冷冷的风 发表于 2008-5-22 18:40:28

基础知识6---电台

1、什么是电台?

   对于频率/频点/频道,倒是可以简单说说:想想大家平常听的97.4MHz、103.9MHz,就是频率了,说频点可能也对,说频道有点牵强了,但经常就有说“103.9频道、97.4频道”,实在是有点误导的嫌疑。

   实际上我们天天听的广播就是无线电,只不过那个是“广播电台”发射的,广播电台功率狂大,发射天线位置狂好,覆盖范围狂广,于是在它覆盖范围内的接收机(就是收音机)都可以接收到它的信号,并转换成声音播放出来。

   现在设想一下你和你的朋友车上/手里各有一台收音机(radio receiver)+“微型广播电台”(radio transmitter),就是大家正在讨论的电台(Radio Tranciver?可能拼错了,反正这个词也是造出来的)了。同时兼备发射和接收的功能,于是可以互相通话。

2、频率/频点/频段

   97.4MHz是音乐台的频点,103.9MHz是交通台的频点,438.500MHz就是北京业余无线电爱好者可以合法使用的发射接收频点了(400MHz频段的,再其他频段也有业余HAM的合法频点,这里先不多说)。

   显然大家的调频(FM)收音机是不支持430多兆赫兹接收的(好象调频部分是86-107MHz之间),所以,如果想在438.500MHz频率上发射和接收信号,得要有专用的设备,也就是需要大家平常所说的“支持业余频段的车台/手台/基地台”等等。

   如果说438.500是频点的话,那么它同时是属于430MHz频段的,我们大概把430.000-439.999叫做业余400M频段,也叫业余70CM(厘米)波段(是波长的说法),因为这个频段主要是分配给业余无线电爱好者使用的。初级HAM接触比较多的可能还有大家常说的“2米波段”或者叫“140兆赫兹业余频段”就是144.000-145.999MHz之间,也是HAM可以使用的频率范围。

3、业余电台/专业电台/收音机的区别及简单概念

   收音机就不多说了,用来收听其他发射台发射出来的信号,通常的收音机与调频(FM)/调幅(AM)两个部分,是两种不同的调制方式,相对应的,在电台里,也有这两种调制方式的,大家常用的是支持430MHz段的的调频电台,而空管则是使用的110MHz-130MHz之间频率,采用AM调制方式。

4、再来说“业余电台”、“专业电台”之间的一些简单区别

   有一层意思是说业余电台是支持业余频段的电台——支持(包含)430-440这个频段的电台,专业电台是支持某些其他特定频段的电台;另一层意思是业余电台的功能方面注重于“玩儿”,对各个相关参数的调整,包括调整发射功率/搜索有信号的频点/储存多个不同的频点/可以随时手动变更储存的频点/调整不同的静噪值/有信号强度表等。专业电台则更加注重使用的稳定性,所以对于操作的复杂性要求相对较低,而对电池的耐久/发射功率的稳定等性能要求高些。

   对比起来,专业台通常不能随意调整发射功率/没有搜索频点的功能/能存储少量的频点并只能显示频道号码而不能显示频点值/需要用电脑和专用软件及连线才能调整每个频道内储存的频率及相关功能/通常也没有信号强度表显示。

   并且,专业电台可能在设计上还会增加一些诸如调频什么的保密措施,或者支持集群什么的功能(我懂得也不多了)你就想交通JC,武装JC什么的,他们拿个台子,知道哪个是开关音量,按着哪个说话,拧到几频道是中队,几频道是总台就完了,要是拿个花哨的业余电台给JC,保他晕菜。

5、中继台(Repeater)/差频(OFFSET)与直发

   可能有人会问“中继”是什么?这要是说细了可能会说到波的绕射什么的,我也有点泛晕,早还给物理老师了,咱们就说在高处有那么一个或者一对天线和特殊的收发电台,负责把一个频点上收来的信号转发到另外一个频点上,于是在它覆盖范围内的台子都可以和它联系,也就是通过它和其他人联系。还晕?这么说吧,由于建筑物/地形等的遮挡,在地面上的2个电台之间的信号可能无法互相传送到,但2个人和这个很高的,位置很好的中继台都可以很好的通联,于是大家就通过中继台达到覆盖更广的范围,帮助小功率设备扩大信号的目的。

   通常的中继台需要使用一对频点,一个接收来自使用者手台或车台的信号(上行信号,如北京大家使用的434.750),当收到时,在另一个频点上广播出去(下行信号,如北京的439.750)。由于中继台必须占有比较好的地理位置,同时可能有比较大的发射功率,所以通过中继台能够联络到更多的人。

   差频(OFFSET):刚才说的439.750(使用者收听的频点,下行频点)和434.750(使用者发射的频点,上行频点)之间的差就是差频,我们可以看到,发射的频点是接收频点减去5MHz(-5MHz),所以叫做“下差5MHz”。对于某些电台,可以直接设置上行频点和下行频点然后保存到一个频道里方便使用,有些电台的设置方法则是先设接收频率,然后设置差频数值及方向,保存到某个频道内。设置方法并不重要,只要了解了中继台/差频的概念,剩下的事情可以通过看设备的说明书解决。在这里就不多说了。

   直发:通常电台是在同一个频点上发射和接收的,使用者通过PTT(PUSH TO TALK)键来切换电台的接收/发射状态,按着钮说话,松开听。在一个频点上收和发就是所说的“直发”,比如,438.500、438.550MHz是北京比较常用的业余无线电直发频点。当然,同一个频点上,同时只能有一个设备进行发射,其他设备接收,如果同时2个人都在发射,他们的信号就会互相干扰,也就听不见了。

6、车坛常用业余电台选购要点

   不要迷信MOTO 6200C和一些外国产的FRS(FAMILY RADIO SERVICE)家用小对讲机,那根本就是玩具!为了避免发射设备对其他频率的干扰,这些产品的功率肯定低于0.5瓦,在没有遮挡的情况下可能能够达到直发距离2公里,如果在市区,1公里已经不错了。唯一的优点是,不需要任何执照及验机,即买即用。

   当然,这些小手台都是固定信道固定频率的收发设备,可以理解为“低功率的/不可调整频率的专业电台(家用)”,当然,他们的频率固定在409.***(国产)/462.***/467***MHz(进口)范围,通常只有16个固定频点,且无法更改。如果你的其他业余电台支持这些频点,是可以和他们进行直发通联的,但显然,这些小玩具不能和HAM们进行通联。

   如果要选择电台,一定要满足以下条件:

   所覆盖的频率范围必须支持(包含)430-440MHz;
   必须具备差频功能;
   建议选择可以手置频的机器(就是可以直接选择频点并自由储存到频道里,而不用电脑写频的);
   建议支持亚音功能;
   建议功率25W以上(车台)/支持最大5W(手台),以保证通连的距离。

   有了以上条件,就可以步入业余无线电爱好者(HAM)的行列了(当然,还必须办理相关的合法手续)。

冷冷的风 发表于 2011-9-21 20:59:24

基础知识    最简单的入门教程

加油贝塔 发表于 2011-9-23 21:49:46

感谢分享 学习了

edressc2n 发表于 2011-9-24 17:09:15

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bg6icq 发表于 2011-9-25 07:07:33

感谢分享 学习:handshake

19210701江岸 发表于 2011-9-26 21:49:07

管用的很,实用,明了。

徐州军士长750 发表于 2011-9-28 17:33:21

:victory::victory:

冷冷的风 发表于 2012-2-10 20:29:38

基础知识普及   坚持

黑龙 发表于 2012-2-11 10:40:36

学习了,顶顶:victory::victory::victory:

wh1250 发表于 2012-3-15 21:55:53

:victory::victory::victory:
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