Today to introduce the basic knowledge of antenna, so that we have a preliminary understanding of the antenna. Let's start with a few concepts.

Resonance: any antenna resonates at a certain frequency. We want to receive the signal at that frequency. Antenna resonance is the most basic requirements of the antenna, otherwise, there is no so much attention, casually throw a wire out is not antenna. The main data involved in the antenna resonance problem is the wavelength and a quarter of it. The formula for calculating the wavelength is very simple, 300/f. Where the unit of f is MHz, and the unit of the result is meters. The 1/4 wavelength is called the fundamental oscillator. For example, the dipole antenna is a pair of fundamental oscillators, and the vertical antenna is a fundamental oscillator. But the length of the oscillator in the antenna is not exactly a quarter of the wavelength, because the wave travels through the wire at a different speed than it would in a vacuum, and is generally shorter, so there is a shortening factor. This factor depends on the material.

Bandwidth: this is also an important but overlooked issue. The antenna has a certain bandwidth, which means that although the resonant frequency is a frequency point, the performance of the antenna is about the same within a certain range of the frequency point. This range is the bandwidth. We certainly want the bandwidth of an antenna to cover a certain range, preferably the entire FM radio band we listen to. Otherwise, it would be too troublesome to change the station and change the antenna or adjust the antenna. Antenna bandwidth and antenna type, structure, material are related. Generally speaking, the thicker the tube and line used by the oscillator, the wider the bandwidth. The higher the antenna gain, the narrower the bandwidth.

Impedance: the antenna can be viewed as a resonant loop. A resonant circuit certainly has its impedance. The impedance requirement is matching: the circuit connected to the antenna must have the same impedance as the antenna. Connected to the antenna is the feeder, and the impedance of the feeder is determined, so we want the impedance of the antenna to be the same as that of the feeder. Generally produced feeders are mainly 300 ohms, 75 ohms and 50 ohms, and there are 450 ohms and 600 ohms in the past abroad. The impedance of basic dipole antenna is about 75 ohms, v-type dipole antenna is about 50 ohms, and the impedance of basic vertical antenna is about 50 ohms. Other antennas do not normally have impedance of 50 or 75 ohms, so some means of impedance transformation is needed before connecting them to the feeder.

Balance: symmetrical antenna is balanced, such as dipole antenna, yagi antenna, and coaxial cable is unbalanced, connecting the two, the need to solve the problem of balance imbalance conversion.

Gain: the antenna is a passive device, but the antenna can have gain. This gain is of course relative gain, relative to the basic dipole antenna. The antenna used by FM DX, of course, wants to gain as high as possible. Remember, however, that high gain is often accompanied by narrow band width.

Directional: not all antennas are directional. A pull-rod antenna on a portable radio is directionless. Dipole antenna has weak directivity, yagi antenna can get better directivity. Good directionality provides the ability to concentrate radio waves in the desired direction, and an important ability to partially mitigate the effects of local radio signals. But directional antennas are not always good. When waiting without a target, the directional antenna may cause you to miss the signal on the back of the antenna. So the more reasonable way, is to use a vertical antenna and a pair of directional antenna, with the vertical antenna to wait, hear the signal, and then use the directional antenna turned to the listening.

Elevation Angle: the elevation Angle of the antenna is the elevation Angle of the wave, not the mechanical elevation Angle of the antenna oscillator itself. The elevation Angle reflects which altitude Angle the antenna receives the strongest waves from. For the F layer, we want the elevation to be low, so we want it to be far away, and for the Es layer, we want it to be high, so we want it to be high. The elevation Angle depends on the antenna type and erection height. Generally speaking, the vertical antenna has a low elevation Angle, and the elevation Angle of other antennas varies with the erection height.

Erection height: the antenna has one erection height. This height is actually two heights, one height let's think about its horizontal height, and this height is useful for the local signal, not so useful for DX. The second height, which is often overlooked, is the height from the antenna to the electrical ground. For example, the antenna erected in the reinforced concrete roof, although the house is 20 meters high, but the antenna is only 1 meter away from the roof, so the height of the antenna is only 1 meter. Antenna height has different effects on different antennas, generally affecting antenna impedance and elevation Angle. Generally, we think that the height of the antenna should be above 0.4 wavelength, so it is relatively unaffected by the ground.

Standing bobbi: finally, introduce this feature which is not familiar to Chinese fans. The standing-wave ratio reflects the matching of antenna feed system. It measures the performance of the antenna by the ratio of the energy emitted and reflected when the antenna is used as the transmitting antenna. The standing wave ratio is determined by the impedance of antenna feed system. The impedance of the antenna and the impedance of the feeder are consistent with that of the receiver, and the VSWR is small. The antenna feeder system with high standing-wave ratio loses a lot of signal in the feeder.

The role of celestial modulation:

1. Match the impedance to make the antenna system (antenna + antenna) match the impedance for the transmitter, so as to maximize the radiation efficiency of the antenna cable in the antenna system.

2. Resonant antenna: according to the electromagnetic theory, antenna impedance Z=R+jX is considered as antenna resonance when X = 0. After using the antenna with unnatural resonance, the antenna is adjusted so that Z=R+jX, where X = 0, by adding inductance or capacity.

3. The electrical efficiency of the antenna after day adjustment is relative to the natural resonant antenna. Tuning the antenna to the transmitter is impedance matching, relying on the LC network inside the day adjustment. Since L and C are not ideal elements and consume some energy, the more unnatural resonance of the antenna (especially the farther the equivalent radiation resistance deviates from 50 ohms), the lower the electrical efficiency after the addition of day adjustment.

Fujian jiasida communication technology co., LTD., located in quanzhou city, fujian province, was established in 2009. It is a high-tech company specializing in communication research, development, production and operation of communication antennas and passive devices.

Company has an experienced r&d team, skilled production testers, perfect testing instruments and equipment, committed to the development and production of all kinds of radio antenna, car antenna, omni-directional antenna, power splitters, combiner, road system and platform, and so on related products, and provide personalized custom services according to customer's specific requirements. We in line with take the technology as the core, the science and technology strives for the development, provides the superior high technology product for the user the enterprise culture idea, in fully introduces absorbs the domestic and foreign advanced technology foundation, unceasingly develops the innovation, devotes to the research and development own characteristic new product. At the same time, we will sincerely provide customers with quality service.

今天给大家介绍一下天线方面的基本知识，使大家对天线有初步的了解。下面先来了解几个概念。

共振：任何天线都谐振在一定的频率上，我们要接收哪个频率的信号，就希望天线谐振在那个频率上。天线谐振是对天线最基本的要求，要不然，就没那么多讲究了，随便扔根线出去不也是天线嘛。天线的谐振问题涉及到的主要数据是波长及其四分之一。计算波长的公式很简单，300/f。其中f的单位是MHz，而得到的结果的单位是米。1/4波长是称作基本振子，如偶极天线是一对基本振子，垂直天线是一根基本振子。不过天线中的振子的长度并不正好是1/4波长，因为电波在导线中行进的速度与在真空中的不同，一般都要短一些，所以有一个缩短因子。这个因子取决于材料。

带宽：这也是一个重要但容易被忽略的问题。天线是有一定带宽的，这意味着虽然谐振频率是一个频率点，但是在这个频率点附近一定范围内，这付天线的性能都是差不多好的。这个范围就是带宽。我们当然希望一付天线的带宽能覆盖一定的范围，最好是我们所收听的整个FM广播波段。要不然换个台还要换天线或者调天线也太麻烦了。天线的带宽和天线的型式、结构、材料都有关系。一般来说，振子所用管、线越粗，带宽越宽；天线增益越高，带宽越窄。

阻抗：天线可以看做是一个谐振回路。一个谐振回路当然有其阻抗。我们对阻抗的要求就是匹配：和天线相连的电路必须有与天线一样的阻抗。和天线相连的是馈线，馈线的阻抗是确定的，所以我们希望天线的阻抗和馈线一样。一般生产的馈线，主要是300欧姆、75欧姆和50欧姆三种阻抗，国外过去还有450欧姆和600欧姆阻抗的馈线。基本偶极天线的阻抗是75欧姆左右，V型偶极天线是50欧姆左右，基本垂直天线阻抗50欧姆。其他天线一般阻抗都不是50或75欧姆，那么在把它们与馈线连接之前，需要有一定的手段来做阻抗变换。

平衡：对称的天线是平衡的，如偶极天线、八木天线，而同轴电缆是不平衡的，把这两者连接起来，就需要解决平衡不平衡转换的问题。

增益：天线是无源器件，但是天线是可以有增益的。这个增益当然是相对增益，是相对于基本偶极天线而言的。FM DX所用的天线，当然希望增益越高越好。不过别忘了，增益高往往伴随着带宽窄。

方向性：不是所有的天线都有方向性的。便携式收音机上的拉杆天线就没有方向性。偶极天线有弱的方向性，八木等定向天线可以得到较好的方向性。好的方向性意外着能够集中收集所需方向的电波，还有一个重要的能力就是能部分地减弱本地电台信号的影响。但是定向天线并不是什么情况下都好。当没有目标而等待的时候，定向天线就有可能使你错过天线背面的信号。所以比较合理的方式，是用一个垂直天线和一付定向天线配合使用，用垂直天线等待，听到信号后，再用定向天线转过去对准了听。

仰角：天线的仰角是指电波的仰角，而并不是天线振子本身机械上的仰角。仰角反映了天线接收哪个高度角来的电波最强。对于F层传播，我们希望仰角低，可以传播地远，对于Es层，电波主要是从高处来，我们希望仰角高。仰角的高低取决于天线型式和架设高度。一般来说，垂直天线具有低仰角，其他天线的仰角随架设高度变化。

架设高度：天线有一个架设高度。这个高度实际上是两个高度，一个高度我们考虑它的水平面高度，这个高度对于本地信号有些用，对于DX其实用处不大。第二个常常被忽略的高度是地面高度，是指天线到电气地面的高度。比如架设在钢筋水泥房顶的天线，虽然房子高有20米，但是天线距房顶只有1米，那么这付天线的高度只是1米。天线的高度对不同的天线有不同的影响，一般会影响天线的阻抗和仰角。通常我们认为天线的地面高度应在0.4个波长以上，才比较不受地面的影响。

驻波比：最后介绍这个最不被中国的爱好者熟悉的特征。驻波比反映了天馈系统的匹配情况。它是以天线作为发射天线时发射出去和反射回来的能量的比来衡量天线性能的。驻波比是由天馈系统的阻抗决定的。天线的阻抗与馈线的阻抗与接收机的阻抗一致，驻波比就小。驻波比高的天馈系统，信号在馈线中的损失很大。

天调的作用：

1、匹配阻抗，使天线系统（天调+天线）对于发射机来说是阻抗匹配，这样才能让天线系统中的天线电缆部分辐射效率最高。

2、谐振天线，按照电磁理论来讲天线阻抗Z=R+jX，当Ｘ＝０时视为天线谐振。不自然谐振的天线使用天调后，天调通过加感或加容，使得Z=R+jX中Ｘ＝０。

3、加天调后的天线相对于自然谐振天线的电效率问题，将天线调谐到相对于发射机来说是阻抗匹配，靠的是天调内部的LC网络，有很大一部分功率在天调的L、C内“吞吐”，不辐射电磁波。由于L、C不是理想元件，会消耗一部分能量，因此天线越不自然谐振（特别是等效辐射电阻偏离50欧越远），加天调后的电效率就越低。

福建佳思达通讯科技有限公司坐落于福建省泉州市，成立于2009年，是一家专业从事通讯研究，开发，生产及经营通讯天线及无源器件的高科技有限公司。

公司拥有一支经验丰富的研发队伍，熟练的生产测试人员，完善的测试仪表与设备，致力于开发生产各类对讲机天线，车载天线，全向天线，功分器，合路器，多系统合路平台等等相关产品，并根据客户具体需求提供个性化定制服务。我们本着以技术为核心，科技求发展，为用户提供优越的高科技产品的企业文化理念，在充分引进吸收国内外先进技术的基础上，不断开拓创新，致力于研发自己特色的新产品。同时，我们也将真诚为顾客提供优质的服务。