Rf power measurement of motorola gp328

Discussion in 'Wireless & RF Design' started by sumitdby, Apr 9, 2013.

  1. sumitdby

    Thread Starter New Member

    Aug 1, 2012
    donno whether i m posting on write forum or not but here it is.
    what i am trying to do is i m measuring Transmitting power of RF signal from the device MOTOROLA GP328

    well i have no experience in working with rf signals and there power measurement i need to calculate the same using atmega8
    which i m planing to read the voltage on adc pins and then calculate the power

    i m following the links that i got first while browsing how to calculate RF signals power



    help plzz need to do this asap.....
  2. paulktreg

    Distinguished Member

    Jun 2, 2008
    Are the figures given in the specification not accurate enough?

    Power output 5W - 136 - 174 MHz, 4W - 403 - 470 MHz, 450 - 527 MHz

    It's not easy measuring RF power even for somebody with intermediate electronics know how.
  3. YokoTsuno


    Jan 1, 2013
    This is indeed not that easy.

    It requires an RF power meter like the HP/Agilent 432 or 436, R&S NRP series with thermal or diode sensor or equivalent instrument, or eventually a spectrum analyzer (Channel Power or zero span measurement). There are also thermal or diode sensors with USB interface which can be connected to a PC directly. A calibrated sensor from a reputable brand costs somewhere in between $3,000 and $10,000.

    You also need a proper adaptor to connect the antenna output to a standard N/SMA type connector, these are often propriety connectors.

    An additional problem might be the impedance. These sensors are typically 50 Ohms and the measured power is therefore in a 50 Ohms load. The radiation impedance of the antenna is generally different which means that the power going to the antenna does not necessarily correspond with the measurement. This is not necessarily a problem though if you only want to measure transmitter performance.

    To build and calibrate your own diode detector, like the one in your link, at the given frequencies (400-600 MHz) requires a considerable amount of RF experience
  4. TecknoTone

    New Member

    May 20, 2012
    The RF adaptor for these is interesting. It looks a bit like a 3.5mm jack plug, but what it does when inserted into the antenna mount is to turn the polarity inside out so that what is in normal use the hot feed, becomes ground and the tip becomes hot.
    Sumitdby (no name given), do you actually need to measure the RF output of your radio, or are you just looking for a project to carry out with your Atmega8?
  5. vk6zgo

    Active Member

    Jul 21, 2012
    I'm not sure what frequency this radio operates on,whether VHF or UHF.

    Edit:--I missed this posting

    "Are the figures given in the specification not accurate enough?

    Power output 5W - 136 - 174 MHz, 4W - 403 - 470 MHz, 450 - 527 MHz"

    The Oscilloscope display method in the link won't work at these frequencies,unless you have a very good 'scope.

    The normal way to measure output power with transmitters is to feed the output into a Test load (commonly called a "dummy" load).
    Most Test loads are for 50 Ohm systems.

    Test loads may have a low power probe incorporated (maybe as simple as a resistive tap),at which point,you can have a look at a lower level version of the input signal,using a Spectrum Analyser,voltmeter with a detector,etc.

    Other units have a built in meter scale scaled in watts,so you can read power directly.
    In all cases,these things really read volts,which may be converted by the device,or by you.

    Other Test loads have no monitoring,so you need to use an inline meter,like a Bird 43,an old CB SWR meter or similar.

    Some of the "CB" type devices even though designed for 27MHz,are quite good as high as 146MHz.(no good at 400MHz,though)

    Designs for "Homebrew" RF power meters are common in Amateur Radio publications such as the ARRL Handbook,RSGB Manual,etc.
    Last edited: Apr 21, 2013