一般射频系统｜
工作原理：
系统通过数字电路采集数字信号后经过数模变换器后将数字信号转换为模拟信号，模拟信号经过混频器后增大频率，混频器中使用的载波由本地振荡器产生，因为在电磁波在传输过程中会有损耗，所以需要经过发射功率放大器，并经过带通滤波器得到需要的频率段的信号，由变化的电流经过天线，转换为电磁波辐射出去，在接收端通过带通滤波器得到需要获取的信号，因为电磁波在传播过程中损耗了很多了，所以需要通过功率放大器进行放大，通过和本地振荡器产生的载波信号进行混频，得到原信号和其他信号，然后经过低通滤波器得到原信号，滤去多余信号，最后经过模数变换器传输给数字电路。
Operating principle:
The system collects the digital signal through the digital circuit and converts the digital signal into the analog signal after passing through the digital analog converter. The analog signal increases the frequency after passing through the mixer. The carrier used in the mixer is generated by the local oscillator. Because the electromagnetic wave will be lost in the transmission process, it needs to pass through the transmission power amplifier and the band-pass filter to obtain the signal of the required frequency band, The changing current passes through the antenna, is converted into electromagnetic wave and radiated out. The signal to be obtained is obtained through the band-pass filter at the receiving end. Because the electromagnetic wave is lost a lot in the propagation process, it needs to be amplified through the power amplifier, mixed with the carrier signal generated by the local oscillator to obtain the original signal and other signals, and then the original signal is obtained through the low-pass filter to filter out the redundant signal, Finally, it is transmitted to digital circuit through analog-to-digital converter.
壹
天线和天线方向图
天线是将发射机输出的高频电流能量转换成电磁波辐射出去，或将有用的空间电波信号转换成高频电流能量送给接收机。
天线方向图是用图示的方法来表示天线辐射能量在空间的分布。
The antenna converts the high-frequency current energy output by the transmitter into electromagnetic wave and radiates it, or converts the useful space radio wave signal into high-frequency current energy and sends it to the receiver.
The antenna pattern is a graphical representation of the spatial distribution of antenna radiation energy.
贰
实验仪器
发射机，接收机，对数周期天线，单极天线，射频电缆，转接头 ，固定转台，电动转台
Transmitter, receiver, log periodic antenna, monopole antenna, RF cable, adapter, fixed turntable, electric turntable
叁
测量方法
固定天线法和旋转天线法
固定天线法：待测天线不动，辅助天线绕待测天线转动。
旋转天线法：待测天线绕自己的轴转动，辅助天线不动。
本次实验采用旋转天线法进行天线方向图的测量。
Fixed antenna method and rotating antenna method
Fixed antenna method: the antenna to be tested does not move, and the auxiliary antenna rotates around the antenna to be tested.
Rotating antenna method: the antenna to be tested rotates around its own axis and the auxiliary antenna does not move.
In this experiment, the rotating antenna method is used to measure the antenna pattern.
肆
测量步骤
1.搭建好发射端固定转台、接收端电动转台及天线，对数周期天线作发射，单极天线作接收。
2. 确定最小测试距离（要满足远场测试条件，>3 ）及架设高度（收发天线应处于同一高度上，并将转台调到水平）。
3. 检查周边的反射电平及测试条件（收发天线之间没有反射物和遮挡物，测试时人员尽量远离天线以避免人体对收发信号的干扰）。
4. 确定发射机的频率：将发射机输出1端口连接至校准输入口，将显示模式切换成频率显示，转到频率调节旋钮将端口1的输出信号频率调至2.4GHz。
5. 用射频线缆将发射机端口1连接至对数周期天线。
6. 将接收转台的BNC口连接至接收机背面的角度录取口，将接收转台信号输出口连接至接收机信号输入口2，将接收机的频段切换开关切换至CH1。
7. 开启接收端电动转台，待测单极天线转动方位角，接收机液晶界面上自动记录天线的接收电平值。
8. 待电机自动旋转一周，暂停电机转动，得到被测天线的方向图。
1. set up fixed turntable at the transmitting end, electric turntable and antenna at the receiving end, log periodic antenna for transmitting and monopole antenna for receiving.
2. determine the minimum test distance (to meet the far-field test conditions, >3) and the erection height (the transceiver antenna shall be at the same height, and the turntable shall be adjusted to the horizontal).
3. check the surrounding reflection level and test conditions (there are no reflectors and obstructions between the transmitting and receiving antennas. During the test, the personnel shall try to operate the antenna to avoid the interference of the human body to the transmitting and receiving signals).
4. determine the frequency of the transmitter: connect the transmitter output 1 port to the calibration input port, switch the display mode to frequency display, turn the frequency adjustment knob to adjust the output signal frequency of port 1 to 2.4GHz.
5. connect transmitter port 1 to log periodic antenna with RF cable.
6. connect the BNC port of the receiving turntable to the angle acquisition port on the back of the receiver, connect the signal output port of the receiving turntable to the signal input port 2 of the receiver, and switch the frequency band switch of the receiver to ch3.
7. turn on the electric turntable at the receiving end, the rotation azimuth of the monopole antenna to be measured, and the receiving level value of the antenna will be automatically recorded on the LCD interface of the receiver.
8. after the motor rotates automatically for one cycle, stop the motor rotation to obtain the direction pattern of the antenna to be measured.
伍
实验结果