台灣無線電俱樂部 TAIWAN RADIO CLUB ( TRC )
標題: 線材問題((RG-58A/U RG-58U)) [打印本頁]
作者: BM4JJJ 時間: 2011-11-30 10:55
標題: 線材問題((RG-58A/U RG-58U))
仿間市場都有賣3D黑色 RG-58A/U及RG-58U線材
因為 不知道有沒有族友發表過 印象中 好像有族友發帖過 卻找不到
本人 如果沒有記錯的話
RG-58A/U線材 = 樓下.樓上都可以使用
RG-58U線材 = 樓上可以使用 樓下不能使用
不知道 我寫的是否正確
作者: bm8cfq 時間: 2011-11-30 11:46
RG-58A/U線材 = 阻抗50歐姆 跟一般無線電機的輸出阻抗一樣 衰減量 174 db/km
RG-58/U線材 = 53.5歐姆 用途不知道... 衰減量 138 db/km
作者: cinghuei 時間: 2011-11-30 14:01
RG-58 原先的用途是電腦網路用 大約15年前吧 10MB的網路卡時代
線材分別有 RG-58U A/U B/U C/U
差別在中心的線直徑與線數、還有中心導線的材質、外圍被覆的材質，靜電容量，衰減量有所不同
在特性來說RG-58A/U的線材的靜電電容與衰減率較低 較適合無線電使用
特性表
http://www.100y.com.tw/pdf_file/G-58AU.pdf
作者: tjhu 時間: 2011-11-30 16:59
那可以請問一下 如果用"銀線"會比較好嗎?
那小車 用3D的線可以嗎?
還是可以再粗一點呢?
作者: bx6aaa 時間: 2011-11-30 17:45
fatpet 發表於 2011-11-30 14:01 
RG-58 原先的用途是電腦網路用 大約15年前吧 10MB的網路卡時代
線材分別有 RG-58U A/U B/U C/U
哇............
好棒的資料，我又暗槓了起來了
看到這些資料，想起10年大家在瘋..............RG-213線材的景象了
OM提到RG-58A/U和RG-58/U的阻抗是不一樣的？但從資料看來是一樣的優
作者: bx6aaa 時間: 2011-11-30 17:55
tjhu 發表於 2011-11-30 16:59
那可以請問一下 如果用"銀線"會比較好嗎?
那小車 用3D的線可以嗎?
還是可以再粗一點呢?
線材的好壞當然是影響傳輸中損失因素
但是..........線材粗細？小弟是覺得夠用就好！
一條水管承受10公升水量，給10公升剛好，有必要10公升的水，需要用乘載100公升的水管嗎？
你只是5W功率輸出，需要用到5D，10D，12D.......................嗎？
以上個人見解，不代表正確答案
作者: taiwan 時間: 2011-11-30 20:08
看到我們家專業會員越來越多真是無比高興啊
作者: BV7CW 時間: 2011-12-1 01:38
RG-58 不建議用在VHF 或 UHF 頻段, 給個連結
hamradio.arc.nasa.gov/coaxcableloss.html
不管是RG-58U或RG-58 輸入50瓦,
在(樓下)100MHz時,衰減量達3.8DB 和5.4DB, 輸出端只剩下21瓦和14瓦
在(樓上)400MHz時,衰減量達8.4DB 和3.4DB, 輸出端只剩下7瓦和3瓦
用在700MHz時..... 3瓦和1瓦.
選擇饋線,應注意除了特性阻抗外,衰減量才是第一要考量的,
想想你的車機滿功率輸出,經過100英呎饋現剩下一半時,你能接受嗎?
在條件許可下盡量選擇較粗的饋線,低減衰減量,
讓發射機的每一分功率完整的送到天線才是王道.
(哈!沒有發表URL連結權限,大家委屈一點複製連結吧)
作者: renjong 時間: 2011-12-1 01:50
本帖最後由 renjong 於 2011-12-1 02:08 編輯
小弟親自實測過,結果比理論值還差！
RG58A/U線材,使用M頭連接設備與天線(線本身無續接,僅於發射機/功率表/假負載有接頭)
在 145.00MHz 測試,假負載,50W發射,空曠地,線材完全拉直 無彎曲或打圈圈.
40M長度 假負載端 10w
60M長度 假負載端 1w+
PS.
為了這個實驗,幹掉100M RG58A/U線材.
不要迷信因為是進口的RG58A/U線材線就會比5D 8D好...
[因為我實驗用的就是進口線]
(有圖有真相,1000ft 剪到現在剩下的 正 美國進口RG-58A/U線材！)
真的如 CW 大大所說,
「在條件許可下盡量選擇較粗的饋線,低減衰減量,讓發射機的每一分功率完整的送到天線才是王道.」
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作者: tjhu 時間: 2011-12-1 11:53
bx6aaa 發表於 2011-11-30 17:55
線材的好壞當然是影響傳輸中損失因素
但是..........線材粗細？小弟是覺得夠用就好！
一條水管承受10公升 ...
那請問一下
如果是輸出 25w~100w
一樣嗎? 3D的線就可以了嗎?或是....?
感謝回覆
作者: bx6aaa 時間: 2011-12-1 12:38
tjhu 發表於 2011-12-1 11:53
那請問一下
如果是輸出 25w~100w
一樣嗎? 3D的線就可以了嗎?或是....?
您是問我嗎？
我的答案只適用我自己，如果您要參考也可以！
通常我使用上，單純使用在機器上，我都使用5~8D的，如果需要使用功率放大器(龜龜啦)，我才會考慮到8D以上的線材！
作者: renjong 時間: 2011-12-1 14:43
本帖最後由 renjong 於 2011-12-1 14:43 編輯
tjhu 發表於 2011-12-1 11:53
那請問一下
如果是輸出 25w~100w
一樣嗎? 3D的線就可以了嗎?或是....?
要看您的 長度 及 頻段 !
以我個人來說,
如果是3~5M,145MHz 頻段 ,我可能會接受~
如果是3~20M,HF 頻段 ,我也可能會接受~
您說 25~100.... 是說 275H 嗎?
基地用應該都會超過5M吧! 那就不建議囉~
這也是我自己的考量啦, 重點還是你自己的認定跟接受度囉!
作者: tjhu 時間: 2011-12-1 19:26
i感謝兩位大大的回覆
因為我本來想說 要裝車機在車上
只是認為3D的線好像還不夠粗
怕一直被門壓會斷線的問題
是不是有需要裝到5D的線
所以才特別跟先進們請教
原來還有輸出功率與距離長度的問題呢
感謝 受益良多
作者: whitemagic 時間: 2011-12-1 23:00
傳輸線與功率輸出的關係我見解是這樣
如有與前輩牴觸的還請見諒
選擇傳輸線粗細規格與輸出功率是無關的
傳輸線粗細規格應該是與你的安裝環境及經費考量才有關
假使你是使用手機做基地台用
即使輸出只有5瓦
如果能夠選擇使用更粗的傳輸線來架設
還是有其價值
因為傳輸線規格影響的就是衰減值而已
除去各家做工材料品質的問題不談
通常粗的線材衰減小
細的線材衰減大
使用輸出五瓦的機器
你還是會希望到達頂端天線輻射出去的是越接近五瓦越好
不會因為你只是使用五瓦機器
就認為變成0.5瓦也沒關係
以上是我的見解
而此欄相關文章中也有前輩提到線材的影響就是衰減的問題而已
我的觀念正是與其相同
作者: llsheu 時間: 2011-12-2 01:44
我現在匹配的3D銀線是藍皮包裹，是某位線上先進推薦的，坊間人稱藍精靈，是嗎？
作者: et006623 時間: 2011-12-2 05:46
llsheu 發表於 2011-12-2 01:44
我現在匹配的3D銀線是藍皮包裹，是某位線上先進推薦的，坊間人稱藍精靈，是嗎？ ...
藍精靈
小弟有在C大那看到2種都號稱藍精靈
但是真的含銀高的線,只有一條~
還有銀線真的是含銀線還是全銀線
大功率輸出會不會熔呢?
小弟經C大等友台了解後~嘿嘿嘿
當時生魚片也在現在一起學習~
有空可以請生魚片去C大那拍拍照
PO給友台參考
購買時還是要先做功課~
店家的話,還是先打折.多比較~
小弟都是精挑細選的為TRC友台去把關.
作者: llsheu 時間: 2011-12-3 00:47
感謝TRC先進們熱誠提供資訊，251巨蟹座145.730/431.703，有空大家一起呼一下，謝謝！
作者: renjong 時間: 2011-12-3 07:12
llsheu 發表於 2011-12-3 00:47
感謝TRC先進們熱誠提供資訊，251巨蟹座145.730/431.703，有空大家一起呼一下，謝謝！ ...
我...太遠+條件太爛 呼不到～
殘念～
作者: BM4JJJ 時間: 2011-12-3 08:46
llsheu 發表於 2011-12-3 00:47
感謝TRC先進們熱誠提供資訊，251巨蟹座145.730/431.703，有空大家一起呼一下，謝謝！ ...
收到 應該是431.730吧~~
可以外送阿給 魚丸 到台中嗎??....
中山56的滷肉飯 也很有名 在中山路上 淡水捷運站斜對面..不知道是否還在..我以前在112讀書 都會特地跑到那吃ㄧ碗 凌晨唷
作者: allen333 時間: 2011-12-3 19:52
本帖最後由 allen333 於 2011-12-3 19:54 編輯
訊號線的長度跟波長有沒有關係呢??
作者: et006623 時間: 2011-12-3 19:57
allen333 發表於 2011-12-3 19:52
訊號線的長度跟波長有沒有關係呢??
小弟上次聽到一堂課
說到確實訊號線的長度跟波長是有關係的.
也因此小弟的原本買現成的4M RG58/U車天線用磁座
經計算也縮短為3米6,
現為2米多.....
忘了正確長度= =
作者: allen333 時間: 2011-12-3 20:04
et006623 發表於 2011-12-3 19:57
小弟上次聽到一堂課
說到確實訊號線的長度跟波長是有關係的.
也因此小弟的原本買現成的4M RG58/U車天線用 ...
原來真的有關係啊
不知有沒有算法 ? 查半天只有天線長度的算法 ~
真巧跟版大用的是同一個吸盤座~~
作者: mike1539 時間: 2011-12-3 21:46
我還以為2D銀線很好...想不到線材粗一點比較好
作者: whitemagic 時間: 2011-12-4 23:26
訊號線長度與波長有關係的問題我也有聽過
但沒聽到它的確切機制作用及影響為何
不過就所見過的傳輸線架設等實務經驗
從來也沒有在管傳輸線長度的問題會產生怎樣的波長變化
究竟與頻率的波長匹配與否
多是建立在天線上
一但發現駐波拉高
查到最後都是接頭問題
即使懷疑是傳輸線問題
換了傳輸線後確實解決了駐波拉高的狀況
但更換的傳輸線也是相同的長度(且這種狀況也非常少)
我是還還沒見識過傳輸線影響頻率波長導致怎樣的發射不良問題
故我判斷
傳輸線與波長的關係
於平常的實務應用上是沒有多大意義的
作者: WSUgocouars 時間: 2011-12-5 23:06
本帖最後由 WSUgocouars 於 2011-12-5 23:21 編輯
et006623 發表於 2011-12-3 19:57
小弟上次聽到一堂課
說到確實訊號線的長度跟波長是有關係的.
也因此小弟的原本買現成的4M RG58/U車天線用 ...
有啦, 有關啦..... 網路有資料, 但是都是原文, 用一下軟體翻譯一下..... 要我翻會翻很久!!
出處為 w w w.epanorama.net/documents/wiring/cable_impedance.html
---------------------part 1--------------------------------------------
Cable impedance
This document tries to clear out some details of transmission lines and cable inductance. This document is only a brief introduction to those topics. If you expect to work much with transmission lines, coaxial or otherwise, then it will be worth your while to get a book on that subject. The ideal book depends on your background in phsics or electrical engineering, and in mathematics.
What is the cable impedance and when it is needed?
The basic idea is that a conductor at RF frequencies no longer behaves like a regular old wire. As the length of the conductor (wire) approaches about 1/10 the wavelength of the signal it is carrying - good ol' fashioned circuit analysis rules don't apply anymore. This is the point where things like cable impedance and transmission line theory enter the picture.
The key tenet of all transmission line theory is that the source impedance must be equal to the load impedance in order to achieve maximum power transfer and minimum signal reflection at the destination. In real world case this generally means that the source impedance is the same as cable impedance and the value of the receiver in another end of the cable has also the same impedance.
How cable impedance is defined ?
Characteristic impedance of the cable ratio of the electric field strength to the magnetic field strength for waves propagating in the cable (Volts/m / Amps/m = Ohms).
Ohm's Law states that if a voltage (E) is applied to a pair of terminals and a current (I) is measured in this circuit, the following equation can be used to determine the magnitude of the impedance (Z). The following formula will hold truth:
Z = E / I
This relationship holds true whether talking about direct current (DC) or alternating current (AC).
Characteristic Impedance and is usually designated Zo or "Zed nought". When the cable is carrying RF power, without standing waves, Zo also equals the ratio of the voltage across the line to the current in flowing in the line conductors. So the characteristic impedance is defined with the formula:
Zo = E / I
The voltages and currents depend on the inductive reactance and capacitive reactance in the cable. So the characteristic impedance formula can be written in the following format:
Zo = sqrt ( (R + 2 * pi * f * L ) / (G + j * 2 * pi * f * c) )
Where:
R = The series resistance of the conductor in ohms per unit length (DC resistance)
G = The shunt conductance in mhos per unit length
j = A symbol indicating that the term has a phase angle of +90 degres (imaginary number)
pi = 3.1416
L = Cable inductance per unit lenght
C = Cable capacitance per unit lenght
sqrt = square root function
For materials commonly used for cable insulation, G is small enough that it can be neglected when compared with 2(3.1416) f C. At low frequencies, 2(3.1416) f L is so small compared with R that it can be neglected. Therefore, at low frequencies, the following equation can be used:
Zo = sqrt ( R / (j * 2 * pi * f * C))
If the capacitance does not vary with frequency, the Zo varies inversely with the square root of the frequency and has a phase angle which is -45o near DC and decreases to 0o as frequency increases. Polyvinyl chloride and rubber decrease somewhat in capacitance as frequency increases, while polyethylene, polypropylene, and Teflon* do not vary significantly.
When f becomes large enough, the two terms containing f become so large that R and G may be neglected and the resultant equation is:
Zo = sqrt ( (j * 2 * pi * f * L) / (j * 2 * pi * f * C) )
Which can be simplified to the form:
Zo = sqrt ( L / C )
Cables characteristics at high frequencies
At the high frequencies you can't look at the cable as a usual cable. On higher frequency it works as a waveguide. Characteristic impedance is specific resistance for electro-magnetic waves. So: It's the load the cable poses at high frequencies. Thehe high frequency goes (dependent of cable of course) usually from 100kHz and up.
If you feed a sinusoidal electrical AC signal of reasonable frequency into one end of the cable, then the signal travels as an electrical wave down the cable. If the cable length is an extremely large number of wave-lengths at the frequency of that AC signal, and you measure the ratio of AC Voltage to AC current in that traveling wave, then that ratio is called the characteristic impedance of the cable.
In practical cables the characteristic impedance is determined by cable geometry and dielectric. The cable length has no effect of it's characteristic impedance.
作者: WSUgocouars 時間: 2011-12-5 23:14
----------------------------------- part 2 ------------------------------
What does the coaxial cable model look like ?
The coax is represented schematically by a series of capacitors and inductances, a sort of odd filter arrangement, the particular values unique to the particular coax type. At a given frequency, if correctly chosen, that arrangement passes most of the signal; while at higher frequencies, that arrangment attentuates signal.
How does coaxial cable chacteristics define the impedance ?
The length has nothing to do with a coaxial cable impedance. Characteristic impedance is determined by the size and spacing of the conductors and the type of dielectric used between them. For ordinary coaxial cable used at reasonable frequency, the characteristic impedance depends on the dimensions of the inner and outer conductors, and on the characteristics of the dielectric material between the inner and outer conductors.
The following formula can be used for calculating the characteristic impedance of the coaxial cable: (formula taken from Reference Data for Radio Engineers book published by Howard W. Sams & Co. 1975, page 24-21)
impedance = (138 / e^(1/2)) * log (D/d)
Where:
log = logarithm of 10
d = diameter of center conductor
D = inner diameter of cable shield
e = dielectric constant (= 1 for air)
In a nut shell the characteristic impedance of a coax cable is the square root of (the per unit length inductance divide by the per unit length capacitance). For coaxial cables the characteristic impedance will be typically between 20 and 150 ohms. The length of the cable makes no difference whatsoever in regard to the characteristic impedance.
If the frequency is much too high for the coaxial cable, then the wave can propagate in undesired modes (i.e., have undesired patterns of electric and magnetic fields), and then the cable does not function properly for various reasons.
How is the impadance of balanced pairs ?
Characteristic impedance is determined by the size and spacing of the conductors and the type of dielectric used between them. Balanced pair, or twin lines, have a Zo which depends on the ratio of the wire spacing to wire diameter and the foregoing remarks still apply. For practical lines, Zo at high frequencies is very nearly, but not exactly, a pure resistance.
The following formula can be used for calculating the characteristic impedance of balanced pair near ground: (formula taken from Reference Data for Radio Engineers book published by Howard W. Sams & Co. 1975, page 24-22)
impedance = (276 / e^(1/2)) * log ((2D/d) * (1 + (D/2h)^2))^(1/2))
Where:
log = logarithm of 10
d = wire diameter
D = distance between wires in pair
e = dielectric constant (= 1 for air)
h = distance between balanced pair and ground
Not that this formula is only valid for unshielded balanced pair when D and h are order of magnitude larger than d. If the twisted pair is far away from ground (h is nearly infinite), the the effect of the ground is neglegtible and the impedance of the cable can be approximated with simpler formula (my own derivation from formula above):
impedance = (276 / e^(1/2)) * log ((2D/d)
For twin line Zo will be typically between 75 and 1000 ohms depending on the intended application. The impedance of typical old telephone pair in the telephone poles in the air has characteristic impedance of around 600 ohms. The telephone and telecommunication cables in use have typically a characteristic impedance of 100 or 120 ohms.
What kind of electrical model I can use for long coaxial cable ?
If you know the imductance and capacitance of certain lenght of cable you can use the following electrical model for it:
L L L / / L
---+uuuu+-+-+uuuu+-+-+uuuu+--/ ... /+uuuu+---
| | | / / |
--+-- --+-- --+-- --+--
C --+-- C --+-- C--+-- C --+--
| | | / / |
----------+--------+------+-/ ... /------+---
/ /
For this model it is a beneficial to know an useful impedance equation which described the relation of impedance, capacitance and inducatance:
Z = sqrt ( L / C )
The equations and model are based on the fact that for "long" cables you can calculate the cable impedance with the following model:
L L L / / L
---+uuuu+-+-+uuuu+-+-+uuuu+--/ ... /+uuuu+->
| | | / / |
--+-- --+-- --+-- --+--
C --+-- C --+-- C--+-- C --+--
| | | |
Z = jwL + [(1/jwC) || {(jwL + [(1/jwC) || ...
=Z
Since the chain is infinite, the terms on the right are just equal to Z. You get a nice quadratic.
"long" isn't real restrictive so as to be in the wavelength or better ballpark.
作者: WSUgocouars 時間: 2011-12-5 23:18
----------------- part 3 ----------------------------
Can I measure the cable impedance using multimeter ?
Cable characteristic impedance is a cable characteristics which is only valid for high frequency signals. Multimeters use DC current for resistance measurements, so you can't measure the cable impedance using your multimeter or other simple measurement equipments. It is usually best to check the cable type (usually printed on cable) and it's characteristics impedance from some catalogue instead of trying to measure it.
How can I measure cable impedance ?
A relationship exists which makes determination of Zo rather simple with the proper equipment. It can be shown that if, at a given frequency, the impedance of a length of cable is measure with the far end open (Zoc), and the measurement is repeated with the far end shorted (Zsc), the following equation may be used to determine Zo:
Zo = sqrt ( Zoc * Zsc )
Where:
Zoc = impedance of a length of cable is measure with the far end open
Zsc = impedance of a length of cable is measure with the far end shorted
NOTE: The Zoc and Zsc measurements both have magnitude and phase, so the Zo will also have magnitude and phase.
High frequency measurements of Zo are made by determining the velocity of propagation and capacitance of the cable or by reflectometry.
When cable impedance effects the signal ?
In order for a cable's characteristic impedance to make any difference in the way the signal passes through it, the cable must be at least a large fraction of a wavelength long for the particular frequency it is carrying.
Most wires will have a speed of travel for AC current of 60 to 70 percent of the speed of light, or about 195 million meters per second. An audio frequency of 20,000 Hz has a wavelength of 9,750 meters, so a cable would have to be four or five *kilometers* long before it even began to have an effect on an audio frequency. That's why the characteristic impedance of audio interconnect cables is not something most of us have anything to worry about.
Normal video signal rarely exceed 10 MHz. That's about 20 meters for a wavelength. Those frequencies are getting close to being high enough for the characteristic impedance to be a factor. High resolution computer video signals and fast digita signals easily exceed 100 MHz so the proper impedance matching is needed even in shor cable runs.
How impedance matching works
First, you want to drive the cable with an electrical source that has an output impedance equal to the characteristic impedance of the cable, so that all of the source's output power goes into the cable, rather than being reflected from the cable's input end back into the source. Second, you want the electrical load on the output end of the cable, to have an input impedance equal to the characteristic impedance of the cable, so that all of the power goes into the load rather than being reflected from the load back into the cable.
There are many exceptions to this normal driving method, but those are used for for special effects. You can choose an impedance match for maximum power transfer at low bandwidth, or mismatch the impedance for a flatter frequency response. It's the engineer's call, depending on what he wants.
Why impedance matching is needed ?
If you have mismatches between the source's output impedance, the cable's characteristic impedance, and load's input impedance, then the reflections can depend critically on the length of the cable. And if you distort the cable, as by crushing or kinking, or if you install connectors improperly, then you can have reflections, with resulting power loss. And sometimes reflected power can damage the power source if lits of power is sent to the cable (e.g., a radio transmitter). So you need to be careful of impedance mismatches.
An anomoly that is not in all text books is when antenna pushes power back (not a proper termination), it looks at the inside of the shield and the outside, which ever one is lowest gets the power . This means the RF can travel on the outside of the coax. The most difficult concept about coax is the XL,XC do not exist (to your transmitter) if cable is terminated.
Most common reasons for listing a cable impedance is that because of its reliable electrical characteristics, and that very impedance listing. Coax is often is used to carry low level higher frequency signals that are separated. Separations are very expensive in terms of signal loss -- a perfect impedance match will cost you half the signal, and even a slight mismatch is very costly, particularly at antenna strength signals. Carefully matched carriers, like coax, are necessary to preserve signal at reduced noise.
What effect does the nominal capacitance have on the cable's performance or transmission capabilities?
Capacitance of the cable is nothing to do if the coax is terminated. The transmitter will see absolutely no capacitance nor inductance.
And this transmission line characteristic is used to hide capacitance in high frequnecy PCB's. Engineers can design the PCB traces so that they have the proper capacitance and inductance values so that the transmitter will see nothing but a proper impedance transmission line.
作者: WSUgocouars 時間: 2011-12-5 23:19
-----------------------------------part 4-----------------------------------
Why is characteristic impedance important in data transmission?
If a cable is terminated in its matching characteristic impedance you can't tell from the sending end that the cable is not infinitely long - all the signal that is fed into the cable is taken by the cable and the load.
If the impedances are not matches, part of the waves in the cable will be reflected back on the cable connections distorting the outbound waves. When these reflected waves hit the wave generator, they are again reflected and mingle with the outbound waves so that it is difficult to tell which waves are original and which are re-reflections.
The same thing happens when pulses are sent down the cable - when they encounter an impedance other than the characteristic impedance of the cable, a portion of their energy is reflected back to the sending end. If the pulses encounter an open circuit or a short circuit, all of the energy is reflected (except for losses due to attenuation - another subject). For other terminations, smaller amounts of energy will be reflected.
This reflected energy distorts the pulse, and if the impedance of the pulse generator is not the same as the characteristic impedance of the cable, the energy will be re-reflected back down the cable, appearing as extra pulses.
Can I use coaxial cable without impedance matching ?
If the coaxial cable is very short, the cable impedance does not have much effect on the signal. Usually the beast way to transmit signal through coaxial cable is to do the impedance matching, although there are some applications where the normal impedance matching on both ends is not done. In some special applications the cable might be only impedance matched at only one end or intentionally mistached at both ends. Those application are special cases, where the cable impedance is take acount so that the combination of the cable and the terminations at the ends of the cable produce the desired transmission characteristics to the whole system. In this kind of special application the cable is not considered as a passive transmission line, but a signal modifying component in the circuit.
What about the velocity of propogation ratio ?
Velocity of propogation ratio percentage based on the speed of light in vacuum. The percentage tells what is the speed of the signal in the cable compared to the speed of light in vacuum. In coax cable, under reasonable conditions, the propagation velocity depends on the characteristics of the dielectric material.
Why attenuation figures tend to increase with increasing frequency ?
That usually is due mainly to the limited penetration of current into the inner and outer conductors (the skin effect). With increasing frequency, the current penetrates less deeply into the conductors, and thus is confined to a thinner region of metal. Therefore the resistance, hence attenuation, is higher. It also can be caused partly by energy loss in the dielectric material.
How to minimize the attenuation in coax ?
For a line with fixed outer conductor diameter, and whose outer and inner conductors have the same resistivity, and assuming you use a dielectric with negligible loss (such as polyethylene or Teflon in the high-frequency range at least), then you get minimum loss in coax if you minimize the expression:
(1/d + 1)/ln(1/d)
where d is the ratio of inner conductor diameter to outer conductor ID. A spreadsheet or calculator gets you close pretty quickly: D/d = 3.5911 is close. Thr formula was claimed to be derived from the formula for coax impedance versus D/d and a formula for loss that you'll find in "Reference Data for Engineers" published by Howard Sams, on pg. 29-13 in the seventh edition.
The interesting thing to notice is that this minimum loss does not directly yield a line impedance: the line impedance depends on the dielectric constant of the dielectric. For air insulated line, the corresponding impedance is about 76.71 ohms, but if the line is insulated with solid polyethylene, then minimum attenuation is at about 50.6 ohms. So, however it came to be, all the RG-58 we use for antenna feeds and test equipment connections is pretty close to minimum attenuation given the above conditions, and that the dielectric is polyethylene.
But if the line uses foam dielectric with a velocity factor of 0.8, then the impedance of minimum atten would be about 61 ohms. However, that minimum is a pretty broad one, and you don't start loosing a lot till you get more than perhaps 50% away from the optimal impedance.
Note that foam-dielectric line with the same impedance and outer diameter as solid-dielectric line will have lower loss. That's because, to get the same impedance, the foam line will have a larger inner conductor, and that larger conductor will have lower RF resistance, and therefore lower loss.
作者: WSUgocouars 時間: 2011-12-5 23:20
------------------------------------part 5 -------------------------------------------
Typical cable impedances
What are typical cable impedances ?
The most typical coaxial cable impedances used are 50 and 75 ohm coaxial cables. 50 ohm coaxial cables might be the most commonly used coaxial cables and they are used commonly with radio transmitters, radio receivers, laboratory equipments and in ethernet network.
Another commonly used cable type is 75 ohm ciaxial cable which is used in video applications, in CATV networks, in TV antenna wiring and in telecommunication applications.
600 ohms is a typical impednace for open-wire balanced lines for telegraphy and telephony. A twisted pairs of 22 gage wire with reasonable insulation on the wires comes out at about 120 ohms for the same mechanical reasons that the other types of transmission lines have their own characteristic impedances.
Twin lead used in some antenna systema are 300 ohms to match to a folded dipole in free space impedance (However, when that folded dipole is part of a Yagi (beam) antenna, the impedance is usually quite a bit lower, in the 100-200 ohm range typically.).
Why 50 ohm coax ?
Standard coaxial line impedance for r.f. power transmission in the U.S. is almost exclusively 50 ohms. Why this value was chosen is given in a paper presented by _Bird Electronic Corp._ Standard coaxial line impedance for r.f. power transmission in the U.S. is almost exclusively 50 ohms. Why 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 power-carrying 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 ohms.
Cable capacitance and characteristic impedance
Take a chunk of coax, connected to nothing. The center conductor and shield form a capacitor. If you charge that capacitor up to 100V, then short the shield to the center conductor, What is the current flow?
It is not infinite (or determined by parasitic resistance anc reactance ) like a "normal capacitor" but it is determined by the characteristic impedance of the line. If it is 50 ohm line charged to 100V then the current WILL be 2Amps. (100/50) It will be a square pulse, and temporal width (time duration, pulse width whatever you choose to call it) will be determined by the length of the line (around 1.5 nS/foot depending on line's velocity factor).
This method can be used for example to generate current pulses to semicondictor lasers. To get the pulse lengths longer than easily availabe with practical coaxial lines you can use use lumped impedance near-equivilant.
Using coaxial cables in applications
What happens if I use 50 ohm cable for vidoe application which needs 75 ohm cable ?
If 50 ohm cable sees a 75 ohm load (the receiver), a substantial part of the signal will be reflected back to the transmitter. Since the transmitter is also 75 ohm, this relected signal will be substantially reflected back to the receiver. Because of the delay, it will show up as a nasty ghost in the picture. Multiple ghosts like this look like ringing. Also, the reflections cause partial signal cancellations at various frequencies.
How can I convert cable impedance values ?
The cable impedance itself can't be converter unless you replace the whole cable with new one which has the right impedance. If you absolutelu need to use the existing cable for your application then there is one way to use the exiting cable: impedance converters. There are transformers which can make the cable look like different impedance cable when those are installed to both ends of the cable.
In some application it is possible to resistive adapers to convert the cable impedances. Those adapters are simpler than transformers but typically have a noticable signal loss in them (typically around 6 db for 75 ohm to 50 ohm conversion).
Impedance of circuit board traces
High speed signals can be routed on a circuit board if care is taken to make the impedance of the traces match the source driver impedance and the destination termination impedance. A microstrip line will exhibit a characteristic impedance if the thickness, width, and height of the line above the ground plane are controlled.
Characteristic impedance formula:
Z = (87 / sqrt( Er + 1.41 )) * ln( (5.98*h)/(0.8*w + t))
Where:
Er = dielectric constant (4.8 for typical fiberglass board)
h = height of the dielectric (fiberglass board thickness between trace nad ground plane)
t = thickness of the copper material in microstrip
w = width of the copper material in microstrip
The dielectric constant, Er, for typical 0.062" fiberglass board is 4.8. Using a trace thickness of 0.00134" gives a line width of 109 mils for a 50 ohm microstrip.
When routing circuit board traces, differential pairs should have the same length trace. These trace lines should also be as short as possible.
Impedance matching between different impedances
If two cables with different impedances are connected togerther or a cable is connected to a source which has different impedance then some kind of impdance matching is needed to avoid the signal reflections in the place where the cables are connected together.
Using transformer for impedance matching
The most classical method for matching different impedances is to use a matching transformer with proper impedance tranfer ratio. The impednace tranfer ratio of a transformer is determined by using the formula:
Za / (Na^2) = Zb / (Nb^2)
Where:
Za = input impedance
Na = number of turns on input coil
Zb = output impedance
Nb = number of turns on output coil
The equation can be converted to format:
Zb = Za * (Nb/Na)^2
From that equation you can see that Nb/Na is same as the transformer voltage transferrign ratio between primaty and secondary. This means that when you know that ratio you can use the equation without knowing the exact turns ratio.
Impedance matching netweork usign resistors
The matching network shown below can be used to match two unequal impedances, provided that Z1 is grater than Z2.
____
----|____|---+---------
R1 |
| |
Z1 | | R2 Z2
|_|
|
-------------+----------
The resistor for this circuit can be calxulated using the following equations:
R1 = Z1 - Z2*R2 / (Z2+R2)
R2 = Z2 * sqrt(Z1) / (Z1-Z2)
The table below will show some precalculated values for some most common interfacing situations:
Z1 Z2 R1 R2 Attenuation
(ohm) (ohm) (ohm) (ohm) (dB)
75 50 42,3 82,5 5,7
150 50 121 61,9 9,9
300 50 274 51,1 13,4
150 75 110 110 7,6
300 75 243 82.5 11,4
As you can see from the table the cost of simple resistor based impedance matching is quite large signal level attenuation in the conversion process.
--------------------------------------------------------------------------------
Comments
I have received the following comment on the cable impedance equation:
I have read your document, which is I must say very well written. I found a small mistake there though in "How does coaxial cable chacteristics define the impedance ?". Your formula is *impedance = (138 / e^(1/2)) * log (D/d)*, but this is only true for ideal conductor. Speed of wave in copper is less than in vacuum, and it equals to about 248827740 m/s; this means that it should be multiplied by a factor of 0.83. So the formula should look like this:
impedance = ( (138/e^1/2) * log(D/d) ) * 0.83
作者: et006623 時間: 2011-12-5 23:54
WSUgocouars 發表於 2011-12-5 23:20
------------------------------------part 5 -------------------------------------------
Typical cabl ...
老大看完也暈了～
還是感謝老大的文章～
有助晚上睡眠～～
７３
作者: WSUgocouars 時間: 2011-12-6 00:06
et006623 發表於 2011-12-5 23:54
老大看完也暈了～
還是感謝老大的文章～
有助晚上睡眠～～
費玉清的晚安曲唱著........ 再說一聲..... 明天見..... night...night....
作者: BM4JJJ 時間: 2011-12-6 15:07
WSUgocouars 發表於 2011-12-6 00:06
費玉清的晚安曲唱著........ 再說一聲..... 明天見..... night...night....
....當兵時後 晚安曲 哎呀 好懷念!!!~~~
作者: bx6aaa 時間: 2011-12-6 16:04
whitemagic 發表於 2011-12-4 23:26
訊號線長度與波長有關係的問題我也有聽過
但沒聽到它的確切機制作用及影響為何
不過就所見過的傳輸線架設等 ...
學術上的資料我是沒有啦！因為我不是電子專科
所以我只能用我碰到過的實務上提出來討論！
線的長短或是線長經過轉和，確實會影響波長匹配的
1.曾經使用過不同長短的或不同粗細的傳輸線做機器與駐波比表的連接測試，同一頻率它的駐波是不一樣的
2.架設室外天線纜線隨地丟置與纜線經過整理(沿屋角轉和)，測試駐波是不一樣的
所以纜線的長短是會影響駐波的
不過.........也不排除您說的接頭問因素。
因為自已製作接頭的水準不一，因此應該也會影響測試結果
作者: allen333 時間: 2011-12-7 20:51
查了一下 RG-58U 的資料 ...
衰減量是 148db/km
好像都優於一些號稱進口銀線 ...
算是一條經濟又好用的線
作者: BV7CW 時間: 2011-12-10 06:59
二篇文章推薦給大家
1. 電纜的速度係數 bv3fg.tripod.com/cqm/24/24068.htm
2 傳輸線特性介紹 bv3fg.tripod.com/cqm/11/11051.htm
應該對饋線的了解有幫助
VY 73
作者: allen333 時間: 2011-12-22 22:06
BV7CW 發表於 2011-12-10 06:59
二篇文章推薦給大家
1. 電纜的速度係數 bv3fg.tripod.com/cqm/24/24068.htm
2 傳輸線特性介紹 bv3fg.tripod ...
感謝分享~
每衰減 3db 功率減半
作者: et006623 時間: 2011-12-22 22:11
http://bv3fg.tripod.com/cqm/24/24068.htm
電纜的速度係數No.24 1994 Dec. p68~71, by 文伯銓 / KM2X
業餘無線電有很多實用而有趣的原理、定律和公式，明瞭理論之後，舉一反三，對於 很多有關無線電的事物和現象，可作一適當的分析和解釋。本文就來探討電纜線的「 速度係數」 (VELOCITY FACTOR) ……
電波速度因媒介物質而異 凡是參加過業餘執照考試的同好，相信都會試著牢記下列公式，以應付可能的試題：
| 波長λ (單位公尺 ) = 電波速度 (每秒 300,000,000 公尺 ) ÷ 週率 f( 單位 HERTZ) |
這個重要的波長與週率變換公式，是以電波在真空環境中行進的速度為基礎，即每秒 三億公尺，或每秒環繞地球七圈半。
在實際使用時，要注意的一點是，電波在其他物質中，如電線等，其進行速度便會慢 下來，慢多少則依其導線的構造和使用的絕緣物質，而各有不同。
這和光線很相似，光線在真空中的速度和電波相同，也是每秒三億公尺，但在水中便 慢多了；在玻璃中，因其密度比水高，速度又更慢。
這個速度減慢的程度，稱之為「速度係數」 (VELOCITY FACTOR)，通常是以百分比或 小數點來表示。在設計天線系統，尤其是高頻率輸電纜時，如果不考慮速度係數而不 修正其差異，輕則失調，重則失效。
導線構造影響速度係數 舉例來說，電波在以空氣作絕緣體的赤裸銅線中進行，其速度極近似在真空中的速度 。如以這種赤裸銅線作為天線，其實際的長度與計算出來的長度相差極微，但是，若 用包有絕緣物體的導線，便需要作修正了。
有人做了一根長度為一個波長的方形天線 (1 λ QUAD ANTENNA)，實際的導線長度要 比計算出來的要短 3~5%，才能在所要求的週率上諧振 (RESONANCE)。這 "3~5%" 便 是這一種導線的速度係數。
再看常用的 RG8 等同軸輸電纜 (COAXIAL CABLE)，它的速度係數從表一得知是 0.66 。這意思是說，假定一個電波計算出來的長度是 10 公尺，進入 RG8 電纜時，其長 度僅須 6.6 公尺便可，其理由即是因為速度係數。
 | A: 這是一個 7.5MHz 的電波在真空中進行的情形，其波長是 40 公尺。這電波從 a 點開始行至終點 b，即完成一個週波，所在的時間是一百萬分之 0.1333 秒 (1 ÷ 7.5MHz=0.1333μS)。
|
 | B:這是同一電波在 RG8 同軸電纜行走的情形。其完成一個週波 (即從 a' 至 b' 點 )的時間不變，仍然是 0.1333μS，由於這電波在 RG8 內行走，其速度要比在 真空中慢得多。雖然走得慢，但仍在規定的時間內 (0.1333μS) 完成一週波 (CYCLE)。所以這週波便短了。RG8 的速度係數是真空的 66%，這週波在 RG8 內的 長度便為 40 × 0.66=26.4 公尺。
|
| 圖一：速度係數 |
再舉例解釋，假設有一個週率是 7.5MHz( 即每秒振盪 7500,000 次 )的電波，則它 在真空中的長度剛好是 40 公尺 (300,000,000 ÷ 7,500,000= 40)。當這電波走進 RG8 時，它的長度則變為 26.4 公尺 (40 × 0.66=26.4)，雖然它的頻率仍然是 7.5MHz。縮短的理由是因為它在 RG8 內走得比在真空中來得慢的緣故 (見圖一 )。
電氣長度較電波長度短 上文提及速度係數依電線的構造而各有不同。同一週率的電波在不同的電線中，如果 以公尺來度量，其電波的長度也因線而異，令人無所適從。
於是，學術界為了方便起見，創立了所謂「電氣長度」 (ELECTRICAL LENGTH)，而以 波長 (WAVE LENGTH，簡寫為λ )作單位。
舉例來說，假使有人說天線最低的有效高度是半個波長，這在 2O 公尺 (14MHz) 波 段而言，半個波長是 10 公尺；若以 80 公尺 (3.5MHz) 波段來說，則是 40 公尺高 ，簡單明瞭。
速度係數很有實用價值，下列公式是用來計算電線、電纜的實際長度。所得答案是一 個波長的電線的長度。如用來計算 1/2 或 1/4 波長的電線時，耍依 1/2 或 1/4 比 例來減短：
λ = 299.79 ÷ f × V.F.
λ = 一個波長電線的長度 (單位公尺 )
f = 使用週率 (單位 MHz)
V.F.= 速度係數的英文縮寫
(見表一)
299.79 = 電波和光波在真空中的速度；最新測定的數字是每秒 299,792,500 公尺。
利用速度係數改變長虔 筆者在架設天線時，經常應用到這個公式。有二個較突出的例子，誌之如下:
- 有一次我架設了一個用於 14MHz 波段的四元件 (即四支 )垂直導向天線，天線本 身架設後，固定不動，然後用數根長度為 1/4 波長的 RG8 同軸電纜作為相位線 ，接於諸元件間，再用開關來變更各元件的電氣相位 (PHASE)，以達到轉變電波 方向的目的。這幾根相位線 (PHASING LINES) 的長度，據上述公式計算出來再被四除 (因所要的 長度是四分之一波長 )，答案是每根 3.53 公尺長。
1/4 λ =299.79 ÷ (4 × 14.0) × 0.66=3.53 公尺
- 我在住家的屋頂上，豎立了一根用於 80 公尺 (3.5MHz) 波段的垂直天線。這天 線需要用四根 1/4 波長的人工地線網 (RADIALS)。書本上說用赤裸銅線，每根的 長度算出來是 21 公尺。寒舍的屋頂是 7 × 20 公尺，所以，這四根地網線在屋頂上，轉彎抹角如蛛網一般 ，既要顧到機械性的穩固和安全度，又要考慮電線靠得太近或互相交錯而生破壞性的 感應，相當惱人。後來靈機一動，試用閑置一旁的舊 RG8 同軸電纜來代替，每根的 長度只需 14.1 公尺，處理起來方便得多；經實際 ON AIR 試驗，效果同前一樣。後 來介紹給其他電台使用，訊號報告也同樣好。
類型 | 總阻
(ohm) | 速度係數
% | 每公尺電容量
(pF/meter) | 外徑
(mm) | 最大電壓
容限(V) | 每百公尺衰減量 (dB/100meter) |
30MHz | 150MHz | 400MHz |
| RG-8 | 52 | 66 | 96.8 | 10.3 | 4000 | 4.3 | 10.5 | 18.7 |
| RG-8A | 52 | 66 | 96.8 | 10.3 | 5000 | 4.3 | 10.5 | 18.7 |
| RG-8 FOAM | 50 | 80 | 83.3 | 10.3 | 1500 | 3.0 | 6.9 | 9.7 |
| RG-11 | 75 | 66 | 67.6 | 10.3 | 4000 | 4.3 | 10.5 | 18.7 |
| RG-11A | 75 | 66 | 67.6 | 10.3 | 5000 | 4.3 | 10.5 | 18.7 |
| RG-11 FOAM | 75 | 80 | 55.4 | 10.3 | 1600 | 3.0 | 6.9 | 9.7 |
| RG-58 | 53.5 | 66 | 93.5 | 5.0 | 1900 | 6.9 | 19.7 | --- |
| RG-58A | 53.5 | 66 | 93.5 | 5.0 | 1900 | 8.9 | 22.3 | --- |
| RG-58 FOAM | 53.5 | 79 | 93.5 | 5.0 | 600 | 6.9 | 15.4 | 21.1 |
| RG-59 | 73 | 66 | 68.9 | 6.1 | 2300 | 6.9 | 15.4 | 21.1 |
| RG-59A | 73 | 66 | 55.4 | 6.1 | 2300 | 6.9 | 15.4 | 21.1 |
| RG-59 FOAM | 75 | 79 | 68.9 | 6.1 | 800 | 5.2 | 11.5 | 18.7 |
| RG-213 | 50 | 66 | 101.1 | 10.3 | 5000 | 4.3 | 10.5 | 18.7 |
| RG-214 | 50 | 66 | 101.1 | 10.3 | 5000 | 4.3 | 10.5 | 18.7 |
| 9913 (BELDEN) | 50 | 84 | 78.7 | 10.3 | --- | 2.3 | 5.2 | 7.0 |
| 9914 (BELDEN) | 50 | 78 | 85.3 | 10.3 | --- | 2.3 | 5.2 | 7.0 |
| OPEN WIRE | --- | 97 | --- | --- | --- | 0.3 | 0.85 | --- |
75Ω XMTG
TWIN LEAD | 75 | 67 | 62.3 | 19.0 | --- | --- | --- | --- |
300Ω
TWIN LEAD | 300 | 82 | 19.0 | 5.8 | --- | --- | --- | --- |
| 300Ω TUBULAR | 300 | 80 | 15.1 | 4.6 | --- | 1.8 | 4.2 | --- |
TV TYPE OPEN
WIRE 1/2-INCH | 300 | 95 | --- | --- | --- | 0.6 | 2.3 | --- |
TV TYPE OPEN
WIRE 1-INCH | 450 | 95 | --- | --- | --- | 0.6 | 2.3 | --- |
附註：
- 業餘電台應儘量用粗的電纜，如 RG-8、RG-11 等；雖然 RG-58、RG-59 的容電 能力為 300WATTS，但用粗纜可減少損失。
- RG-214 及 BELDEN 廠的 9913、9914 是用雙層金屬網包裹，防漏性更好。
- 衰減度 (ATTENUATION) 週率度越高，衰減越烈。本表列出 30、150、400MHz 三 個週率的衰減度。在 150MHz 以上，有些衰減極鉅，根本不適用。
- 衰減度以 dB 為單位，每 3dB 電力增或減一倍。如某一段線的衰減度是 6dB， 以 100W 電力輪入，輪出的一端只剩下 25WATTS，因為 100W 減半 (3dB) 是 50W，再減半成 25W。(見本刊 13 期 73 頁 dB 一文 )。
- 衰減度與長度成正比。本表所列是 100 公尺的損失，如用 300 公尺電纜，將表 中 dB 數乘三。如用 50 公尺線，則將 dB 數除二，
- 同軸電纜工作的溫度範圍是 "-65℃ ~ +80℃ "。
表一 輸送電纜特性
作者: et006623 時間: 2011-12-22 22:12
http://bv3fg.tripod.com/cqm/11/11051.htm傳輸線特性介紹No.11 1993 Sep p51~55, by 劉克正 / BV2CL 台北93-32信箱
| 在業餘無線電的領域裡，傳輸線必須將發射功率傳送到天線端，並將天線端所接收到的微弱訊息，傳送到接收機；所以，使用良好的傳輸線可以增進收發效率。在此討論傳輸線的特性、電波反射及負載阻抗： |
傳輸線的種類 現在最常使用於無線電的傳輸線是同軸電纜線 (Coaxial Cable)，傳遞頻率約為 1000MHz 以下的頻帶；其次是平行線，廣泛應用於電視天線的傳輸，價格非常低廉，早期業餘活動中常有所見。在微波領域 SHF 頻帶 (3GHz-30GHz) 以上，就要利用導波管 (Waveguide Tube) 做為傳導，將導波管內外鍍以金銀，以保持信號不致散失。
這些傳輸線和一般所用的電線是有所不同的，主要是傳送信號頻率的不同。當信號頻率愈高時，電線中的分佈電容、導體的電感特性，都會成為特性改變的因素。
同軸電纜線特性- 中心導體供電，外部隔離網接地，兩者之間為電介質，常為泡棉、空氣... 隔離性良好的材質。
- 隔離效果較佳，受環境影響較小。
- 傳輸效果較差，傳輸線損耗較大。
- 傳輸特性阻抗常為 50-150Ω，業餘活動使用 50Ω，電視系統使用 75Ω。
- 常用規格有美規 RG 系列編號，如： RG-58、RG-8 等。日規編號為 5D-2V、8D- FB、10D-SFA 等，中間有 "D" 者為 5OΩ；若是 3C-2V、5C-FB 等，中間為 "C" 者，則是電視專用之 75Ω同軸線，不能隨意在業餘系統中應用。
平行線特性- 兩條線平行寬度和使用頻率有相關性；頻率愈低，寬度增大，用於業餘 HF 頻帶時較不方便。
- 兩條平行線的信號呈反相，互相差 180°，藉以減少干擾的程度。
- 隔離效果不如同軸線良好，易干擾外部，或受外部干擾傳輸線上的信號；雨水附著其上，傳遞效率會降低。
- 傳輸效果較同軸線優良，因為導體之間分佈電容較小。
- 傳輸特性阻抗為 150Ω ~ 600Ω，常用為 300Ω。
此兩種傳輸線優缺點比較下，以同軸線有較長的壽命和不錯的特性，選購上以使用目的為主要考量，特性要符合使用時可能達到之額定值以上才可。
傳送損耗因素銅質損失 一、I2R 損失：傳輸線中導體內電阻值在電流流經時，即消耗部份功率：
導體內阻值為 R，導體長度為 L，導體之截面積為 A，金屬電阻係數為ρ (唸成 Rho)，銅≒ 1.7x10-6( 歐姆 / 公分 )，則：
二、集膚效應 (Skin Effect)：當傳輸線中導體的訊號頻率上升至百 MHz 的時候，導體內部電流分佈呈現不平均狀態，導體截面的外圈集中多數的電流，中心部份僅有少數電流流經，在 V/UHF 傳送大功率時，會使同軸線中心導體表面發黑氧化，使用一段時間後，必須更換。
介質損失 同軸線中間的絕緣材質，如泡綿、空氣等，介於導體和隔離網之間，在金屬平面間會產生電容效應，此絕緣性材料相等於電容的電介質，希望是漏電程度愈小愈好，最好的選擇是空氣同軸線。
輻射損失 傳送訊號波長愈短，愈容易從導體向外擴散幅射，同軸線隔離綱要抑制幅射造成損失，故高頻信號需利用緊密、雙層隔離編織網的同軸線。
等效電路
 |
圖 1. 傳輸線等效電路，為單位元素無限延伸的組合。
[ 圖 1 ] 是一種簡略說明同軸線的等效電路，它可以代表傳輸線的特徵，整段同軸線可看成它的無限串聯延伸：R 為導體電阻 (Ω：歐姆 )；L 為導體的電感效應之電感值 (H：亨利 )；C 為絕緣材料之間導體的電容量 (F：法拉 )；G 為絕緣材料中漏電之電導值 (：姆歐 ) 。以理想無損耗的傳輸線而言，R 和 G 都為零，即導線無電阻和完全不漏電之電容，則信號可無限傳遞。
在數學推導得到傳輸線阻抗為 Z0：
 | | → [ 公式 2 ] |
而理想傳輸線可簡化成 Z0'：
 | | → [ 公式 3 ] |
 |
圖 2. 天線調諧器之功用和連接
從 [ 公式 3 ] 獲知傳輸線阻抗和內部電感量及電容量佔多數的關係，若同軸線阻抗為 50Ω，發射機輸出阻抗為 50Ω時，且天線於發射頻率的特性阻抗也呈 50Ω，為同軸線內部不會產生反射信號的含義。
由於天線並不會保持 50Ω 阻抗，它隨著頻率而有改變，如 [ 圖 2 ] 所示，當天線不為 50Ω時，利用天線調諧器改變 L 和 C 值，使傳輸線二段阻抗由原先 50Ω改成和天線相近的阻抗，達到互相匹配，以獲得最大傳輸功率和減少反射信號。
 |
圖 3. 輸入信號和負載在同軸線的連接。
反射係數 [ 圖 3 ] 中，反射係數Γ (Gmma) 定義為反射信號 Er 和輸入信號 Ei 的比值
| ErΓ= ---- Ei | | → [ 定義 1 ] |
| EL Ei+Er則：ZL= ---- = ------- IL Ii+Ir | | |
| Er 1 + ---- Ei= --------------- Ii Ir ---- + ---- Ei Ei | | ( 提出 Ei ) |
| 1 + Γ= --------------- 1 Γ ---- - ---- Zo Zo | | |
| 1 + Γ= Zo˙ -------- 1 - Γ | | → [ 公式 4 ] |
| [ 公式 4 ]可改變形式為： | | |
| ZL - ZoΓ= ----------- ZL + Zo | | → [ 公式 5 ] |
當傳愉線特性 Z0 為 50Ω時，Γ能的值介於 -1~1 之間，Γ =0 時表示傳輸線不反射信號，此時傳統線特性 Z0 為 50Ω，且天線端 ZL 也為 50Ω。
當輸入信號 Ei 和反射信號 Er 相會時，就會產生合成作用，出現合成波 EL，其關係式為 EL=Ei+Er，合成波有兩種影響：
一、增加能量消耗：輸入信號 Ei 希望能傳送至天線端 ZL 部份，過程中不損失為最佳，但現在 Z0 和 ZL 不相同，即產生反射信號；除了不匹配，還產生削減作用；若反射信號功率過大，甚至燒毀發射機末級功率放大元件。
二、傳輸線諧振：(1) 反射信號 Er=0，傳輸線稱為非諧振線，適用於一般傳遞用途。
(2) 反射信號 Er ≠ 0，即傳輸線 Z0 和天線阻抗 ZL 不相等，傳輸線就成為諧振線。
駐波比 駐波比 "Standing Wave Ratio"，縮寫為 SWR，有時針對電壓討論有 VSWR，而業餘常用到的駐波比表，是經由下列公式產生：
| EL (max)SWR ＝ ------------ EL (min) | | → [ 定義 2 ] |
| EL (max) Ei + ErSWR ＝ ------------ ＝ ----------- EL (min) Ei - Er | | ( 提出 Ei ) |
| | Er | 1 + ------- | Ei | 1 + | Γ | ＝ --------------- ＝ ------------- | Er | 1 - | Γ | 1 + ------- | Ei | | | → [ 公式 6 ] |
| | Γ | ＝ ρ ( Rho )，介於 0 ~ 1 | | → [ 定義 3 ] |
| 1 + ρ所以，SWR ＝ ------------ 1 - ρ | | → [ 公式 7 ] |
駐波比可能數值：
ＺL＝Ｚ0，Γ＝０，ρ＝０，ＳＷＲ＝１
ＺL≠Ｚ0，－１＜Γ＜１但Γ≠０，０＜ρ＜１，１＜ＳＷＲ＜∞
駐波比和天線阻抗換算 從 [ 公式 7 ] 和 [ 公式 5 ]知：
| 1 + ρ 1 + | Γ | ZL - ZoSWR ＝ ---------- ＝ -------------，和 Γ＝ ----------- 1 - ρ 1 - | Γ | ZL + Zo |
一、若 Γ 介於 0 ~ 1 之間，則 Γ = ρ，且：
|
| ZL - Zo 1 + ----------- ZL + Zo ZL + Zo + ZL - Zo ZLSWR ＝ ------------------- ＝ --------------------- ＝ ------ ZL - Zo ZL + Zo - ZL + Zo Zo 1 - ----------- ZL + Zo |
| 即：ＺL＝ＳＷＲ˙Ｚ0，當０＜Γ＜１ → [ 公式 8 ] |
[ 公式 8 ] 說明反射係數為正數值時，且使用 SWR 表測得之 SWR 值乘以同軸線阻抗 Z0=50Ω，即得天線阻抗 ZL。
二、若Γ介於 -1 ~ 0 之間，則Γ = -ρ，
| Zo - ZL 1 + ----------- Zo + ZL Zo + ZL + Zo - ZL ZoSWR ＝ ------------------- ＝ --------------------- ＝ ------ Zo - ZL Zo + ZL - Zo + ZL ZL 1 - ----------- Zo + ZL |
| 即：Ｚ0＝ＳＷＲ˙ＺL，當－１＜Γ＜０ → [ 公式 9 ] |
當 SWR 表指示為 1.5 時，天線阻抗值可能為 1.5 x 50 = 75Ω，或 1/1.5 x 50 ≒ 33.3Ω，需視反射係數Γ來判斷，利用一部示波器 來測量輸入信號 Ei 和接至負載端 ZL 端電壓 EL ，即可知Γ的正負性。
欲求得正確天線阻抗 ZL，以業餘角度而言，可利用雜訊電橋配合無線電機量測出 (請參考本刊第 8 期 101-103 頁 )
 |
圖 4. 電流流向路徑
天線匹配和效率 [ 圖 4 ] 中，電流 Ｉ＝ Ｅ／（Ｚ0＋ＺL），天線所獲得之功率為：PZL=I2˙ZL
在不同天線阻抗時所獲得之效率η (Eta)
SWR 值為 1.5 時，天線 ZL=75Ω或 33.3Ω，可求天線效率η，利用 [ 公式 10 ]：
所以 SWR=1.5 時，到天線之效率是相同的，皆為最大值的 96%，且有 4% 因為不匹配而產生反射功率；若從發射機輸出 100W 功率，有 96W 送至天線，其餘 4W 造成功率放大元件額外的熱能。
利用 [ 公式 8、9、10 ] 製成下表：
| SWR | 天線阻抗Ω | 天線吸收率 | 天線反射率 |
| 1.0 | 50 | 100% | 0% |
| 1.1 | 55 或 45.5 | 99.8% | 0.2% |
| 1.2 | 60 或 41.7 | 99.2& | 0.8% |
| 1.3 | 65 或 38.5 | 98.3% | 1.7% |
| 1.4 | 70 或 35.7 | 97.2% | 2.8% |
| 1.5 | 75 或 33.3 | 96.0% | 4.0% |
| 1.6 | 80 或 31.3 | 94.7% | 5.3% |
| 1.7 | 85 或 29.4 | 93.3% | 6.7% |
| 1.8 | 90 或 27.8 | 91.8% | 8.2% |
| 1.9 | 95 或 26.3 | 90.4% | 9.6% |
| 2.0 | 100 或 25 | 88.9% | 11.1% |
| 3.0 | 150 或 16.7 | 75.0% | 25.0% |
| 4.0 | 200 或 12.5 | 64.0% | 36.0% |
| 5.0 | 250 或 10 | 55.5% | 44.5% |
駐波比和天線阻抗效率換算表
一般操作時，若功率在 100W 以上、SWR 超過 2.0 以上時，必須要警覺發射部份，它可能有 11.1% 的反射功率，可利用天線調諧器保護機子，但調諧器免不了會消耗功率，根本之道還是做好天線部份匹配的工作，不可降低天線的增益、方向性等，否則天線會成為假負載的一種，當然欲達成 DX 主動攻擊，更需具備 12 米以上的鐵塔，操作 10、15、20 米波段才能得心應手，南征北討。
諧振線的應用
 |
圖 5. 傳輸線長度影響阻抗
一、當負載 ZL = 0 時，稱為短路諧振線，且
ZS = 0Ω(短路)：長度 L 為 0, λ/2, λ ...
ZS = ∞Ω(開路)：長度 L 為 λ/4, 3λ/4, 5λ/4 ...
雖然 ZL 被短路；了，但從傳輸線可得開路的結果。
二、當負載 ZL = ∞ 時，稱為開路諧振線，且
ZS = 0Ω(開路)：長度 L 為 0, λ/2, λ ...
ZS = ∞Ω(短路)：長度 L 為 λ/4, 3λ/4, 5λ/4 ...
相同地，ZL 開路，卻可從 ZS 端得短路或開路的阻抗。
三、負載為 ZL，傳輸線長度 L 為 λ/4 的偶數倍時：
ZS = ZL，當 L 為 0, λ/2, λ ...
ZS與傳輸線阻抗Z0無關。
 |
圖 6. λ/4 長之傳輪線應用於阻抗匹配。
四、負載為 ZL，傳輸線長度 L 為 λ/4 的奇數倍時：
ZS = Z02 / ZL，當 L 為 λ/4, 3λ/4, 5λ/4 ...
[ 圖 6 ] 利用 75Ω傳輸線來匹配輸出阻抗為 50Ω的發射機，此傳輸線長度可為 λ/4, 3λ/4, 5λ/4 …，都有相同特性。
ZL = Z02 / ZS = 752 / 50 =112.5Ω
當ZL = 112.5Ω左右時，可以達成匹配
作者: luck8465 時間: 2011-12-26 00:24
提示: 作者被禁止或刪除 內容自動屏蔽
作者: 大頭熊 時間: 2011-12-26 12:37
感謝分享
作者: wisejames615 時間: 2012-8-24 18:18
renjong 發表於 2011-12-1 01:50
小弟親自實測過,結果比理論值還差！
RG58A/U線材,使用M頭連接設備與天線(線本身無續接,僅於發射機/功率 ...
佩服!最起碼要剪一條60M的線來測...
作者: ggyy 時間: 2012-8-24 20:34
看完這些後,TRC 真有很大大是高手中的高手,感謝分享~~~~~~
作者: peprhr 時間: 2012-8-30 07:52
renjong 發表於 2011-12-1 01:50
小弟親自實測過,結果比理論值還差！
RG58A/U線材,使用M頭連接設備與天線(線本身無續接,僅於發射機/功率 ...
所以，實驗用的RG-58是用進口貨，測出來的結果，比5D、8D的差??
作者: WSUgocouars 時間: 2012-8-30 08:07
peprhr 發表於 2012-8-30 07:52
所以，實驗用的RG-58是用進口貨，測出來的結果，比5D、8D的差??
RG-58 約是 3D 規格. 當然效能會比5D、8D差呀!
作者: peprhr 時間: 2012-8-30 08:12
WSUgocouars 發表於 2012-8-30 08:07
RG-58 約是 3D 規格. 當然效能會比5D、8D差呀!
原來如此，感謝解說。
作者: spp_1967 時間: 2012-8-30 21:07
50歐姆和53.5歐姆對無線電機的發射有什麼影響?哪位才學之士能為我們做學理上的解說 ,而非臆測假想之言.進而增長我們的知識呢?在此感恩啦!因為腦殘我買到RG58U了啦
作者: cdr500 時間: 2012-9-8 22:43
果然是長知識了~~
作者: redcrab 時間: 2019-3-2 22:22
以前只知道天線的重要性，從沒在意過原來傳輸線也是如此的重要，看完這篇文章真的是獲益良多，只是可能時間的關係，都無法看見圖片了，這樣感覺好像只有吸收一部份而已，有點可惜！
作者: cptsou 時間: 2019-4-1 23:05
感謝分享
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