|
Why 50 ohm coax ? (taken from http://www./Misc/Electronics/docs/wiring/cable_impedance.html) Standard coaxial line impedance for r.f. power transmission in the this value was chosen is given in a paper presented by Bird Electronic Corp. Different impedance values are optimum for different parameters. Maximum power-carrying capability occurs at a diameter ratio of 1.65 corresponding to 30-ohms impedance. Optimum diameter ratio for voltage breakdown is 2.7 corresponding to 60-ohms impedance (incidentally, the standard impedance in many European countries). Power carrying capacity on breakdown ignores current density which is high at low impedances such as 30 ohms. Attenuation due to conductor losses alone is almost 50% higher at that impedance than at the minimum attenuation impedance of 77 ohms (diameter ratio 3.6). This ratio, however, is limited to only one half maximum power of a 30-ohm line. In the early days, microwave power was hard to come by and lines could not be taxed to capacity. Therefore low attenuation was the overriding factor leading to the selection of 77 (or 75) ohms as a standard. This resulted in hardware of certain fixed dimensions. When low-loss dielectric materials made the flexible line practical, the line dimensions remained unchanged to permit mating with existing equipment. The dielectric constant of polyethylene is 2.3. Impedance of a 77-ohm air line is reduced to 51 ohms when filled with polyethylene. Fifty-one ohms is still in use today though the standard for precision is 50 ohms. The attenuation is minimum at 77 ohms; the breakdown voltage is maximum at 60 ohms and the powercarrying capacity is maximum at 30 ohms. Another thing which might have lead to 50 ohm coax is that if you take a reasonable sized center conductor and put a insulator around that and then put a shield around that and choose all the dimensions so that they are convenient and mechanically look good, then the impedance will come out at about 50 ohms. In order to raise the impedance, the center conductor's diameter needs to be tiny with respect to the overall cable's size. And in order to lower the impedance, the thickness of the insulation between the inner conductor and the shield must be made very thin. Since almost any coax that *looks* good for mechanical reasons just happens to come out at close to 50 ohms anyway, there was a natural tendency for standardization at exactly 50 ohm. 為什么RF電路的特性阻抗大多選擇50歐姆?( A lot of people ask, so here's the answer to the eternal question, "How did 50 ohms get to be the standard RF transmission line impedance?" Here are a few stories. Bird Electronics will send you a printed copy of their version if you ask for it. This from Harmon Banning of W.L. Gore & Associates, Inc. cable:There are probably lots of stories about how 50 Ohms came to be. The one I am most familiar goes like this. In the early days of microwaves - around World War II, impedances were chosen depending on the application. For maximum power handling, somewhere between 30 and 44 Ohms was used. On the other hand, lowest attenuation for an air filled line was around 93 Ohms. In those days, there were no flexible cables, at least for higher frequencies, only rigid tubes with air dielectric. Semi-rigid cable came about in the early 50's, while real microwave flex cable was approximately 10 years later.Somewhere along the way it was decided to standardize on a given impedance so that economy and convenience could be brought into the equation. In the 無限長傳輸線上各處的電壓與電流的比值定義為傳輸線的特性阻抗，用Ｚ0 表示。 同軸電纜的特性阻抗的計算公式為 ： Ｚ0＝〔60/√εr〕×Log ( D/d ) [ 歐] 式中：D 為同軸電纜外導體銅網內徑；d 為同軸電纜芯線外徑；εr為導體間絕緣介質的相對介電常數。通常Ｚ0 = 50 歐 ，也有Ｚ0 = 75 歐的。 由公式不難看出，饋線特性阻抗只與導體直徑D和d以及導體間介質的介電常數εr有關，而與饋線長短、工作頻率以及饋線終端所接負載阻抗無關. 2、饋線的衰減系數 信號在饋線里傳輸，除有導體的電阻性損耗外，還有絕緣材料的介質損耗。這兩種損耗隨饋線長度的增加和工作頻率的提高而增加。因此，應合理布局盡量縮短饋線長度。 單位長度產生的損耗的大小用衰減系數 β 表示，其單位為 dB / m （分貝／米），電纜技術說明書上的單位大都用 dB / 設輸入到饋線的功率為Ｐ1 ，從長度為 L（m ） 的饋線輸出的功率為Ｐ2 ，傳輸損耗TL可表示為： TL ＝ 10 ×Lg ( Ｐ1 /Ｐ2 ) ( dB ) 衰減系數為： β ＝ TL / L ( dB / m ) 例如， NOKIA 7 / 而普通的非低耗電纜，例如， SYV-9-50-1， 900MHz 時衰減系數為 β ＝ 20.1 dB / 3、匹配概念 什么叫匹配？簡單地說，饋線終端所接負載阻抗ＺL 等于饋線特性阻抗Ｚ0 時，稱為饋線終端是匹配連接的。匹配時，饋線上只存在傳向終端負載的入射波，而沒有由終端負載產生的反射波，因此，當天線作為終端負載時，匹配能保證天線取得全部信號功率。當天線阻抗為50歐時，與50歐的電纜是匹配的，而當天線阻抗為80歐時，與50歐的電纜是不匹配的。 假如天線振子直徑較粗，天線輸入阻抗隨頻率的變化較小，輕易和饋線保持匹配，這時天線的工作頻率范圍就較寬。反之，則較窄。在實際工作中，天線的輸入阻抗還會受到四周物體的影響。為了使饋線與天線良好匹配，在架設天線時還需要通過測量，適當地調整天線的局部結構，或加裝匹配裝置。 4、反射損耗 前面已指出，當饋線和天線匹配時，饋線上沒有反射波，只有入射波，即饋線上傳輸的只是向天線方向行進的波。這時，饋線上各處的電壓幅度與電流幅度都相等，饋線上任意一點的阻抗都等于它的特性阻抗. 而當天線和饋線不匹配時，也就是天線阻抗不等于饋線特性阻抗時，負載就只能吸收饋線上傳輸的部分高頻能量，而不能全部吸收，未被吸收的那部分能量將反射回去形成反射波。 5、電壓駐波比 在不匹配的情況下, 饋線上同時存在入射波和反射波。在入射波和反射波相位相同的地方，電壓振幅相加為最大電壓振幅Ｖmax ，形成波腹；而在入射波和反射波相位相反的地方電壓振幅相減為最小電壓振幅Ｖmin ，形成波節。其它各點的振幅值則介于波腹與波節之間。這種合成波稱為行駐波。 反射波電壓和入射波電壓幅度之比叫作反射系數，記為 R 反射波幅度 R ＝ ───── 入射波幅度 波腹電壓與波節電壓幅度之比稱為駐波系數，也叫電壓駐波比，記為 VSWR 波腹電壓幅度Ｖmax （ＺL－Ｚ0） （1 + R） VSWR ＝ ───────────── ＝ ───── ＝ ───── 波節電壓輻度Ｖmin （ＺL＋Ｚ0 ） （1 - R） 終端負載阻抗ＺL 和特性阻抗Ｚ0 越接近，反射系數 R 越小，駐波比VSWR 越接近于１，匹配也就越好。 名詞注解： 饋線是配電網中的一個術語，它可以指與任意配網節點相連接的支路，可以是饋入支路，也可以是饋出支路。但因為配電網的典型拓撲是輻射型，所以大多饋線中的能量流動是單向的。但為提高供電可靠性，配網結構變化很復雜，功率的傳輸也并非絕對是一個方向。所以粗略地說，配電網中的支路都可稱之為饋線。 對電視天線饋線（室外天線到電視機之間的連線）一般的要求是：能有效地傳送天線接收的電視信號、畸變小、損耗小、抗干擾能力強，饋線與天線之間、與電視機信號輸入端之間應有良好的阻抗匹配。這些要求普通導線不具備。普通導線對電視信號的高頻衰減嚴重，抗干擾能力差，容易受到各種外來高頻信號的干擾。同時，普通導線的特性阻抗不定，很難滿足阻抗匹配要求，如果用普通導線作為電視機天線饋線，當天線上感應到的信號經普通導線傳向電視機時，在電視機輸入端因阻抗不匹配將產生反射，被反射回去的信號在普通導線與天線之間又由于阻抗不匹配而發生反射，多次反射的結果會使屏幕上圖像嚴重重影，無法正常收看，并對電視接收天線的性能造成一定損害。常用的電視天線鍋線主要有兩種，一種是特性阻抗為75歐的同軸電纜饋線，另一種是特性阻抗為300歐的平地扁饋線。有的電視機沒有300/75歐阻抗變換器，使用這兩種饋線，一般都能滿足天線、饋線、電視機信號輸入端阻抗匹配的要求，獲得最佳接收效果。在選拔天線饋線時，一方面要注意觀察電視機天線插孔處的阻抗標記，另一方面要考慮天線本身的阻抗特性，使天線具備的特點與實際需要吻合。75歐圓饋線與300歐扁平饋線兩者比較，前者因有金屬屏蔽層，抗干擾能力好，傳輸損耗小，在需要饋線較長的情況下比較適用，但價線較貴，不易配接；后者價值雖便宜，但由于無屏蔽作用，抗干擾能力不如前者，容易拾取雜波干擾，影響接收質量，且傳輸損耗大些，在不需要很長饋線的情況下，可考慮選用。 |
|
|
