh, Mr. Satish Lokhande, Mr H.S. Kale and NCRA workshop

members for being supportive throughout our project.

Last but not the least, we thank all our fellow STP students.

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CONTENTS.

Page no. 1. Introduction to RFI

1.1 What is RFI? 5 1.2 Sources of RFI. 5 1.3 Effects of RFI. 6

2. Choice of antenna 7 3. Biconical antenna

3.1 Introduction to biconical antenna. 8 3.2 Physical characteristics of the biconical antenna. 8 3.3 Biconical antenna design considerations. 9

4. Variation in parameters and effects

4.1 Length of the antenna. 11 4.2 Radii of cone elements. 15 4.3 Spacing between the cones. 20 4.4 Number of elements. 25

5. Optimized Design

5.1 Optimized simulation. 28 5.2 Final Results. 29 5.3 Radiation pattern at different frequencies. 31 5.4 Conclusions. 34 5.5 Future scopes. 34

6. Introduction to Discone antenna 6.1 Brief History 35 6.2 Physical aspects 35 6.3 Mounting and dimensions 35 6.4 Operation 37 6.5 Our design and construction details 37

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7. WIPL_D simulations 7.1 The main criteria for judging the performance of the 39 Simulations.

7.2 Simulation model and the results. 39 7.3 Radiation patterns at different frequencies. 42 7.4 Design parameters varied during the simulations. 50 7.5 Variations in cone length. 51 7.6 Variations in disc radius. 53 7.7 Variations in cone base radius. 55 7.8 Variations in spacing between cone’s apex and the disc. 57 7.9 Variations in the cone’s upper radius. 59 7.10 Conclusions of the simulated results. 60

8. Fabrication and testing.

8.1 Design and results of the fabricated discone antenna. 61 8.2 Fabricated design and construction details. 61 8.3 A block diagram showing the test apparatus for testing 62

the discone. 8.4 Practical results. 63 8.5 A block diagram showing the arrangement for testing 65

the antenna radiation pattern. 8.6 Final conclusion. 67 8.7 Future scopes of improvement. 67 8.8 Photographs showing the discone antenna and the 67

experimental setup.

• References 69

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CHAPTER 1 INTRODUCTION: 1.1… WHAT IS RFI?

RFI stands for Radio Frequency Interference. It is defined as the impairment of the reception of a wanted radio signal caused by an unwanted radio signal. The range of radio frequency is from 3 Hz to 300GHz. These are described in table 1. BANDWIDTH DESCRIPTION FREQUENCY RANGE

Extremely Low Frequency (ELF) 3 Hz- 30 Hz Super Low Frequency (SLF) 30 Hz– 300 Hz Ultra Low Frequency (ULF) 300 Hz – 3000 Hz Very Low Frequency (VLF) 3 KHz - 30 KHz Low Frequency (LF) 30 KHz - 300 KHz Medium Frequency (MF) 300 KHz - 3000 KHz High Frequency (HF) 3 MHz - 30 MHz Very High Frequency (VHF) 30 MHz - 300 MHz Ultra High Frequency (UHF) 300 MHz - 3000 MHz Super High Frequency (SHF) 3 GHz – 30 GHz Extremely High Frequency (EHF) 30 GHz – 300 GHz 1.2… SOURCES OF RFI:

The radio frequency spectrum is divided into several bands for commercial applications. When these bands interfere with the frequencies in other bands, RFI is produced.

Not only commercial sources, but also natural sources cause RFI. For e.g.: Lightning radiates radio energy across all bands from VLF (3 KHz-30 KHz) to microwaves (3 GHz-300GHz). Other weather conditions, known as spherics, also generate significant energy which often results in small electrical discharges, generating RFI.

Other than the commercial and natural sources, objects used in daily life also produce RFI. Examples of which include welding equipment, fluorescent lights, loose electrical connections, defective AC power plugs which are among the many sources of electrical interference.

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Table 2 lists the several commercial bands and their frequency of operation.

NAME OF THE SERVICE FREQUENCY RANGE Radio Navigation & maritime/aeronautical mobile

9 KHz – 540 KHz

AM Radio Broadcast 540 KHz – 1630 KHz Shortwave Broadcast Radio 5.95 MHz – 26.1 MHz TV Band 1 – Channels 2 – 6 54 MHz – 88 MHz FM Radio Broadcast 88 MHz – 108 MHz Radio Astronomy Band 150 MHz – 153.6 MHz TV Band 2 – Channels 7 – 13 174 MHz – 216 MHz Radio Astronomy Band 230 MHz – 235 MHz Fixed mobile Broadcasting 235 MHz – 273 MHz Radio Astronomy Band 322 MHz – 328.6 MHz Aircraft Transmission 328 MHz – 335 MHz TV Band 3 – Channels 14 – 70 470 MHz – 806 MHz Radio Astronomy Band 604 MHz – 616 MHz CDMA Transmission 824 MHz – 896 MHz GSM Transmission 900 MHz & 1800 MHz Aeronautical Radio Navigation 960 MHz – 1164 MHz Radio Navigation satellite 1164 MHz – 1215 MHz Earth Exploration Satellite 1215 MHz - 1300 MHz Radio Location 1350 MHz – 1400 MHz Radio Astronomy Band 1400 MHz – 1427 MHz Broadcasting satellite 1427 MHz – 1429 MHz

1429 MHz – 1452 MHz 1452 MHz – 1492 MHz 1492 MHz – 1518 MHz

1.3… EFFECTS OF RFI: Interference may cause erratic program operation or it might cause errors in data processing. In imaging, erratic or undesired colour shifts might occur. Communication can be disrupted or completely unintelligible. Surveillance cameras and associated video m onitors may show shadowy background patterns. Sometimes the audio system may reproduce noise from fluorescent lights or diathermy equipment. RFI may be used to jam mobile phone communication. In case of radio astronomy, the signals received are from distant celestial objects. So the radio frequencies received from them are weak in power. They stand the possibility of being overshadowed by the interference from nearby RFI sources such as mobile phone signals, radio transmission signals etc. which are quite high on dB scale.

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CHAPTER 2 CHOICE OF ANTENNA: In general any antenna can be used for RFI monitoring, depending upon the application, ranging from dipole to log periodic dipole antennas. Here we were supposed to build an omni directional and wide band antenna. Omni directional antennas include: dipole antenna, biconical antenna, discone antenna and others. Wide band antennas include log-periodic dipole antenna, yagi-uda antenna, discone antenna, biconical antenna and others. We zeroed in on biconical and discone antennas as they are both omni-directional and wide band antennas. Due to lack of time, we could fabricate only one of them – the discone antenna. Its description is given in chapter 6. Also these antennas are light in weight. (Initially we were supposed to build a light weight antenna so that it could be mounted on existing 45 m dish. Later this criteria was not considered.)

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CHAPTER 3 BICONICAL ANTENNA: 3.1… INTRODUCTION TO BICONICAL ANTENNA.

A biconical antenna consists of an arrangement of two conical conductors, which is driven by potential, charge, or an alternating magnetic field (and the associated alternating electric current) at the vertex. The conductors have a common axis and vertex. The two cones face in opposite directions. Biconical antennas are broadband dipole antennas, typically exhibiting a bandwidth of 3 octaves or more.

Fig 1: A typical biconical antenna using radial elements.

The antenna exhibits a dipole-like radiation pattern, i.e., a toroidal shape with an omni-directional pattern in the H-plane. Since it is horizontally polarized, it is useful for monitoring the interference from TV and radio since the frequencies from these sources are horizontally polarized.

3.2… PHYSICAL CHARACTERISTICS OF THE BICONICAL ANTENNA:

The skeletal biconical antenna structure consists of wire cages mounted on either side of the support. Fig. 1 shows this structure. Each cone of the antenna is formed from elbow-shaped wires arranged around a single straight wire along the axis. The angle between each bent cone wire and the central wire is 30° and the angle at each bend is 90°.

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Therefore the entire structure of each cone may be defined by a single cone length dimension, as shown in following figure.

Fig 2 : dimensions of wire biconical antenna.

3.3… BICONICAL ANTENNA DESIGN CONSIDERATIONS:

Several parameters affect the performance of a biconical antenna. Among these are:

• Length of the antenna. • Spacing between the cones. • Radii of the cone elements. • Flare angle of each cone. • Number of elements.

Biconical antenna was designed for a frequency range of 400 MHz – 1750 MHz and the above parameters were changed to see their effects on

• Return loss of antenna. • Voltage Standing Wave Ratio (VSWR).

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• Input impedance. • Smith Chart. • Radiation Pattern.

All the above parameters were changed and their effects were seen on the performance of the antenna. Based on these results, the dimension of the parameter that gave the best performance over wide bandwidth was selected. Hence an optimized model was prepared which is described in chapter 5.

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Chapter 4 VARIATION OF PARAMETERS AND EFFECTS: 4.1… VARIATION IN LENGTH OF ANTENNA:

The relation between the length of the antenna and the wavelength in our design is given by L=0.36*lambda (in mm) Thus the length of one cone l = 0.18 * lambda (in mm) We started off with a design for tuning frequency of f = 500 MHz, i.e. lambda = 600mm So, l = .18 * 600 l =108 mm l ~ 110mm. Thereafter we tested the antenna for l = 80, 95, 110, 120, 130 mm and the results were as shown below.

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• ON INPUT IMPEDANCE OF ANTENNA.

As seen in the figure, the input impedance increases with the increase in frequency. It is shot down for l = 110 mm since the rest of the dimensions of the antenna like its width were designed for the same. But we chose l = 95mm for the reason that the radiation pattern for this length is better over a wider frequency range compared to that for l = 110mm. This is shown in the following figure.

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• ON RETURN LOSS.

The graph for return loss shows that as the length is increased, the tuning frequency goes on decreasing.

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• ON VOLTAGE STANDING WAVE RATIO (VSWR).

The VSWR chart also goes on bettering around the tuned frequencies depending upon the length.

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4.2… VARIATION IN RADII OF ELEMENTS:

The radii of the cone elements were kept and tested at r = 11, 12, 13, 14 and 15 mm. At lower radius, like r=6 mm, the return loss is quite high. Also the performance degrades for higher frequencies for r>15. So the radii was varied between the above mentioned range. The results follow:

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• ON INPUT IMPEDANCE.

The graph shows that as the radii increases, the input impedance goes on decreasing. It can be seen that the performance is best for r = 15 mm. But we didn’t opt for this radii as the performance started to deteriorate at higher frequencies as shown in the graph for return loss. Whereas for r = 13 mm, the bandwidth was quite broader. So we went with this radii. Thus a trade off exists between input impedance and high frequency response.

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• ON RETURN LOSS

The return loss improves for lower frequencies with increase in radii but deteriorates at higher frequencies.

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• ON VOLTAGE STANDING WAVE RATIO (VSWR).

The VSWR also shows a similar pattern as for return loss. It improves for lower frequencies but degrades at higher frequencies.

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• SMITH CHART FOR VARYING RADII.

The smith chart shows that as the radii is increased, the antenna starts matching. But at higher frequencies the matching becomes more and more inductive with increased radii.

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4.3… VARIATION IN SPACING BETWEEN THE CONES.

The spacing between the cone was simulated for s = 3, 4, 5, 6 & 7 mm. The results are shown below:

• ON INPUT IMPEDANCE

The input impedance goes on increasing as the spacing between the cone goes on increasing. Thus the spacing should be kept as less as possible keeping in mind other parameters.

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• ON RETURN LOSS

The return loss decreases for lower frequencies but improves for higher frequencies with decrease in spacing. Or in other words, the tuning frequency shifts from lower to higher.

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• ON VOLTAGE STANDING WAVE RATIO (VSWR).

The VSWR also shows the similar pattern as for return loss.

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• SMITH CHART FOR VARYING SPACING BETWEEN THE CONES.

The smith chart shows that frequencies start shifting from capacitive loading to inductive loading with increase in spacing. Also the bandwidth decreases with increase in spacing.

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4.4… VARIATION IN THE NUMBER OF ELEMENTS IN THE CONE:

The biconical antenna requires only minimal number of elements. This number depends on the ground reflection. For strong ground reflection, the number of elements should be more. Else a lesser number of elements will serve the purose. Here we designed and simulated the antenna for n = 4, & 6 elements. 2 element bicone is basically a bowtie antenna. But the results were simulated for n=2 also for reference. The results are shown:

• ON INPUT IMPEDANCE

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As can be seen in fig, a 2 element bicone gives a bit higher impedance than that for n=4 & 6. Although there is no significant difference between the performance for n = 4 & n = 6 in all the parameters. • ON RETURN LOSS

The effect of number of increasing the number of elements is the slight shift in the tuning frequency. As the number of elements is increased, the tuning frequency decreases. Also the return loss increases (since return loss is a negative quantity) by a small amount with increase in number of elements.

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• ON VOLTAGE STANDING WAVE RATIO (VSWR).

VSWR doesn’t show any marked difference for n =4 and n=6 as stated before.

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• SMITH CHART FOR VARYING NUMBER OF ELEMENTS.

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CHAPTER 5 OPTIMIZED DESIGN: 5.1… SIMULATED DESIGN:

From the above design considerations, the dimensions of the parameters which gave the best results were selected. Hence the optimized design is as follows.

Fig 3: Final optimized design. Here,

• Length of the antenna L = 190 mm. • Width of the antenna w = 78 mm. • Spacing between the cones s = 5 mm.

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• Radii of the cone elements r = 13 mm. • Flare angle of each cone = 60 degrees. • Number of elements n = 4.

5.2… RESULTS OF THE FINAL OPTIMIZED DESIGN:

• INPUT IMPEDANCE

• RETURN LOSS:

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• VOLTAGE STANDING WAVE RATIO (VSWR):

• SMITH CHART:

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5.3… RADIATION AND THETA PATTERN AT DIFFERENT FREQUENCIES:

AT 400 MHz:

AT 923.5 MHz

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AT 1006 MHz

AT 1199 MHz

AT 1309 MHz

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AT 1502 MHz

AT 1612 MHz

At 1722 MHz

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AT 1750 MHz

5.4… CONCLUSIONS:

� The bi-conical antenna designed using WIPL-D is optimised for frequency range

400 MHz – 1600 MHz.

� The effect of each dimension of the bi-conical antenna on input impedance, return loss, bandwidth and radiation pattern is presented.

� The antenna is omni-directional in Horizontal Plane for RFI monitoring with

uniform radiation pattern over the designed frequency band. 5.5… FUTURE SCOPES:

� The antenna could be constructed and the performance could be compared with the simulation results.

� Increasing Flare angle of the cone could increase the bandwidth of the antenna.

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CHAPTER 6 Introduction to Discone antenna: 6.1…Brief History The Discone antenna was invented and patented by Armig G.Kandoian of New York in 1943.The invention relates to radio antennas and in particular to broadband antennas for operation at UHF. The objective of his invention was to build an antenna for UHF in aircraft communications. For this he needed a structure of great rigidity, low wind resistance and a wide bandwidth. Hence he invented the so called discone antenna which gets its name from its distinctive shape. 6.2… Physical aspects The discone is normally a biconical antenna with one of the cone replaced by a disc. The ideal discone antenna consists of three major components :the disc, the cone and the insulator, the dimensions of which describe its radiating and receiving properties. The discone is said to have a broadband characteristics with wide bandwidth (1:3 in transmitting mode and 1:10 in receiving mode with a VSWR of < 2.5:1 in the frequency range of interest) and a relatively low angle of radiation and reception. It is ideal for VHF/UHF application as its greatest sensitivity is parallel to the surface of the earth but

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as the frequency increases the angle of radiation also increases slightly .The broadband characteristics owes to the typical dimensions of the discone. Although it is widely used for receiving applications, the discone is less commonly used for transmitting. This is because although it offers a wide bandwidth, it is not optimized for a particular band of frequencies and is less efficient than many other RF antennas 6.3… Mounting and dimensions The disc is mounted at the apex of the cone in spaced relation therewith such that there is an insulating separation between the cone’s apex and the disc. The insulator size governs a number of factors of the performance of the antenna like overall frequency range of the particular RF antenna design. The dimensions of the discone elements should have a relation with the operating frequency range which are as follows: Upper disc diameter (D) = 0.7 times the quarter wavelength of the antenna’s minimum cutoff freq Cone slant height (L) = Quarter wavelength of the antenna’s minimum cutoff freq Cone upper diameter (d1) = Diameter of the coaxial cable used for feeding the antenna Cone base diameter(d2) = Depends on the flare angle Cone flare angle (θ) = 25 to 40 degrees Cone and disc separation (S) = 20% of coaxial line diameter

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Cone

Disc

θ

D

L

S (disc and cone separation)

d1

d2 Typical discone antenna construction

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6.4… Operation The discone has an omnidirectional radiation pattern in the azimuthal plane(horizontal plane) and has a vertical polarization hence can be used for RFI monitoring as most of the man-made interference signals are vertically polarized.The discone has an input impedance of nearly 50 ohms and hence can be fed through a 50 ohms coaxial cable with the center conductor connected to the disc and the outer conductor (braid) connected to the cone. Actual impedance varies from 50 ohms depending on the cone angle, frequency, and disc to cone spacing.The energy from the feeder meets the antenna and spreads over the surface of the cone from the apex and towards the base until the vertical distance between the point on the cone and the disc is a quarter wavelength. In this way the energy is received or radiated efficiently. With the feed point at the top of the antenna the current maximum point is also at the the top .It is found that below the minimum cutoff frequency the antenna presents a very bad mismatch to the feeder. But, for the designed frequency range( 10 times the lower cutoff freq. the antenna maintains a reasonable match to the 50 ohms feeder ).

6.5… Our Design and Construction details Since our frequency range of interest was from 100 MHz to1600 MHz we calculated a design as follows: Lower cutoff frequency(fmin) = 100 MHz( λ=3m) Upper disc diameter = 0.7 times λ/4 at fmin = 0.7*( 0.750mm) = 525mm Cone length= λ/4 at fmin = 3/4mm = 750mm Cone upper diameter = 15.875mm Cone and disc separation = 0.2*15.875mm =3.1mm. Cone skirt angle = 60 degrees (half skirt angle = 30 degrees) From this we get the base radius = 375mm. Note It is true that a continuous structure discone antenna will always have a better radiation pattern, but for designing and fabrication at lower frequencies (VHF band) it is always better to use a structure having radial elements i.e. the disc and the cone approximated by wires (8 to 16 in numbers), as constructing a continuous surface for frequencies equal to and lower than 300MHz leads to impractical models. A discone structure using radial wires is as shown below:

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As we go to higher frequencies above VHF band(above 300MHz) the dimensions of the discone antenna are such that it is feasible to construct a continuous surface model.

The discone with 8 radial elements

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CHAPTER 7 WIPL_D Simulations: In order to understand the performance of the designed Discone antenna, it was necessary to simuate it using some software.We used the WIPL-D Pro–3D Electromagnetic Solver for the simulation. 7.1… The main criterion for judging the performance of the simulation resuts: 1. Return loss(S11) below -10 dB i.e. VSWR of 2:1 for entire frequency range of 100 MHz to 1600 MHz 2. An overall input impedance of 50 ohms for the entire frequency range. 3. A good radiation pattern (omnidirectional with vertical polarization) for the entire frequency range with a minimum of gain . 7.2… Given below are the simulation model and their results obtained from the WIPL_D Pro CAD software. The preview window showing the simulation model:

The simulated Discone with its dimensions

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Simulation plot showing the return loss (in dB)

Simulation plot showing the VSWR

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Simulation plot showing the input impedance variation with frequency

The Smith Chart showing the tuning of different frequencies

Fig Smith Chart

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7.3… Radiation patterns at different frequencies At 100 MHz

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At 235MHz

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At 327 MHz

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At 610 MHz

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At 1060 MHz

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At 1420 MHz

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At 1600 MHz

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At 2000 MHz

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The plot showing the omnidirectional pattern of the simulated discone at different frequencies

7.4… Design parameters varied during simulation

• Cone length(L) • Diameter of the Upper disc(D) • Diameter of the cone’s base(d2) or the cone flare angle(θ) • Diameter of the cone’s upper truncated section(d1) • Spacing(s) between cone and disc

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7.5… Variations in cone length Return Loss Vs Frequency plot obtained for different cone length

VSWR vs Frequency plot obtained for different cone length

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Input impedance vs Frequency obtained for different cone lengths

Effects of variation in cone length With the variation in cone’s length the return loss characteristics change.When the length is increased with respect to the designed dimension (750 mm) the return loss also increases as can be seen from the plots.The input impedance also increases .There is not much change in the radiation pattern by changing the length.

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7.6… Variations in Disc radius Return Loss Vs Frequency obtained for different disc radius

VSWR Vs Frequency obtained for different disc radius

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Input impedance vs Frequency obtained for different disc radius

Effects of variation in disc radius By varying the upper disc radius the return loss characteristics change i.e. by decreasing the disc radius the return loss for lower frequencies is less (better) but the VSWR increases. On increasing it the gain increases but the input impedance also increases. Hence, it was decided to keep the disc radius equal to 290mm according to the proposed design.

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7.7… Variations in Cone Base radius Input impedance vs Frequency obtained for different cone base radius

VSWR Vs Frequency obtained for different cone base radius

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Input impedance vs Frequency obtained for different cone base radius

Effects of Variations in Cone’s Base radius By varying the cone’s base radius(i.e. the flare angle) the return loss characteristics change i.e. by decreasing the cone’s base radius(decreasing the flare angle), the return loss is more for lower frequencies and less for higher frequencies .Similarly VSWR is more for lower frequencies and less for higher frequencies( for less base radius of cone). Input impedance is also more for lower base radius and as it is increased, input impedance increases. The gain is constant for lower frequencies for different base radius but for higher frequencies as the it is is increased, gain increases.

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7.8… Variations in spacing between cone’s apex and the disc Return Loss Vs Frequency obtained for different cone and disc spacing(s)

VSWR Vs Frequency obtained for different cone and disc spacing(s)

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Input impedance vs Frequency obtained for different cone and disc spacing(s)

Effects of Variations in spacing between cone’s apex and the disc This is infact the most important parameter on which the radiation properties depend. The lesser the cone and disc separation(s in mm) the better the return loss and input impedance of the discone. For fabrication and practical mounting consideration we kept the separation distance to be 2mm with a dielectric spacing in between.

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7.9… Variation in the cone’s upper radius Return Loss Vs Frequency obtained for different cone upper radius

VSWR Vs Frequency obtained for different cone upper radii

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Input impedance vs Frequency obtained for different cone upper radius

Effects of Variation in the cone’s upper radius Return loss first decreases with increase in cone upper radius upto 6mm, later it increases. It shows the best VSWR for s=2mm.Input impedance decreases with increasing cone upper radius. 7.10… Conclusion of the simulated results

1. The return loss(S11) is below -10 dB for the frequency range 100 MHz to 2GHz. 2. The VSWR value is below 2.5 and above 1 for the same frequency range. 3. The input impedance remains around 50 ohms(except at some points) for the

entire frequency range. Hence this shows that the discone can be fed directly through a 50 ohm coaxial cable.

4. The radiation pattern is best at 100 MHz and upto 500 MHz. Above this side lobes start to appear.

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CHAPTER 8 Fabrication and testing: 8.1…Design and results for the fabricated discone antenna From the simulations and the comparison plots it was observed that the discone showed the optimized performance for the following dimensions: Upper disc diameter (D) = 540mm Cone slant height (L) = 750mm Cone upper diameter (d) = 8mm Cone flare angle (θ) = 30 degrees Cone and disc separation (S) = 2mm A prototype antenna with a continuous surface was constructed ( as it was easy and less time consuming to fabricate ) and tested for its performance. 8.2… The fabricated design and construction details

The cone’s body was constructed using thin aluminium sheet and the disc was made of brass. For feeding the discone with a 50 ohms cable and to support the whole structure

Brass disc

Teflon Insulator

Aluminium cone

Aluminium Tubing

Type N connector

Brass Rod inner conductor

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the cone was supported on an aluminium tubing of 5/8 inch outside diameter and 19/32 inch inside diameter. The inner conductor used was a brass rod of diameter ¼ inches.The inner conductor is separated from the outer tubing using three dielectric(PVC) beads at three different positions. This section of coaxial line that results has an impedance of 52 ohm approximately. The length of the conductor was kept 1m.The aluminium tubing was soldered to the cone and the inner brass rod was connected to the upper disc through a screw. The disc is insulated from the cone using a Teflon spacing of 2mm. The discone was fed through a coaxial cable (50 ohms impedance) and tested for the various parameters using Agilent Technologies E5070B( 300 KHz to 3Ghz) network analyzer. 8.3… A Block diagram showing the test apparatus for testing the discone The discone

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The discone was tested for its return loss parameters using a network analyzer with the set up as shown above. 8.4… The following results of plots were obtained: The measured return loss plot of the fabricated discone.

Plot of return loss (S11 in dB)

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The graph showing the comparison of the simulated and measured results of plots :

Conclusion of the plot The return loss comparison plot shows that the two results (simulated and measured ) match approximately except that the measured return loss is more (above -10 dB) for certain frequencies which can be due to mismatch in the feeding point and the input impedance of the discone.

Return Loss Performance - Comparision with Simulation

-35.00

-30.00

-25.00

-20.00

-15.00

-10.00

-5.00

0.00

1.0E+08

2.0E+08

3.0E+08

4.0E+08

5.0E+08

6.0E+08

7.0E+08

8.0E+08

9.0E+08

1.0E+09

1.1E+09

1.2E+09

1.3E+09

1.4E+09

1.5E+09

1.6E+09

1.7E+09

1.8E+09

1.9E+09

2.0E+09

Frequency in Hz

Po

wer

in d

B

Measured

Simulated

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8.5… Block Diagram showing the arrangement for testing the antenna radiation pattern

A set up for testing the radiation pattern of the discone was made with the above block diagram illustrating it. A Log Periodic Dipole Array (LPDA)antenna was used to receive the power transmitted from the discone (The power level given to the discone from signal generator was 0 dBm) in four directions around it.The received power levels from the LPDA was measured on a spectrum analyzer along four directions around the discone(i.e. 0˚,90˚,180˚ and 270˚) and a polar plot was plotted.

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Radiation pattern polar plots at different frequencies:

Conclusion from the polar plot Plot shows the omnidirectional radiation pattern. Gain decreases with increase in frequency.

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8.6… Final Conclusion

• The simulated and measured results match to some extent. • The return loss is greater for the measured antenna compared to simulated result. • The dielectric spacing(s)between cone and disc may affect the return loss.

8.7… Future scopes of improvement • The antenna feeding could be improved for better matching. • Discone using radial elements could be light in weight. • Use a whip at the top of disc could improve low frequency response. • Parabolic discone for high gain. • Novel type double discone for wider bandwidth.

8.8…Photographs showing the discone antenna and the experimental setup Test set up for return loss measurement

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Test set up for radiation pattern measurement

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References

� Antenna Design and Theory, C.A. Balanis. � The development of an optimal model of a skeletal biconical antenna, S.M.

Mann and A.C. Marvin. � Poptronix Electronix Handbook,William Sheets and Rudolf Graf. � A UHF Discone antenna for scanners, William Sheets and Rudolf Graf

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