RF oscilator 33MHz to RF detector through pcb coplanar capacitor issue Reflectometry tree Sensor

Discussion in 'Wireless & RF Design' started by vitoa, Jul 28, 2015.

  1. vitoa

    Thread Starter New Member

    Jul 28, 2015
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    Hi, i'm developing some sensor based on RF principles (Reflectometry Sensor)
    First all have a (0,7mm*10mm)water sealed test pcb with two 2mm*10cm cooper strips with 1mm distance between them ( coplanar capacitor ), changing its value with surround moisture (variable coplanar capacitor )

    This pcb dual striped probe is to measure moisture inside tree trunk, so the ideia is to create a RF field sorrounding this plates to measure some volume inside tree trunk.
    [​IMG]

    In order to build a simple,low power oscilator and rf detector circuit I used two IC for this porpouse
    One is a precise 33MHz oscilator outputing 3000mV pp wave between ground and Vout
    To save time on a peak detector circuit with skottkey diode I found some simple RF power detector IC LTC5507.
    It will provide an output voltage proporcional to rf in, according datasheet -18dbm(0.08Vpp) to 14dbm(3.17Vpp) input wil have almost linear Vout from 200mV to 2000mV, perfect for microntroler adc.

    One strip is connected to 33MHz@3Vpp oscilator, the other strip connects to rf detector ic.
    This ideia is to do one strip work as antena emitter and the other strip anntena receptor, the higher the moisture inside tree trunk the higher refletance of signal get back to detector strip, increasing output voltage on rf detetor.

    I need some help to know if this ideia is correct for my porpouses.
    Need also to calculate strips
    width dimensions to have it optimized to 33MHz with a lenght of 10cm to go in tree trunk .
    I will consider worst humidity case same as probe soaked into water, this case reflectance is maximum and best humidity case free air, almost no reflectance so detector should ouput minimum.

    Any help is welcome,
    Best regards,Vito
     
  2. BR-549

    Well-Known Member

    Sep 22, 2013
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    With those dimensions, I can not imagine your sensor.

    Do you have a scaled photograph or drawing?

    What exactly are you measuring? I understand that the parameter is moisture, but the moisture of what? And what for?
     
    Last edited: Jul 28, 2015
  3. vitoa

    Thread Starter New Member

    Jul 28, 2015
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    Hi, this principle is used to measure soil moisture by some comercial Reflectometry soil sensors
    One electrode emmits RF and other electrode receives and mesure signal received
    The more moisture on soil the more reflected signal gets back to receiver electrode.

    I have some draw
    [​IMG]
     
  4. BR-549

    Well-Known Member

    Sep 22, 2013
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    Are you measuring soil?

    What are you measuring?

    Are you trying to measure the moisture of a hole, in a tree?

    What parameter of the tree are you really trying to measure?
     
  5. vitoa

    Thread Starter New Member

    Jul 28, 2015
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    The measuring of the soil is the same principle, so lets see it as a soil moisture sensor.
    water content in soil will reflect waves back to rf detector strip, increasing Vout

    According to my theory the moisture surrounding probe would reflect back signal into rf detector strip.
    For example probe soaked into water will act like a wave guide , the signal emitted is almost all reflected by water to rf detector probe , this scenario will get the maximum out.

    But dont know if 33MHz is enough to get this effect..
     
  6. BR-549

    Well-Known Member

    Sep 22, 2013
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    I'm not to hip on tree study, but I believe you will have problems.

    You will have to find the spectrum of the moisture in the wood fiber that reflects. If you assume they are the same as soil, I believe it's in the microwave range.

    Plus you have the hole interface and damage to the tree to deal with.

    Why not send audio and super audio signals thru it and see what can be determined?

    Even higher if needed. You could do a sweep at each install, to tune the tree, sorta speak.

    One might monitor density changes in real time, without any interference.

    Portable moisture sensors are ok for seeing what area of a field is drying.

    That's because we are comparing grab samples to grab samples.

    If I were going to install a sensor, I would want better than grab sample results.

    Is it density or flow you want to monitor?

    Other fields of research have used this technique for years.

    Surely someone has sounded a tree. Probably years ago.
     
  7. vitoa

    Thread Starter New Member

    Jul 28, 2015
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    Hi, have changed design to non invasive methood

    [​IMG]
     
  8. BR-549

    Well-Known Member

    Sep 22, 2013
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    Ok. You will need to find if anyone has done this. If not, you will have to sweep to see if you can fine a RF sweet spot.

    The only one I heard of was for soil and that was in the microwave range. But they are always coming up with new stuff.

    Then you will need a way to correlate the signal to the moisture content.

    And I have no idea of the power required to ring a tree, or ring water in a tree.

    Have you decided on a sample rate? Once/hour perhaps? Are you just sampling one tree, or an array?

    Are you looking for daily patterns or seasonal?
     
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