Log Periodic New

LOG-PERIODIC ANTENNAS  • It is an antenna configuration, which closely 

parallels the frequency independent concept.• Because the entire shape of it cannot be solely specified by angles, it is not truly frequency independent .

• Impedance is periodic with logarithm of frequency. 

• It is non linear array i.e. spacing of the elements as well as their dimensions are unequal.

LOG-PERIODIC ANTENNAS

• Its excitation is uniform.• Frequency independent antenna.• It can be used to receive a good number of TV channels 

without any deterioration of received field strength.• Entire structure will never radiate. Depending on the 

frequency a particular dipole is active and other are not in resonance hence they don’t radiate.

A plot of the input impedance as a function of logarithm of the excitation frequency shows a periodic variation.

LOG-PERIODIC ANTENNAS

• In spherical co-ordinates (r, θ , Ф ) the shape of the structure can be written as

• θ = periodic function of [ b ln (r)]

• θ = θ o sin [ b ln ( r/ ro )]• from above equation it is clear that the values of θ are

repeated whenever the logarithm of the radial frequency ln(ω) = ln(2πf ) differs by 2π /b.

• The performance of the system is then periodic as a function of the logarithm of the frequency; thus the name logarithmic (Log)periodic.

LOG-PERIODIC ANTENNAS

• a log-periodic antenna (LP, also known as a log-periodic array) is a broadband, multi-element, unidirectional, narrow-beam antenna that has impedance and radiation characteristics that are regularly repetitive as a logarithmic function of the excitation frequency. 

• The individual components are often dipoles, as in a log-periodic dipole array (LPDA). Log-periodic antennas are designed to be self-similar and are thus also fractal antenna arrays.

• The length and spacing of the elements of a log-periodic antenna increase logarithmically from one end to the other.

LOG-PERIODIC ANTENNAS• Major advantage

– independence of radiation resistance and radiation pattern to frequency

• Large bandwidth ratio– highest to the lowest operating frequency of 10:1 or greater

• Gain– unidirectional/bidirectional and low-to-moderate– higher gain may be achieved by adding more arrays

LOG-PERIODIC ANTENNAS

• Radiation pattern – maximum radiation is outward from the small end

• Antenna input impedance– varies periodically the log of frequency

• Often used in HF & VHF communication

A planar log periodic structure 

LOG-PERIODIC ANTENNAS

• It consists of a metal strip whose edges are specified by the angle α/2 . 

• In order to specify the length from the origin to any point on the structure, a distance characteristic must be included.

• The element spacing also decrease towards the front of the array where the smallest elements are located.

LOG-PERIODIC ANTENNAS

• Basic geometric structure is repeated but with changing size of structure.

• Dimensions increases in proportion to distance from origin.• It has number of dipoles of different length and spacing.• It is fed by balance two wire transmission line which is 

transposed between each adjacent pair of dipoles.

LOG-PERIODIC ANTENNAS• It is fed at narrow end.• Dipole length increases along antenna such that phase angle 

remains constant.• Length and spacing are graduated in such a way that certain 

dimensions of adjacent elements bear a constant ratio with each other.

• simpler, lighter in weight, cheaper, and less wind resistant antenna.

• Non planar geometries in the form of a V, formed by bending one arm relative to the other, are also widely used.

• If the wires or the edges of the plates are linear (instead of curved), the geometries reduce, respectively, to the trapezoidal tooth log-periodic structures.

• Strong current concentration at or near edges of conductors.

• These simplifications result in more convenient fabrication geometries with basically no loss in operational performance.

• Experimentally it is proved that performance of planar as well as wire antenna is almost identical.

• the geometric ratio of the log-periodic structure by

• width of the antenna slot by

• The geometric ratio τ defines the period of operation.

• if two frequencies f1  and f2  are one period apart, they are related to the geometric ratio τ

Dipole Array• It consists of a sequence of side-by-side parallel linear

dipoles forming a coplanar array. Although this antenna has slightly smaller directivities than the Yagi -Uda array (7-12 dB), they are achievable and maintained over much wider bandwidths.

• While the geometrical dimensions of the Yagi - Uda array elements do not follow any set pattern, the lengths (ln ’s), spacings (Rn ’s), diameters (dn ’s), and even gap spacings at dipole centers (sn ’s) of the log-periodic array increase logarithmically as defined by the inverse of the geometric ratio τ .

• Another parameter that is usually associated with a log-periodic dipole array is the spacing factor σ

• Straight lines through the dipole ends meet to form an angle 2α which is a characteristic of frequency independent structures.

Feed Elements of LOG-PERIODIC ANTENNAS

• There are two basic methods, as shown in Figures 11.9(b) and 11.9(c), which could be used to connect and feed the elements of a log-periodic dipole array. In both cases the antenna is fed at the small end of the structure.

• The currents in the elements of Figure 11.9(b) have the same phase relationship as the terminal phases. If in addition the elements are closely spaced, the phase progression of the currents is to the right. This produces an end-fire beam in the direction of the longer elements and interference effects to the pattern result.

• It was recognized that by mechanically crisscrossing or transposing the feed between adjacent elements, as shown in Figure 11.9(c), a 180◦ phase is added to the terminal of each element. Since the phase between the adjacent closely spaced short elements is almost in opposition, very little energy is radiated by them and their interference effects are negligible. However, at the same time, the longer and larger spaced elements radiate.

• The mechanical phase reversal between these elements produces a phase progression so that the energy is beamed end fire in the direction of the shorter elements. 

• The most active elements for this feed arrangement are those that are near resonant with a combined radiation pattern toward the vertex of the array.

Yagi uda array

Yagi Fundamentals

• A Yagi-Uda array consists of 2 or more simple antennas (elements) arranged in a line.

• The RF power is fed into only one of the antennas (elements), called the driver.

• Other elements get their RF power from the driver through mutual impedance.

• The largest element in the array is called the reflector.

• There may be one or more elements located on the opposite side of the driver from the reflector. These are directors.

Yagi uda array

• Only the driven element is connected directly to the feeder; • The other elements couple to the transmitter power through 

the local electromagnetic fields which induce currents in them.

•  The spacing and length of the various components significantly affect the performance characteristics of the antenna.

The 2 element Yagi• The parasitic element in a 2- element yagi may be a

reflector or director• Designs using a reflector have lower gain (~6.2 dBi)

and poor FB (~10 dB), but higher input Z (32+j49 )• Designs using a director have higher gain (6.7 dBi)

and good FB (~20 dB) but very low input Z (10 )• It is not possible simultaneously to have good Zin, G

and FB

The 3 element Yagi

• High gain designs (G~ 8 dBi) have narrow BW and low input Z

• Designs having good input Z have lower gain (~ 7 dBi), larger BW, and a longer boom.

• Either design can have FB > 20 dB over a limited frequency range

• It is possible to optimize any pair of the parameters Zin, G and FB

Larger yagis (N > 3)

• There are no simple yagi designs, beyond 2 or 3 element arrays.

• Given the large number of degrees of freedom, it is possible to optimize BW, FB, gain and sometimes control sidelobes through proper design. (although such designs are not obvious)

• Good yagi designs can be created using antenna modeling software

• Difference between yagi uda and log periodic

• The Yagi consists of one driven element, plus one or more shorter elements acting as directors placed in front of it, plus one or more longer elements acting as reflectors placed behind it. 

• Technically, the driven element is cut and the feed system is matched for a specific frequency . The farther from that frequency, the more the performance of the Yagi degrades.

•  While only one element of the Yagi–Uda array is directly energized by the feed line, while the others operate in a parasitic mode, all the elements of the log-periodic array are connected. 

• The Log Periodic array consists of a uniformly tapered array of many elements. 

• The lengths of the elements and the spacing between them decrease by a constant ratio as you move down the array, and all of the elements are electrically connected to the feed line. The objective is an array that's very broad in frequency. 

• The idea is that at any given frequency within the total range of the array, the element closest to a resonant length will radiate/receive best, those in front of it will function as directors, and those behind it will function as reflectors

• Then the basic antenna, driven element with both Reflector and Director is called three elements Yagi-Uda, with increased 

directivity or beam Gain.

More than one parasitic element should be axially added in the front of the driven element and each one is called director. As the reflector, the directors (D1…Dn) has not wired directly to the feed point. As the number of director grow, it increase the directivity as the beam gain of the Yagi-Uda system array.

2 element yagi

• Yagi-Uda is the correct complete name for any antenna with three or more elements in the array.

•  Yagi is the name for the two elements, a driven element and the reflector, or the driven element and one director alone.

•  Yagi is well-accepted term for any linear end-fire array like the Yagi-Uda defined above. 

• The television antenna set, FM and amateur radio antennas commonly named Yagi are really Yagi-Uda antenna when three or more elements in the system array.

Log periodic Yagi-uda