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Wire Antennas Monopole Antenna
Long Wire or Traveling wave Antennas
Yagi Uda Antenna
Reference: C. A. Balanis, J.D. Krauss
Prof. D. Kannadassan
Image theory, an intro
A positive charge+Q (or a charged body) is located a height h from perfect ground, then an image as -Q will form at a depth of h.
A dipole will form, and the filed lines will be closed.
Both the poles will contribute the field present at point P,
located at (x, y, z)
Perfect conducting plane
Monopole antenna - Theory
Theory and derivation of
Monopole and Dipole are
completely same, but the
Power output will always
half to monopole.
And Radiation resistance
will be half of that for
From the ground, at a very small height,
a /4 antenna is place vertically, called
AM, FM applications.
Long wire antennas
Long wire antennas - Theory
Consider a thin wire of length L is horizontally placed at a height from
ground plane. At one end a RF source is connected and other end is
terminated by the characteristics impedance of the wire Z0.
When the wire is excited with a sinusoidal signal, the wave should travel
along one direction and will not create any standing wave so called
We are gonna see the expression for electric field at a point P - located at a
distance of r from the wire and angle of with respect to the length of
Travelling wave structure E field
The retarded current in the wire shall be described as
Where: v=p.c or p=v/c
p is the ratio of the wave velocity at the wire to the free space, called Relative
phase velocity. Used to vary with the attenuation constant.
Z0 Wave direction
We can find the radiation pattern as similar to the dipole case, will result:
Here, is wave impedance.
Maxima direction: Also called Elevation angel (max, m, , )
For 3 and 5
Minor lob directions, Directivity
m=0 m=1 m=2
Bidirectional long wire antenna
The maxima direction oriented with the direction of wave in the wire, so
by introducing wave on both the direction we can introduce Bi-
This can be possible by Open circuited long wire or Un-terminated, so
called Stand wave antenna.
By terminating with Z0, we can get Uni-directional radiation
=2 where is max of single long wire
Based on the principle of Traveling wave and Vee antenna, Rhombic
antenna is a very high directive antenna has Diamond or Rhombus shape
Working of Rhombic antenna
By properly selecting the tilt angle, the
rhombic antenna will give additive effect
of radiation pattern of each long wire
The radiation mechanism is basically
depends on two factors:
Tilt angle ()
Height above the ground (h)
These are design parameters of antenna
Due to ground effect, the maximum
radiation is elevated about an angel ()
Design equations of Rhombic antenna
BBL field equation: (Bruce, Beck and Lowry 1935)
From this equation, we can deduce the condition to get the maximum
power direction with respect to height h and length of line L
About Dr. Bruce, E: http://ieeexplore.ieee.org/iel5/10933/35478/01685103.pdf
Original Article: www.alcatel-lucent.com/bstj/vol14-1935/articles/bstj14-1-135.pdf
Maxima with height
Maxima with Length L
Finally, the design formulae are: (also called BBL formula)
Design a rhombic antenna to operate at 20MHz when the angle of
elevation angle =10o.
Numerical Problems and Review Questions
Explain about long wire and Rhombic antenna with its radiation pattern
Design a rhombic antenna to operate at 20MHz when the angle of elevation
Explain resonant and non-resonant modes of Long Wire Antenna.
If we assume the average beamwidth of rhombic antenna as 10o, then
design an antenna system such that it will radiate maximum power over the
ranges from 10o to 40o for the operating frequency of 10MHz
Explain the working of Open circuited long wire antenna and V antenna
with radiation pattern
Yagi-Uda Antenna A high frequency and high directive Parasitic array antenna
Prof. S. Uda (japan) was invented this antenna by 1927, and collaborated with H.
S. Uda, "High angle radiation of short electric waves". Proceedings of the IRE, vol. 15, pp. 377-385,
After the invention, more than 40 researcher were studied on the improvement.
Latest article (2011): Application of bacteria foraging algorithm for the design
optimization of multi-objective Yagi-Uda array
Shintaro Uda Hidetsugu Yagi
Principle A folded dipole or ordinary half wave dipole is centered between two types of parasitic
elements, called: Directors and Reflectors.
The coupling (capacitive) effect between the parasitic elements and active element(dipole), the directional properties are improved a high with endfire pattern
Reflector: about 5% greater length than the active element, will reflects the power radiation at backward direction.
Directors: 5% lesser length than the active element, will create a converging mechanism and increase the directivity along the forward direction.
Spacing between the directors and reflector are depending on the optimality, in most of the case, the spacing should from 0.3 to 0.4 (at 1927, the spacing was /10)
12 to 20 element yagi-uda antennas are optimum and have improved directivity
Radiation properties Basically End-fire radiation pattern, with high directivity (less HPBW)
Due to the ground and parasitic element, the pattern maxima at elevation will not be
at 90o (along the axis), but 45 to 60 degree elevated so.
We can show that, while increasing the directors, the gain and directivity
will improve, however the side lobs will degrade the performance by
attracting the noise in unwanted direction.
Backward gain (or back gain)
Front to Back ratio (diff of F.gain and B.gain)
Magnitude of side lobes
Bandwidth, quality factor
Simulation 9GHz Yagi Uda
Atleast 1 or 2
Due to inductive effect at dipole
and capacitive effect at parasitic elements
@ 8.8GHz max
Front to back ratio=40-25 = 15dB
Front to back ratio=42-17 = 25dB
Optimized Design of N-element Yagi-Uda.
For the frequency of operation f0, the will be estimated.
Reflector: (mostly 1 element) length of 0.5 with spacing of 0.25 to 0.3 from dipole.
Dipole length (active element): 0.475
Directors (N-2 elements) 0.405 with spacing of 0.3 to 0.4 between each element.
To match the dipole, usually the QWT section be utilized.
Example (from Balanis)
Various Yagi Uda antenna
Smallest Yagi Uda antenna!! Ivan S. Maksymov et al, Optical Yagi-Uda nanoantennas, http://arxiv.org/pdf/1204.0330.pdf