Antenna Balun

[요약

ㅇ벌룬의 필요성: 대칭적 전류(전압)이 여기 되어야 하는 안테나를 unbalanced line으로 급전 시

ㅇ파장이 짧은 경우: 구조를 서서히 대칭적으로 변환 (전송선 형태)

ㅇ파장이 긴 경우: 전송선 구조 사용 시 크기가 커서 비현실적. 페라이트 초크 등 다른 방법 사용

ㅇ벌룬의 종류: 참고문헌

- 페라이트 비드 choke = common-mode choke: Brown, Counselman

[Summary

[Balun

- No radiation from the transmission line feeding an antenna

- Common mode on a transmission line: it radiates and receives.

- Most outdoor antennas (e.g. amateur radio antennas) are unbalanced by their surroundings, even when fed by a balanced source and line; unequal capacitances to nearby conductors, unequal inductance coupling to nearby conductors; trees, buildings, towers, terrain.

- Balanced circuit: The impedance of each conductor to the reference plane are equal.

- Coaxial cable

Differential-mode power: all inside the coax

Common-mode power: outside the coax. A ferrite core surrounding coax sees only the common mode power and field.

Skin effect splits the shield into two conductors: Inner skin carries differential-mode current. Outer skin carries common-mode current (current due to imbalance)

- Common mode choke

A coil of coax at the antenna

A stack of ferrite beads around the coax; why many beads? → to increase the resistance to the common mode to sufficiently high level

Multiple turns of transmission line through a toroid or stack of toroids

Most 1:1 baluns are common-mode chokes.

Reduce receive noise

Minimize antenna interaction

- Common-mode choke design

A good common-mode choke should present a high real impedance (not reactive) to the common-mode current.

Reactance can resonate with the transmission line.

1) Ferrite beads

W2DU, W0IYH balun

Single bead impedance: #73 mix, 0.19" long (which fits RG58 or RG303)

50 #73 beads

Single #43 bead: predominantly inductive below 25MHz. Very sensitive to the feedline legth. Inductance resonates with a capacitive line. Increasingly resistive above 25MHz and much less sensitive to feedline length.

Single #31 bead (fits RG8)

2) Coaxial chokes

DX Engineering 50Ω choke balun

Coax connector + Shield box (connected to the outer conductor of the coax and the ground line of the twin line) + 3 thick toroidal cores of ferrite (coax passes through it many times) + Twin-line terminal (one is connected to the coax inner conductor, the other to the outer conductor)

DX Engineering 200Ω - 50Ω choke balun

Ferrite choke + 4:1 transformer

W2FMI choke balun

- Heating of the ferrite by common current

Large R, small common-mode current and power

For HAM bands, R > 5k gives reasonable margin.

Coaxial cable wound around a ferrite toroid.

Impedance increases as the square of the number of turns

Capacitance between turns

Resonance frequency drops.

1-2 turns for VHF, 4-7 turns for HF

Spacing between turns:

Close spacing lowers resonant frequency (large capacitance and inductance). Close spacing below 10MHz. Wide spacing best above 10MHz.

- Choke measurements

Very difficult to measure. Traditional reflection measurements don't work. Stray capacitance of fixture causes additional errors.

3) Twin lead chokes

Twin lead has 30-40% leakage flux (of transmit power) plus the common mode voltage

Much more likely to overheat

Must use low loss core #61, #67

- Single wire choke on a #61 toroid

[Glossary

common mode

differential mode