Folded Dipole Antenna

BHAVIN V KAKANIIT-NU

Introduction

• A folded dipole is a half-wave dipole with an additional wire connecting its two ends.

• It’s a widely used aerial, not only on its own but also as the driven element in other antenna such as yagi antenna.

• Two half wave dipoles have been folded and joined together.

• One is continuous while other is split at the centre.

• The separation distance is assumed to be small relative to wavelength.

• The split dipole is fed with a balanced transmission line

• The voltages at the ends of 2 dipoles are same.

• Two dipoles are found to be in parallel essentially.

• Radiation pattern of conventional half-wave dipole and folded dipole are almost same.

• Major difference is in terms of input impedance.• Input impedance of folded dipole is much higher than half

wave dipole.• Other 2 parameters relatively different from half wave are

directivity and Bandwidth.• main reasons for using the folded dipole aerial is the increase

in feed impedance that it provides.• In free space, this gives an increase in feed impedance from

73Ω to around 300Ω .

• Because the folded dipole forms a closed loop, one might expect the input impedance to depend on the input impedance of a short-circuited transmission line.

• Because the folded dipole is "folded" back on itself, the currents can reinforce each other instead of cancelling each other out, so the input impedance will also depend on the impedance of a dipole antenna

Antenna Impedance

• The input impedance of the folded dipole is defined by the ratio of voltage to current at the antenna feed point.

• The current on the folded dipole can be decomposed into two distinct modes:– An Antenna mode (currents flowing in the same direction yielding

significant radiation)– Transmission line mode (currents flowing in opposite directions

yielding little radiation)

• Superposition of the two modes yields the folded dipole input voltage V on the left wire and zero on the right wire.

• The transmission line current It in both antenna conductors must be the same in order to satisfy Kirchoff’s current law at the ends of the antenna.

• The total antenna current Ia must be split equally between the two antenna conductors to yield the proper results for the radiated fields

• The total folded dipole input current can then be defined as the sum of the transmission line and antenna currents

• the folded dipole input impedance may be written as

• The folded dipole impedance is determined by relating the transmission line and antenna mode currents to the corresponding input voltage.

• Insert an equivalent set of voltage sources into the transmission line mode

• Equivalent to set of two shorted transmission lines of length l/2.

• Both of the shorted transmission lines are driven with a source voltage of V/2 across its input terminals.

• The voltage and current for the transmission lines are related by

• The general equation for the input impedance of a transmission line of characteristic impedance Zo and length l terminated with an load impedance ZL is

• For the shorted line, ZL = 0 and the length is l/2 so that

• The characteristic impedance of the two wire transmission line is

• The folded dipole antenna current can be related to an equivalent dipole by

• Zd is the input impedance of a dipole of length l and equivalent radius ae.• The equivalent radius is necessary because of the close proximity of the

two wires (capacitance) which alters the current distribution from that seen on an isolated dipole.

• The impedance Zd is given by

• Given the relationships between the transmission line and antenna mode currents and voltages, the input impedance of the folded dipole can be written as

• For the special case of a folded dipole of length l = λ/2, the input impedance of the equivalent transmission line is that of a shorted quarter wavelength transmission line (open-circuit).

• The impedance of the half-wave folded dipole becomes

• The half-wave folded dipole can be made resonant with an impedance of approximately 300 which matches a common transmission line impedance (twin-lead).

• The half-wave folded dipole can be connected directly to a twin-lead line without any matching network

• Dipole antenna possesses “Impedance Transforming Property”• It can be done not only by increasing the dipoles in antenna but

also by changing radii of the conductors in the FD antenna• Unequal radii results into unequal current into different

conductor and that gives transformation ratio of 1.5-20. • The folded dipole has a larger bandwidth than a dipole of the

same size.• The folded dipole antenna is resonant and radiates well at odd

integer multiples of a half-wavelength (0.5, 1.5, ...).

Conclusion

– Basic ½ dipole folded to form complete circuit

– Core to many advanced antennas

–Mechanically more rugged than dipole

– 10% more bandwidth than dipole

– Input impedance 292

– Close match to std 300 twin lead wire transmission line

–Use of different diameter upper & lower arms allows variable impedance

552.14 dB Dipole

3600 dB Isotropic

Beamwidth -3 dB

Gain (over isotropic)

ShapeName Radiation Pattern

20

30

50

200

25

14.7 dB

10.1 dB

-0.86 dB

3.14 dB

7.14 dB

Parabolic Dipole

Helical

Turnstile

Full Wave Loop

Yagi

Biconical Horn

1515 dBHorn

360x20014 dB