1 Introduction

LPDA (log periodic dipole antennas) is a kind of common MIMO Directional Antenna ultra wideband antenna, which is composed of basic unit dipole. Its structure has self similarity and its electrical performance can be basically stable in a wide frequency band [1-2]. Generally, log periodic antenna is made of cylindrical vibrator, which is large in volume, heavy in weight, difficult in fabrication and limited in precision. The so-called printed log periodic antenna (plpda) is made on the microstrip substrate by modern lithography technology, which has the advantages of light weight, small size, easy loading, high precision and good consistency.

In the 1980s, the design of double-layer printed log periodic antenna appeared in foreign countries [3], printing the printed vibrator on two substrates respectively, feeding through the microstrip line in the middle of the two plates; in addition, the structure of MIMO Directional Antenna single-layer printed log periodic antenna was proposed and experimental research was carried out [4-5]. It is found that the bandwidth of LPDA can be further expanded and its size can be reduced. In this paper, by selecting a high dielectric constant dielectric substrate and adopting the structure of closely arranged vibrator, the double-layer printed log periodic antenna designed in this paper can achieve a working bandwidth of 1.5ghz-18.5ghz in the size of 60mm × 60mm × 2.054mm, which has good miniaturization and ultra wideband characteristics. The optimized antenna has stable standing wave and gain characteristics in the working frequency band, low processing difficultyIt has high precision and good realization value.

2 principle of LPDA

as shown in Figure 1, LPDA is composed of N parallel dipoles arranged in a certain proportion and fed by a pair of two-wire transmission lines. In order to make the pattern point to one end of the short vibrator, the adjacent two oscillators are fed by cross feed, and the assembly line terminal connected with the long / short vibrator is sometimes connected with an impedance element or a branch node, so as to reduce the terminal reflection. The antenna structure parameters are defined as follows:

period rate or scale factor

(1)

opening angle or structure angle

(2)

where ln is the length of the nth vibrator, RN is the distance from the nth vibrator to the antenna vertex o. When the antenna is fed by the long or short end of the oscillator, the energy supplied by the signal source is transmitted along the assembly line, and each vibrator is excited in turn. Only the part of the vibrator whose length is close to the resonant wavelength can excite a large current and form an effective radiation to the space. Generally, this part of the vibrator is called the effective area or the radiation area, and the part between the feeding point and the radiation area is called the transmission area. The part whose length is a little greater than or much greater than the resonance length and the part of its assembly line are called the non excited region.

Fig. 1 Structure of LPDA antenna

3 miniaturized printed LPDA antenna design. Its structure is a double-layer microstrip board, in which the strip feeder is between the two sidesThey are the basic vibrator unit and feeder of the antenna, and the end of the middle feeding belt line is short circuited with the antenna on one side. The specific structure and components are shown in Figure 2.

In order to realize miniaturization, Rogers 6010 with high dielectric constant is selected as the material of microstrip plate, and the dielectric constant is 10.2; the thickness of single-layer dielectric plate is 1mm, and the thickness of copper-clad plate is 0.018mm.

Figure 2 printed log periodic dipole antenna structure

where:

1-feed port

2-impedance matching section

3-feed wire

4-upper microstrip vibrator

5-lower microstrip vibrator

6-dielectric substrate

7-short-circuit terminal

3.2 Parameter calculation

select the working frequency band of the antenna as 1.5GHz-18.5GHz. The length of the shortest vibrator is brazilianhairtop obtained from the material characteristics of the dielectric substrate and the highest working frequency

(3)

select the dielectric substrate with high dielectric constant to greatly reduce the length of the vibrator. In addition, in order to shorten the axial dimension, the antenna angle

is set as a larger angle and selected In order to further realize miniaturization, a compact arrangement of vibrators is realized with appropriate scale factor. Select,

; calculate the length, width and spacing of each patch unit through formula (1) - (3), and calculate the width W of the feeder belt line.

According to the calculation size, the antenna model is established by HFSS software, which is used for simulation and optimization. Power grid in source)