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Transmission Lines....

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What are the different types of TLs used????I know of 2...
1.Co-axial cables
2.Parallel wire lines
Are there any other types???
I am not very sure about the structure of parallel wire lines....All i know is that there are 2 wires running parallel (well almost parallel ) n supported by insulators at certain distances....Is there anything else worth knowing????
 

RadioRon

Well-Known Member
There are a number of transmission line structures that are used very heavily on printed circuit boards. These include micro-stripline, stripline, coplanar line and a few other variants.

For very high powered transmitters, like those at the BBC shortwave service, for example, you will find an unusual kind of coaxial line with a square pipe, actually more like waveguide only larger, with a metal center conductor suspended in the center of it and the inside of the pipe is a vacuum.

There are some really exotic types that are quite interesting. For example, it is possible to guide a microwave signal along a single bare wire, which essentially makes it a single wire transmission line. I believe this sort of thing is used in some wire guided missile applications.

In general, though, for hooking up RF signals other than on a circuit board, the coax cable and balanced wire line are the two dominant types.

At microwave frequencies, typically above about 6 GHz, waveguide is the most popular.
 
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stevez

Active Member
On the hobby/amateur radio side I see coaxial cables and the parallel lines (often called ladder line).

Usually the impedance of the coax is 50 ohms or 75 ohms, 50 ohms being more familiar to me. RG-8, RG-8X and RG-58 are common designations as is Belden's 9913. There are hardlines that are in use by some.

Parallel line used to be two wires separated by plastic or wood insulator so that the characteristic impedance meets the requirements. 300 ohm twinlead was used for television applications but can be used for other things. I think you can still purchase 600 ohm ladder line but many people make their own. I use 450 ohm ladder line where I can for work below 30 mHz because it is very low loss.
 

crutschow

Well-Known Member
Most Helpful Member
Twisted pair (which is a type of parallel wire line) is also used as a transmission line, usually for digital signals. It's used in ethernet (CAT5/6) and HDMI cables, for example.
 
Thnx for all the information guys....now i would lyk to know a bit about the manner in which energy is transferred through such lines....
In case of co-axial cables,current flows through both the cables in opposite directions...n there is a voltage difference between the cables....So there is a H field n a E field in the space between the cables....n their interaction causes the transfer of energy....
In case of parallel wire lines the same phenomenon occurs,except tht now the current flows through one line and returns through the other...and the voltage difference exists between these lines...
This is what i know.....did i miss out on anything????n more importantly m i wrong anywhere?????
 

crutschow

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For both coax and parallel wires there is a voltage difference that transfers the energy. For coax it's between the center conductor and the shield, which is at AC ground potential. For parallel wires it is typically done differentially with each wire alternately going above and below AC ground potential due to the AC signal.

In each case the energy is propogated by an E-H field down the cable. The energy is carried by the inherent capacitance and inductance of the cable (which determines the cable's characteristic impedance)..
 
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unclejed613

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For both coax and parallel wires there is a voltage difference that transfers the energy. For coax it's between the center conductor and the shield, which is at AC ground potential. For parallel wires it is typically done differentially with each wire alternately going above and below AC ground potential due to the AC signal.

In each case the energy is propogated by an E-H field down the cable. The energy is carried by the inherent capacitance and inductance of the cable (which determines the cable's characteristic impedance)..

the shield is at ground potential wherever it's connected to ground, but that doesn't mean it's always at ground potential, especially at odd multiples of 1/4λ.

another thing to know about transmission lines (it becomes very important whan using transmission lines as antenna elements or resonant sections) is it's velocity factor,(as a percentage od "c", the speed of light) which varies with construction and materials of the line itself. you can have 3 different grades of RG-8 side by side, and have 3 different velocity factors. with really cheap and lossy materials and a poor shield braid, the velocity factor can be as bad as 50 ot 60%. well made coax cable with good low loss materials and 100% shield coverage can have a velocity factor as high as 90 or 95%. what this means with transmission lines, is that for a 1/4λ tuning stub, you have to calculate it for 1/4λ * v. so for a 1/4λ stub at 75Mhz, λ= 1 meter. to find the length of a piece of transmission line needed for a 1/4λ stub, we multiply by the velocity factor (we'll use 70% for an example). so 1m*0.70= 70cm of cable for our tuning stub.

btw, the usual characteristic impedance of twisted pair is 100 ohms IIRC, and the velocity factors are different as you go from CAT3 cable to CAT5 and CAT6. i once saw a very useful chart that showed the characteristic impedance of various types of transmission lines and wiring methods (including wire wrap with and without ground planes, as well as PC board techniques).... i'll see if i can find it on the web, but i originally saw it in print somewhere....
 

unclejed613

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desn't look like i'll find it easily on the web. i'm about 80% sure it was in an app note for ECL chips back in the 80's, but it's a bit tedious wading through the results of google searches on the subject...... i do remember it listed the impedances of things like wire wrap, ribbon cable, obviously coax and twisted pair, and a lot of other wiring methods, etc... there were quite a few surprising items on the list.
 
desn't look like i'll find it easily on the web. i'm about 80% sure it was in an app note for ECL chips back in the 80's, but it's a bit tedious wading through the results of google searches on the subject...... i do remember it listed the impedances of things like wire wrap, ribbon cable, obviously coax and twisted pair, and a lot of other wiring methods, etc... there were quite a few surprising items on the list.

I just need an elementary knowledge about TLs (for the tym being)....so i dont think the impedance chart is necesary at ths point of tym....so u need not lose sleep over it....:)

I have certain doubts regarding parallel wire lines...
1.Are parallel wire lines shielded????If they arent,wont the EM fields generated around them radiate into space and possibly interfere with other EM signals???
2.What are the advantages of using twisted pairs over parallel wire lines???
 

unclejed613

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twisted pairs are a form of parallel wire lines. no, they are not shielded, but their noise immunity comes from their being balanced (i.e. a differential signal path) rather than unbalanced (a single-ended signal path). rather than having a single ended line with a shield, a parallel line or twisted pair has a balanced current path, so even though both conductors may pick up interference, it's eliminated by cancellation. a parallel wire line can also be formed with shielded twisted pair, shielded parallel line (also known as "twinax), or dual coax lines.
 
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Warpspeed

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I just need an elementary knowledge about TLs (for the tym being)....so i dont think the impedance chart is necesary at ths point of tym....so u need not lose sleep over it....:)

I have certain doubts regarding parallel wire lines...
1.Are parallel wire lines shielded????If they arent,wont the EM fields generated around them radiate into space and possibly interfere with other EM signals???
2.What are the advantages of using twisted pairs over parallel wire lines???

1/ Parallel wires are not normally shielded, and yes they radiate external electric and magnetic fields. By twisting the wires, the fields are also twisted, and at a distance these fields cancel.

2/ Two parallel lines run side by side over any distance will mutually couple, causing all sorts of interference problems. Something like computer ribbon cable can be particularly awful in this respect if used incorrectly.

By using twisted pairs, particularly when many twisted pairs are bundled into a multi pair cable, this mutual coupling tends to cancel out, even though the twisted pairs are tightly packed very close together..
 

unclejed613

Well-Known Member
Most Helpful Member
i've seen ribbon cable made of twisted pairs, usually used for SCSI drives that use differential signals. here's a picture with twisted pair spectra-strip cable on top, and standard parallel spectra-strip cable on the bottom
 

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