Design of Log Periodic Dipole Array ( Lpda ) with Cross-shape Eaching for Uhf-vhf Application ( 200-1500 mhz ) With Band Notch of ( 800 mhz-1000 mhz )

Computer analysis reveals that the anomalous behavior of symmetric Log Periodic Dipole Array (LPDA)Antennahas infloenced by different parameters . The most important is mainly due to the radiation of the dipole elementsin the anomalous radiating region which is excited by the transmission line resonance of the feed structure. In the designby changing the element spacing , changes of length of each element in the LPDA antenna and define cross eaching in an boom of an antenna in middle of the specified element the result is the return loss less from 200MHZ to 1500MHZ other than the notch between 850MHZ 950MHZ according to the simulation results.The log-periodical Dipole Antenna (LPDA) is a kind of ultrawide-band antennas. To achieve the purpose of miniaturization of the LPDA in radiate direction, this paper presents anunusual structure logperiodically loaded with slot cross-shape design.

GHz and 5.250 GHz-5.825GHz) 5 .Thus, designing UWB antennas with multiple frequency notched bands for blocking the interference between the UWB systems and the existing narrow bandwireless communication systems has become a challenge research topic 6,7 .Frequency bands allocated to various types of radio services in India but for mobile communication in Mhz 806-960 Used by GSM and CDMA mobile services(In India GSM technology works in the frequency bands of 900 and 1800 MHz and CDMA technology works in the 800 MHz band) 4 .

Design consideration the LPDA
CST2102 antenna design software uses a wire antenna of a constant diameter for each element to calculate antenna characteristics.Actually, the Log Periodic Dipole Array antenna can be determine by the following equation : In the first step, the scale factor, spacing factorand the number of the dipole elements should be determined.The longest element length in LPDA is given by ... (1)   where λL is the longest operating wavelength and K1 is the lower truncation constant.We determine K1 from the empirical equation Wavelength at 200MHz: λ = 150cm, shortest operating wavelength.
We calculate the upper truncation constant from * The shortest element length is L U , = [1]  ... (3)   where λ U is the shortest operating wavelength.
The value of L U =4.866 cm that is calculated from the equation (3).By using the truncation constants and the frequency band edges for calculating the number of dipole required for the a specify design [1]  ...( 4) For a given frequency, F L = F U = f The number of element for the design from Eqn( 4) is,To reach to better result adding the number of element by 2 make the total number of element comes to 27 element we compute the number of elements in the active region from equation( 5) then we get, [1]  ... (5)   the no. of element in active region is counted as 10.
By increasing the number of elements in the active region we get increases in a gain of LPDA even we can say gain have a perpendicular relation with log τ.
We combine Eqs.ln= τ and to determine the virtual apex distance [1]  ...( 6) ... (7)   =253.6cm The axial length of the antenna is the difference between R1 and RN : ...(8) axial length=235.39Finally, once the dimensions of the longest dipole element are determined, the dimensions of other dipoles can becompute the dimensions of the antenna from the equations by using an integer number of dipoles ,for better implementations in practical for number of element the diameter of dipole has holed the same so practically it will be easier 6 .
Fig. 1 shows the layout of the proposed LPDA.It consists of 26 dipole elements.This antenna is defined as reference antenna for the comparison.
In Table .1describethe parameters of the LPDA obtained by fine optimizing the dimensions of the LPDA with the goal of impedance matching within the standard UWB frequency band.The simulated voltage standing wave ratio (VSWR) of the LPDA before implementing of crossshape slotis shown in Fig. 2. In this paper, cross-shaped slotare used to create notched band instead of shunt open circuit stub.Compared to other shaped slot ,cross-shaped slot has more flexibility to choose the location and more cross-shaped slot can be easily inserted into the pro-posed LPDA [5].Due to the mechanism of LPDA, the location of cross-shaped slot should be prior to the resonated dipole element in order to obtain better band-notched performance.Fig. 3 shows the proposed LPDA incorporated with a cross-shaped slot, which can be equivalent to the shunt open circuit stub but with much compact layout.The physical length of the cross-shaped slotis about cross-wavelength at the notched frequency such that the slot resonates and stops the signal.Total signal reflection is expected at the notched frequency.
Voltage standing wave ratio(VSWR) of the LPDA is shown after using cross shape slot and some minor changes in Fig. 4.

CONCLUSION
The purpose of this study is to gain more complete understanding of the LPDA antenna with goal of developing and notch bandwidthwith optimized design procedure.This is achieved by making distance between the elements on resonating region as well as changes on width of elements, this discussion consider more in defining an cross-shape slot to Following the development of the design procedure mention above ,successfully designed log-periodic antenna for frequency independent operation with range of 200-1500MHZ, An average of -14 dB of S11 is obtained over the entire bandwidth other than a notch band.Results indicate that the notch can be implemented in LPDA, if proper care is taken while fabricating the mechanical structure of the antenna .

Fig. 2 :
Fig. 2: Voltage standing wave ratio before implementing of cross-shape slot Design issue of the cross-shape slot To realize band-notched characteristic, band-stop filter should be incorporated into the antenna design.A shunt open circuit stub is the simplest way to realize the band-stop effect.However, the shunt connection will occupy booms area.For the notched band antenna design,shunt open circuit stub with the length should be used, but it will result in strong mutual coupling between the stub, which leads to the increase of design complexity.