실습 04 – 반파장 다이폴

1. 이론

ㅇ 반파장 다이폴은 길이가 0.35-0.48파장(다이폴 단면에 따라 변함)인 금속의 중간에서 급전하는 안테나이다.

ㅇ 가장 기본적인 안테나로서 기본형과 응용구조가 널리 사용된다.

ㅇ 다이폴 설계 시 핵심은 대역폭, 급전, 다이폴의 크기이다.

ㅇ 다이폴의 전류분포: 중심을 기준으로 대칭적 정현파 분포

current along halfwave dipole versus length current distribution on dipole antennas

ㅇ 다이폴의 입력 임피던스: 임피던스는 주파수에 따라 변하며 허수부가 0이 되는 최초 공진은 다이폴 길이가 반파장 부근일 때 발생. 이 때 입력 저항은 약 72W

ㅇ 다이폴의 방사패턴: 3dB 빔폭 78°

radiated fields from dipole antenna power plot of dipole's radiation pattern

3d radiation pattern for one wavelength dipole radiation plot for 1.5 wavelength diple antenna

ㅇ 다이폴의 이득(지향도)

– 길이가 매우 작을 때: D = 1.50 = 1.76 dBi

– 반파장 다이폴: D = 1.64 = 2.15 dBi

directivity of dipole versus length

ㅇ 상용제품 예:

1) 기지국용 광대역 다이폴(220-400MHz)

2) 통신용 광대역 다이폴(138-1000MHz)

Wideband Antennas_1206

2) Dipole

- A dipole is a two-piece straight metal antenna fed at the center.

- When a half-wave dipole is placed along the z-axis, at far fields we have .

- The radiation pattern of a half-wave dipole resembles the number 8 with a maximum occurs in the direction normal to the dipole axis while a null occurs in the direction of the dipole axis.

- Half-wave beamwidth and gain of a half-wave dipole are 78º and 2.1dB respectively.

Fig. 5.4 A dipole and its radiation pattern.

Fig. 5.5 A 2-m dipole antenna.

- The dipole's impedance changes with the frequency. Its first resonance is at the frequency where its total length is about a half wavelength (to be more exact, L = 0.468λ). At resonance the dipole's input impeance is about 70+j0 (Ω).

input impedance of dipole antennas

Fig. 5.6 The dipole input impedance versus the length.

- A dipole is often fed by a coaxial cable. In this case there is an imbalance in the magnitude of currents on two halves. Even with this current imbalance, one can still use a coaxial cable feeding for the sake of simplicity.

Fig. 5.6 A coax-fed dipole and the current imbalance.

2. 실습

1) Draw the geometry.

- Draw the following geometry of a dipole with dimensions:

d = 0.5mm (You will fabricate the dipole using a thin wire used for wiring components on the breadboard)

L = λ/2 @ 1.575GHz = 95.2mm

g = 0.1 mm

- Be sure to make the dipole axis be in the z direction. In this case, the E-plane is zx- or yz-plane and the H-plane is the xy-plane.

Dipole antenna 2011.04.06.wmf

2) Simulate and tune the antenna.

- Use a discrete port (a delta-gap source) for the excitation.

- Frequency range: 0.2 to 3.2GHz.

- With the initial dimension, your antenna will resonate at 1.46GHz. Using the following formula based on

the theory of frequency scaling, we find the dipole length for a resonance at 1.575GHz.

- Now adjust the dipole length to 88.2mm and do the simulation again.

3) Analyze the result and write a report.

- Analyze the result, that is, see simulation results and form your opinions on them.

- In making graphs, make sure that sizes of axis labels, scale values are big enough.

○ Near field at 1.575GHz:

- Observe the electric field around the dipole using stationary and animated views.

- Observe the magnetic field (or the equivalent electric current) around the dipole. The surface electric current density on a conductor is related to the magnetic field

- Plot the magnitude and phase of J_z along the surface of the dipole on the same graph.

○ Input impedance:

- |S₁₁|(dB) from 0.2 to 3.2GHz. Find the impedance bandwidth (|S₁₁| < -10dB) with two vertical cursors.

- Plot the impedance locus on the Smith chart from 1.0 to 2.0GHz.

- Plot R_in and X_in from 0.2 to 3.2GHz on the same graph. Mark R_in and X_in at 1.575GHz with a vertical cursor.

○ Gain and directivity:

- G(dB) and D(dB) from 0.2 to 3.2GHz by 0.2GHz step on the same plot.

○ Gain pattern at 1.575GHz:

- G_abs: 3D, E-plane polar pattern, H-plane polar pattern

- G_θ: 3D, E-plane polar pattern, H-plane polar. Mark the 3dB beamwidth on the polar plot.

- G_φ: 3D, E-plane polar pattern, H-plane polar pattern

- Write a full report using graphs from the simulation software.

- Give the antenna geometry, dimensions and the method of excitation. Include the coordinate axis.

- Present graphs described in the above and give your assessment of the results.

- Make a hand-written report on the spot as follows.

- Describe the antenna geometry, dimensions and the feeding method. Include the coordinate axis.

- Plot the magnitude of J_z along the surface of the dipole. Give your analysis of the result.

- Input impedance:

- |S₁₁|(dB) = ( ) @ 1.575GHz

- Impedance bandwidth (-10dB reflection)

- Z_in = ( ) + j( ) ohms @ 0.2, 1.0, 1.575, and 2.0GHz

- Gain and directivity:

- G_θ(dB) = ( ) @ 0.2, 1.0, 1.575, 2.0GHz. Give your analysis of the result.

- D_θ(dB) = ( ) @ 0.2, 1.0, 1.575, 2.0GHz. Give your analysis of the result.

- Gain patterns at 1.575GHz:

- Plot G_θ(dB), G_φ(dB) on E-plane. Give your analysis.

- Plot G_θ(dB), G_φ(dB) on H-plane. Give your analysis.

- E-plane beamwidth = ( ) deg.

4) Simulation results.

- Electric field E around the dipole

- Magnetic field H around the dipole

- Current density J on the dipole surface

- Magnitude and phase of J_z along the surface of the dipole

(Question) In a discrete port, what is the terminal voltage or current?

- Terminal voltage and current of the discrete port vs frequency

- Reflection coefficient: bandwidth = 146.2MHz (9.3%)

- Input impedance locus on the Smith chart

- Input impedance . Inaccurate results below 1GHz. Why is the accuracy below 1GHz poor? We have to increase the accuracy setting in the 'Solve' - 'Transient Solver Parameters' menu. Increase the accuracy from -30dB to -60dB for example.

- Input impedance . Accurate results.

- Gain and directivity. vs frequency. Inaccurate below 1GHz.(MWS Transient Solver accuracy setting: -30dB)

- Gain and directivity. vs frequency. Inaccurate below 0.4GHz.(MWS Transient Solver accuracy setting: -60dB)

- Gain and directivity. vs frequency. Accurate results. (MWS Transient Solver accuracy setting: -60dB). The absorbing boundary is too close to the antenna. We have to increase the distance from the dipole to the absorbing surfaces of the rectangular box enclosing the dipole.