arduino antenna analyser project, impedance measurements "how to", need help

Discussion in 'The Projects Forum' started by ninjaman, Sep 11, 2014.

  1. ninjaman

    Thread Starter Member

    May 18, 2013
    306
    1
    hello

    I am doing an HNC in electronics at college and I have a unit called project. the project can be anything and I chose an antenna analyser. using arduino and modules/components because its cheap, simple(ish) and (fairly) easy. (hopefully, though its not looking good)
    I have a pro mini 328 which is a cheap and small arduino, a dds module, an lcd display and some idea about the swr bridge.
    I know that swr is a ratio of forward and reflected power and I should be able to get this working. my idea is the arduino automatically finds the lowest swr and shows the impedance. I was going to use a wheatstone bridge to measure the impedance with the antenna as the unknown leg. the dds would send a signal through the bridge to the antenna, then the reflected signal should show. (writing this im getting a little confused) I think that the bridge wont measure swr so I will have to make an swr circuit prior to the bridge.
    I know I want the swr to measure forward and reflected waves. this is to operate at from 3-30 Mhz by the way.
    so the dds just provides a signal, it should be low power. I haven't read much about the module im using. its the cheap one off ebay. its just called ad9850 signal generator, it goes to 40Mhz.
    I have a sark 100 which uses an ad9851 and is multiplied upto 180Mhz which is something to do with stability. im not sure what stability issues I might have with the ad9850. im not sure what to look for here. there is already an antenna analyzer using arduino but its all in Italian and I wanted to do a something different.
    so, if anyone could point me in the direction of websites/books that explain some of the following:
    • how to measure impedance, I have looked at a few things that confuse the hell out of me
    • measuring swr at hf, I found a few but if there are more informative sites I would like to see them
    • dds module, creating a signal and power levels, stability issues and what to look for, what can affect them

    so its just suggested reading really. the problem im having with some of the information about impedance measuring is that its way above my head. from what i have read and understood(sort of) is that the forward voltage is compared to the reflected voltage and the reflected current. i have read about the voltage and current in an antenna and that as one goes up the other goes down and that changes the impedance. so the impedance changes along the length of the antenna. though measuring this impedance and getting an accurate reading is a little baffling after reading some of the sites. these are sites that provide information about their own products and components.
    thanks
    simon
     
  2. DickCappels

    Moderator

    Aug 21, 2008
    2,647
    632
    You asked a lot, and I think this might be of some hope. The article at the URL below will show you how to measure SWR with analog circuits and a PIC -I plan to use the same scheme with an ATMEGA48 next month, since the math and analog circuitry are the same. If you find the SWR and are sure of the source impedance, then you can calculate the total impedance of the load from that information.

    http://www.pi4zlb.nl/Zelfbouw/Diswamtr/DigiSwrWatt.html
     
  3. ninjaman

    Thread Starter Member

    May 18, 2013
    306
    1
    "The meter described here works from about 28MHz through to 430MHz with 5 or 10 watts required at 28MHz and only a watt or so at 430."
    im not sure what this is saying. I have studied amateur radio but nothing is making sense with this. I have read this sort of thing a few times. power change over frequency.
     
  4. DickCappels

    Moderator

    Aug 21, 2008
    2,647
    632
    The point is to be sure to have enough voltage for the analog-to-digital converter to give high enough resolution measurements for your calculations to have an acceptable error. In out case, we are using a dual opamp set up as a pair of noninverting amplifiers with adjustable gain so that the larger signal will be near the maximum measurable input voltage. You so do the same.
     
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