What would happen if a coaxial cable is terminated with a 50 ohm resistor?

Discussion in 'General Electronics Chat' started by c1rcu1ts, Mar 30, 2016.

  1. c1rcu1ts

    Thread Starter Member

    Oct 19, 2013
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    What would be the consequences of this and why?

    Thanks
     
  2. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Depends on many things, like the characteristic impedance of the cable, the output impedance of the signal source, the length of the cable, etc. You question needs much more detail.

    Is this a homework question?

    ak
     
  3. rudha13

    New Member

    Sep 6, 2015
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    hello,
    firstly, I suggest you to elaborate more on the question. Is the cable open on both the ends? If yes, then it follows the characteristics of an open circuit. Maybe then I can try to explain my understanding...
     
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    If its for a radio, ok, if its for a TV no!
     
  5. rudha13

    New Member

    Sep 6, 2015
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    Well, whatever be the device for which the co-axial cable is used, if its an open circuit (when nothing is connected at the end of the cable), then reflection of the data is bound to occur. But, they will happen in phases.So, in the positive half cycle of the data waveform, one wave tends to overlap with the negative half cycle part of the data wave thus resulting in cancellation and I believe that this is termed the "round-trip time" as far as I have studied. I remember these from a few articles online and a couple of books and this is what I have understood. Please do correct me if I am wrong. However, I am pretty confident that this is the closest that I could get to the question asked.
     
  6. c1rcu1ts

    Thread Starter Member

    Oct 19, 2013
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    Thanks for the reply and apologies for being vague. One end of the cable is connected to a miniVNA, while the other is open ended. In order to calibrate it, I used a 50 ohm resistor.

    No, this is not a home work question
     
    Last edited: Mar 30, 2016
  7. crutschow

    Expert

    Mar 14, 2008
    12,988
    3,226
    For video and RF frequencies you want to terminate the cable in its characteristic impedance to minimize any reflections of the signal.
    Typical coax characteristic impedances are 50Ω and 75Ω.
    The cable type number is needed to know its impedance (or you can experimentally determine it with a fast pulse generator and a fast oscilloscope).
     
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  8. rudha13

    New Member

    Sep 6, 2015
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    I would also share this piece of information here. The mini VNA antennae analyser has the ability to measure impedence in the range of 1 ohm to 1000 ohms.
     
  9. c1rcu1ts

    Thread Starter Member

    Oct 19, 2013
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    What would happen if higher impedances are attempted to be measured using the miniVNA?
     
  10. ian field

    Distinguished Member

    Oct 27, 2012
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    Some BNC dummy loads/terminations are exactly that.

    It has to be a non inductive resistor like carbon composition, and there's no good way to hang a wire ended component on the end of coax without some hint of mis match.

    The top quality ones are an annular ring of carbon pressed into an end cap, the hole in the middle accommodates the centre pin.
     
  11. Papabravo

    Expert

    Feb 24, 2006
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    If you terminate a 50 Ω cable with a 50 Ω resistor, the minVNA will measure 50 Ω across a wide range of frequencies. Does yours top out at 180 MHz., or do you have the extender that goes up to 500 MHz.? The reason I ask is that at 500 MHz. the inductance of the leads on the 50 Ω resistor might show a significant effect.
     
  12. GopherT

    AAC Fanatic!

    Nov 23, 2012
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