impedance measurement range of a wheat stone bridge

Discussion in 'The Projects Forum' started by ninjaman, Oct 5, 2014.

  1. ninjaman

    Thread Starter Member

    May 18, 2013

    I want to build an antenna analyser using a wheat stone bridge and arduino
    I have read about analysers measuring from zero up to several hundred ohms.
    how does this work?
    I have read that the frequency range of a wheat stone bridge can be up to gigahertz. but it doesn't mention the impedance range. does this have to do with the ADC in the arduino.
    i read about resolution and that for an 8 bit ADC with 255 values, say 10v. the 10v would be divided into 255 and give 0.0392v for each value. so if im right 00000011 (3) would be a voltage of 3 x 0.0392v = 0.1176v?
    does the resolution dictate the impedance range. the arduino has 10 bit ADC with 1023 values and the DDS module has 2vpk-pk. i don't know if i have to use 2vpk-pk or 1vpk when finding the resolution.
    i did this:
    10 bit ADC = 1024 values
    2vpk-pk / 1024 = 1.9mv
    1.9mv per value
    does it have something to do with the reactive components and formulas:
    1/2piFC and 2piFL

    i don't know enough about this to know what to search for.
    any help would be great, a point at some reading material would do it!




    this year i intend to catch santa in the process, i got a tranq-gun and electrified net!
    last year i made a mistake and zapped my brother with a 50,000 volt cattle prod, thought he was santa. and the year before that i shot a bazooka at the neighbours cat (it was throwing a shadow on the wall, i thought it was a reindeer!)
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
    the impedance range wll be determined by the resistance values in the bridge.
  3. ninjaman

    Thread Starter Member

    May 18, 2013
    sorry I meant something else, im trying to understand something I read. this is what I read
    • Usable Measurement Range:
    • SWR: 1.0 to 9.99
    • Impedance: approx. 5 to 2000 ohms

    • it is part of the spec of an antenna analyzer.

    • I didn't understand the part about impedance, the "Impedance: approx. 5 to 2000 ohms" bit

    • though I have thought about it a little and think that it is to do with the SWR. an SWR of 3:1 means 150 ohms. 6:1 is 300 ohms

    • I divided 9.99 by 50 and got 0.1998 which is close to 2000. I think this is the approx. 2000 ohms stated. this is the only way I can think of what the impedance range means. so any antenna with an impedance in that range can be measured by this device.

    • though im stuck now as to what limits the measurements to 2000 ohms. I think maybe the signal voltage, but I think that the signal voltage just supplies a sample that you find the ratio from. im confused. I have read a lot of stuff to do with SWR and impedance but I cant find the answer to this. I think it is a hardware thing to do with range of measurements, like a limitation of components or something.

    • any help would be great!!
  4. DickCappels


    Aug 21, 2008
    There is nothing fundamental that limits the measurement to 2,000 ohms, except that as a practical matter, it should be noted that the impedance of free space is 377 ohms, so its difficult for me to imagine a higher impedance transmission line unless unusual (or impossible) techniques are used.

    You asked enough to fill an entire short course on transmission lines and antennas.

    You might want to start by reading up on impedance.

    After that, understanding reflection coefficients should give you some answers
    The key formula from the reflection article is [​IMG], you can see that the reflection coefficient is a little more complicated than simply the ratio of impedances, but not much more complicated.