Download Folded Dipole Antenna

Folded Dipole Antenna
BHAVIN V KAKANI
IT-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 shortcircuited 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
Name
Gain (over
isotropic)
Beamwidth 3 dB
0 dB
360
2.14 dB
55
Turnstile
-0.86 dB
50
Full Wave Loop
3.14 dB
200
Yagi
7.14 dB
25
Helical
10.1 dB
30
Parabolic Dipole
14.7 dB
20
Horn
15 dB
15
Biconical Horn
14 dB
360x200
Isotropic
Dipole
Shape
Radiation Pattern