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Stubs in Transmission Lines

  1. Jun 1, 2014 #1
    Hello experts!

    What is stubs in regards with transmission lines?

    I don't find any precise and point to point definition of it. Every where book tries to make us understand with a concept of 4 to 5 lines and this makes me confused. Please tell me the very precise and easiest definition of it.

    And why we use them?

    PS: I don't know where to post microwave or telecommunication's question that's why I posted in the General Physics category.

    Thank you.
     
  2. jcsd
  3. Jun 1, 2014 #2

    davenn

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    in a coax transmission line I have used stubs as notch filters

    I would usually connect the stub via a "T" connector ( BNC, TNC, N Type etc)

    The stub would usually be an electrical 1/4 wave long with an open circuit end at the frequency I wanted to notch out

    attachment.php?attachmentid=70210&stc=1&d=1401620498.jpg

    cheers
    Dave
     

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  4. Jun 1, 2014 #3

    davenn

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    is that the type of stub you wanted to know about ?

    Dave
     
  5. Jun 1, 2014 #4
    Yes, coaxial and transmission line too. And I need precise definition too. What you have shown, it is already in book and I have read it. But long description is annoying me. I am not getting any precise definition of stubs either with coaxial or with transmission line. If I get definition then I will go to description and hope fully then I will get it.

    Thank you
     
  6. Jun 1, 2014 #5

    sophiecentaur

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    Transmission lines are a difficult subject and it's hard to give an easy hand-waving explanation. Transmission line theory tell us that they can act as transformers - changing the impedance of a load when 'seen' from the other end of the line. 1/4 wavelength line will turn a short circuit into an open circuit, for instance.
    Hanging a stub on a line will place an impedance 'across' (i.e. in parallel with) the line. A short, open ended or short circuited stub can 'look like' a short circuit or an open circuit or anything reactance in between - depending upon the length. (Putting a resistive termination on the end of the stub can give even more choices.) Adding stubs to a line can improve the match to a load and they are easily adjustable, compared with adding lumped components.
    It all depends upon what you actually want to know.

    And, yes, this is the right forum for the question. :smile:
    I suggest that you do a Google search on 'transmission line stub' along with 'filter' or 'matching' and find something at a level you can appreciate.
     
  7. Jun 1, 2014 #6
    Hi again,
    My question is also same again this time too. What is a stub. You explained its application but I still didn't find any precise definition of the stub. And I have searched the google but I didn't find what I am looking for. That's why I moved to physicsforums.com. I reckon that there experts would help me. But I didn't find what I want. :-(
     
  8. Jun 1, 2014 #7

    jim hardy

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    Have you ever shouted down a pipe and listened to the echo?

    A pressure pulse travels down the pipe and is reflected back to the point of entry.
    Next time you enter a long hallway clap your hands and see if the time delay of the echo is discernible.


    A transmission line stub is similar in principle to that sound propagation down a pipe or hallway.
    An electrical pulse (or sinewave) travels down the stub and is reflected back without much loss of energy.
    Here's the kicker - If it arrives back at the point of entry a half cycle later, then its energy subtracts from(cancels out some of) the incoming energy of next half cycle at that frequency.

    So a stub is often used as a frequency selective device to remove unwanted signal of particular frequency. You'll see them sold in TV shops as "traps" for a specific channel. I used to live within sight of channel 6 TV tower . To receive other stations it was necessary to place a "channel 6 trap" on the incoming line so as to not overload the TV's tuner. I made my own from a piece of TV antenna lead-in wire cut to proper length for channel 6's frequency. Just as in Dave's excellent drawing above except he used better connectors.

    So, a stub is just a segment of transmission line of the length it needs to be. That length depends mostly on the frequency of interest.

    Stubs can do other things too. When in your studies you get to "Smith Chart" , learn to use it well.

    ARRL Antenna Handbook is a great practical reference.
     
    Last edited: Jun 1, 2014
  9. Jun 1, 2014 #8
    @ jim hardy
    Thank you very very much. That is what I am looking for. Very good and easy answer along with the example.

    Thank you very much sir. I am very grateful for this.

    Which edition of ARRL Antenna Handbook should I buy? Should I buy its 2014 edition?
     
  10. Jun 1, 2014 #9
    I want to know about this paragraph. What you have said, I didn't get much. What I have understand is that if we want a specific channel to watch on our TV then we can use stub. Right? And for this we must know the frequency of that TV channel over which it is being broadcasted. Right sir?
     
  11. Jun 1, 2014 #10

    sophiecentaur

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    I wonder how hard you tried. I googled "transmission line stub" and the very first hit was Wkikpedia and the very first sentence was
     
  12. Jun 1, 2014 #11

    sophiecentaur

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    No. Far too simplistic. The selectivity of such a filter would not be good enough unless the stub were specially made. For a filter that narrow at UHF TV frequencies, a cavity resonator would probably be needed. Filtering of TV channels is achieved at an 'intermediate frequency', using Superhetrodyne techniques (All TVs and radios do it this way)
    A notch (excluding) filter has very different requirements from a receiving filter. A UHF channel filter is often made using an interdigital filter which consists of five or six 'fingers' of transmission line in a single cavity- hard stuff.
    I suspect that you are leaping into this subject (a vast and complicated one) and expecting to have an understanding of something that is way down the line in any course on Radio Frequency Engineering. Take it easy and start at the beginning and you will find it a very worthwhile field of study. But, without the basics, it will be very confusing.
     
  13. Jun 1, 2014 #12

    jim hardy

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    Shayann hasn't told us what is his background. I too assume he's leapt in .

    Mine is ninth edition, 1960. You can find old ones on Ebay. I haven't read the newer editions - perhaps some younger radio buffs will help here. Are they still practical ?


    You're approaching it.
    Your second question first:
    A stub is a quarter wavelength long so that when an incoming wave travels down the stub, reflects, then returns to point of entry will have traveled 1/2 wavelength - a quarter wavelength each way.
    That makes its return delayed by a half cycle which for a sinewave is 180 degrees.
    Observe sin(A+180) = -sin(A) , so they'll nearly cancel. That's how a stub can remove most of an unwanted signal.
    Since a wavelength is the distance traveled by a wave in one cycle time, which for radio waves is speed of light/frequency, you must know the frequency to figure out how long is a quarter wavelength.

    Your first question:
    In my example which is real, i used a stub to reduce the strength of an unwanted TV signal by that cancellation method just described.

    As Sophie points out that is unlikely to work with today's tv for the stations have mostly moved up to UHF bands where stations are quite closely spaced. Their wavelengths are so nearly the same that you need more precise filters. My channel 6 was separated from channel 7 by the commercial FM and some aircraft bands. Channel six occupies the frequencies between 80 to 88 megacycles, channel 7 is around 175. Their wavelengths are quite different so a simple trap worked well.
    http://www.csgnetwork.com/tvfreqtable.html

    Take your vacuum cleaner hose and slap your palm over its end. You'll hear a resounding 'thud' as the pressure pulse reverberates from end to end. Try it again on a hose half as long, or with a sock stuffed partway up the hose - the sound will change. That's tuning a stub.

    A transmission line stub can be as simple as a piece of the physical line cut to desired length. TV twin lead is a good example.
     
  14. Jun 1, 2014 #13
    Hi, having some knowledge in software and computer engineering I know, google doesn't open the same page in anywhere in the world. I didn't find any wikipedia stuff on the top of the same searching sir.

    You can ask from any computer engineer that this is true.
     
  15. Jun 1, 2014 #14

    sophiecentaur

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    Ah well - that's life. You do have a point there.
    However, unless you have a different version of Wikipedia in your part of the World you will find many hits within Wiki with descriptions, pictures and examples of 'stubs' in the context of antennae and electronics. Actually, one doesn't need to find a 'definition' of a term in order to get an idea of what it's all about. Searching is a skill that's well worth developing.
    You guys have it a lot easier than I used to, when all that was available was volumes of Abstracts in the library - followed by a wait until the library could order you a copy of a paper (one week turnaround time minimum).
    There is always the good old text book, of course, and that's always been available. The advantage of a text book is that the order that stuff appears tends to be logical and it takes you through the story in a useful way. The position of a topic in a text book often gives a clue as to how advanced or easy it is.
     
  16. Jun 1, 2014 #15

    sophiecentaur

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    @ jim
    The essential difference between a band stop filter and an band pass filter is that you don't particularly care about the shape of the band stop characteristic but a TV channel filter needs to be flat over the pass band; In many antenna combiner circuits, the isolation between different transmitters can be achieved pretty crudely with stubs.

    I still reach behind me for a 1946 ITT Radio Engineer's Handbook, on occasions. It can be marginally quicker than the computer for finding things like wire gauges. Many things haven't changed in decades.
     
  17. Jun 1, 2014 #16

    jim hardy

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    That's sensible . A pass filter shouldn't distort. My stub probably didn't attenuate uniformly across the whole bandwidth of channel 6. But it took the interference out of channels 2,4, 7 and 10.
    Channel 6 was so strong it'd overload the RF stage of many FM radios so you'd get one station all over the radio dial - the 83.25 mhz video carrier sub-heterodyned a local FM station at 94.

    I envy folks with a lot of communications experience. I've not much.
    Maybe i shoulda tuned my stub for that video carrier rather than mid-band?


    old jim
     
  18. Jun 1, 2014 #17

    Averagesupernova

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    I would agree with that.
     
  19. Jun 2, 2014 #18

    sophiecentaur

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    Absolutely. Even the sound carrier (6Mhz away in the PAL system and the highest power at any other frequency) is at 1/5 vision carrier power. Vision side bands are well down. But a simple stub would not be all that sharp (>>1% bw) so the notch frequency would not be too critical.
     
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