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Thread: SWR meter - Do it yourself project

  1. #1
    Engineer for Jesus Christ IBCrazy's Avatar
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    SWR meter - Do it yourself project

    That's right, a DIY SWR meter and it's good for 5.8GHz too! The bad part is you MUST ME A MASTER AT SOLDERING (did I say that loud enough?) in order to build it. It only requires a few parts and is quite simple to work.

    Parts needed:

    1 - SMA directional coupler (you must use one that encompasses the frequency you intend to test)
    1 - Microwave detector diode (standard diodes will not work) - I used an Agligent HSMS-286
    1 - Resistor: Value should be about 1K or so. not that important.
    1 - Capacitor: ~.01 uF works. Anything from .1 to .001uF works, actually.
    1 - A small coaxial cable with SMA end (I used RG316)


    How to build it:

    Basically the entire thing is made at the end of a coaxial cable. It is nothing more than a diode, capacitor, and loading resistor. All you are doing is rectifying the output of the directional coupler.

    1. Strip the coaxial cable. Pull the shield back and twist it up. Trim the insulation off of the center wire as close to the shield as possible. Tin both the shield and the center element with solder. Trim the center element so that the distance from the tip of the element to the shield is less than 1/4" (6mm).

    2. Set you multimeter to diode test. Figure out which end of the microwave diode is which. Put a small dab of soldering flux on the pins of the diode. If it is a multiple diode chip, I suggest trimming off the extra pins. Place the diode on something sticky so it doesn't move during soldering. Heat up the solder on the coaxial cable and touch it to the positive pin on the diode. Do not heat the diode directly. 600-700 degrees is a good temperature. After solder has flowed, remove heat.

    3. Trim your resistor leads very short. Solder the resistor to the shield of the coaxial cable. Now bend the other tab of the resistor to touch the negative side of the diode. Heat this joint and add solder.

    4. Now add your capacitor to the circuit. Be careful not to desolder the connection to the diode. The capacitor should be in parallel with the resistor.

    5. Add your leads. You are measuring the voltage drop across the resistor. The capacitor will stabilize this voltage so it it readable. Lead length is irrelevant since this is now DC.

    6. Once done, cover the meter in some non conductvie glue so it does not pull apart. You can stick it to a small circuit board or piece of wood if you like.


    Calculating SWR:

    First you need to calculate p. p is your reflection coefficient. Vr is reverse voltage, Vf is forward voltage.

    p = Vr/Vf

    Ideally p = 0. Thus perfect SWR is acheived when there is no reverse power.

    SWR = 1+p/1-p

    Below is a video on how to read this meter:





    From the numbers you get, you can calculate your SWR.

    -Alex
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    Last edited by IBCrazy; 14th December 2011 at 10:43 PM.
    If it is broken, fix it. if it isn't broken, I'll soon fix that.

    videoaerialsystems.com - Performance video piloting

  2. #2
    Engineer for Jesus Christ IBCrazy's Avatar
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    Building your own directional coupler

    This one is not easy. It might take me a few days to get to this.

    -Alex
    If it is broken, fix it. if it isn't broken, I'll soon fix that.

    videoaerialsystems.com - Performance video piloting

  3. #3
    Engineer for Jesus Christ IBCrazy's Avatar
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    Wheatstone bridge

    You need to be a true soldering master for this one, but it is far cheaper than buying a directional coupler. Still need to write about this one. be patient.

    -Alex
    If it is broken, fix it. if it isn't broken, I'll soon fix that.

    videoaerialsystems.com - Performance video piloting

  4. #4
    Navigator Enthlapy's Avatar
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    SWEET, been waiting for this! Will his work for a Helical, i hear hey are bit trickier.
    Last edited by Enthlapy; 12th December 2011 at 09:09 PM.
    youtube.com/nue3nthlapy vimeo.com/enthlapy

  5. #5
    Engineer for Jesus Christ IBCrazy's Avatar
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    Yes it will work for a helical. It will work for any antenna. All you need is a transmitter. Like I said, you need to be a master at soldering. Look at how small the diode (small black rectangle) is compared to the head of a pen. And it has 6 pins!!!

    -Alex
    If it is broken, fix it. if it isn't broken, I'll soon fix that.

    videoaerialsystems.com - Performance video piloting

  6. #6
    KK4IRW MASHTON1138's Avatar
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    I know a guy who could use one of these
    Quote Originally Posted by whakahere View Post
    I keep thinking I should give up crashing too ... I guess it is an addiction.
    Quote Originally Posted by CaliDave
    Who sliced open that tauntaun?
    Me----"This is a results may vary type of hobby, get used to it."

    Video Aerial Systems Operations Manager

  7. #7
    Building my Plankton... BloomingtonFPV's Avatar
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    couplers

    will either of these work?
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    Loving my plankton.

  8. #8
    Engineer for Jesus Christ IBCrazy's Avatar
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    ^ yes but only for 2.4GHz. There are a few out there that are 2-6Ghz. I got lucky and found a good old unit that hits all the way up to 8GHz. I love Ebay

    -Alex
    If it is broken, fix it. if it isn't broken, I'll soon fix that.

    videoaerialsystems.com - Performance video piloting

  9. #9
    could quit if I wanted to volto's Avatar
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    You, sir are a genius. Everyone serious about FPV will be building one of these, including me. Thanks for your hard work!
    Don't steal, the government hates competition.

  10. #10
    KB3WHA kenkos68's Avatar
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    Awesome! Can't wait for the details. Now I can go test the plethora of designs I made up in 4NEC2.

    -Ken

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