Antenna itself is an oscillator, but different from ordinary LC oscillation circuit, it is normal deformation of the tank.

Electromagnetic energy from the transmitting antenna radiation, the forward propagation along the surface of all direction. If a conductor is placed in alternating electromagnetic field, due to the magnetic force line conductor, the incentive certain alternating voltage - electromotive force on both ends of the wire, its frequency is the same as the transmitting frequency. If the wire through the feeder are connected to the receiver, the receiver can obtain modulated wave signal of current.   So the wire up to receive the role of high frequency signal current electromagnetic energy is converted into energy, therefore said the wires for the receiving antenna.   Both transmitting antenna and receiving antenna, they all belong to the energy converter, "reciprocity" is the general characteristics of the energy converter.   Also a pair of antenna, it can be used as a transmitting antenna, can be used as a receiving antenna and communications equipment are generally received and sent together with an antenna. As a result, the same antenna is relationship to the transmitting system of effective energy output, and directly affects the performance of the receiving system.

Antenna the reversibility of performance not only in the transmitting antenna can be used as a receiving antenna, receiving antenna can be used as a transmitting antenna, and perform the antenna used for transmitting antenna parameters, and used as the parameters of the receiving antenna in remains the same, this is the reciprocity principle of antenna.

1. The input impedance

To make the antenna to get the most power, should make the antenna and feeder match, just need to know the antenna input impedance. The antenna input impedance Zin as the ratio of the input voltage and input current. The input impedance generally includes input resistance and input reactance.     Input resistance corresponding to the antenna radiation power and power loss of antenna system, namely the Rin = Rro + Rs Rs is calculated from the input end of wear resistance, the input reactance corresponds to the reactive power of induction field around the antenna.

2. The working frequency band

Antenna working frequency band is similar to the meanings of the meanings and circuit of frequency band, it refers to the antenna can meet the requirements of the technology at work frequency range. For there is only one frequency or within easy reach of several frequency communication equipment, the band width of antenna without consideration. But to have two or more frequency, and frequency offset and larger communication equipment, it must consider the band width of the antenna.

天线本身就是一个振荡器，但又与普通的LC振荡回路不同，它是普通振荡回路的变形。

电磁波的能量从发信天线辐射出去以后，将沿地表面所有方向向前传播。若在交变电磁场中放置一导线，由于磁力线切割导线，就在导线两端激励一定的交变电压——电动势，其频率与发信频率相同。若将该导线通过馈线与收信机相连，在收信机中就可以获得已调波信号的电流。因此，这个导线就起了接收电磁波能量并转变为高频信号电流能量的作用，所以称此导线为收信天线。无论是发信天线还是收信天线，它们都属于能量变换器，“可逆性”是一般能量变换器的特性。同样一副天线，它既可作为发信天线使用，也可作为收信天线使用，通信设备一般都是收、发共同用一根天线。因此，同一根天线既关系到发信系统的有效能量输出，又直接影响着收信系统的性能。

天线的可逆性不仅表现在发信天线可以用作收信天线，收信天线可以用作发信天线，并且表现在天线用作发信天线时的参数，与用作收信天线时的参数保持不变，这就是天线的互易原理。

1. 输入阻抗

为使天线能获得最多的功率，应使天线与馈线匹配，就需要知道天线的输入阻抗。天线的输入阻抗Zin为输入端电压与输入端电流之比。即输入阻抗一般包括输入电阻和输入电抗。输入电阻对应于天线辐射的功率和天线系统损耗的功率，即 Rin=Rro+Rs Rs为从输入端计算的损耗电阻，输入电抗对应于天线周围感应场的无功功率。

2. 工作频带

天线工作频带的含义与电路频带的含义相类似，它是指天线在工作时能符合某种技术要求的频率范围。对于只有一个频率或几个频率相距很近的通信设备而言，天线的频带宽度无需考虑。但对于具有两个以上频率，而且频差又较大的通信设备，就不能不考虑天线的频带宽度。