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Stripline SWR, how ?

Discussion in 'Electronic Design' started by tk5ep, May 18, 2006.

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  1. tk5ep

    tk5ep Guest

    Hi,

    I want to make a stripline SWR meter on a PCB that could handle 300W @
    144 MHz. This SWR will be followed by a lowpass filter also made on the
    PCB.

    No need for great accurary, it's intended to detect a high SWR and
    shutdown a supply.

    Has anyone an idea on howto ?

    Thanks,
     

  2. Well that shouldn't be a too big problem with
    the help of a simulation package. You
    need a directional coupler with some directivity
    over a certain bandwidth. Until it does what
    you want, the effort and pcb cost... you possibly
    rather buy one.
    Have a look at the directional couplers at
    minicircuits.

    Rene
     
  3. Steve Burke

    Steve Burke Guest

    My advice is to see if anyone has already done it commercially. If the
    vendors of directional couplers and packaged filters aren't using stripline
    at these freqs and power levels, ask yourself why before investing lots of
    time in it.

    At 300W, even tiny losses will generate lots of heat. An (exceptional)
    directional coupler with only 0.1 dB loss will dissipate 7 watts.

    Stripline at 144 MHz is going to be pretty large. Do you know how much heat
    is present in the frequencies you have to filter out? With a 300W
    fundamental, its possibly many watts heating your filter components.

    Steve
     
  4. tk5ep

    tk5ep Guest

    Thanks,

    You maybe right... It's perhaps easy to build an outboard directional
    coupler and not use the PCB technic... I have a transverter with a PCB
    stripline SWR meter but i have only 25W runing in it...
    What i need are the dimensions of the traces with FR4 PCB...

    Thanks anyway,

    Patrick
     
  5. Guest

    Try this:

    http://www.hp.woodshot.com/

    Steve Roberts
     
  6. Joerg

    Joerg Guest

    Here is some math background to design a stripline directional coupler:
    http://atlc.sourceforge.net/determine_dimensions_of_couplers.html

    IIRC this commercial SWR meter uses stripline coupling:
    http://www.mfjenterprises.com/man/pdf/MFJ-864.pdf

    As Steve has said, make sure you don't cook your board material.

    Regards, Joerg
     
  7. Joerg

    Joerg Guest

    Hello Rene,

    But not at that power range. If they offered that I could have funneled
    quite some sales in their direction.

    Regards, Joerg
     
  8. Andy

    Andy Guest

    Andy writes:

    My suggestion would be to get a copy of the following:

    The ARRL handbook
    The UHF experimenters handbook
    The ARRL antenna handbook

    One of these will have directions for an SWR meter at
    your freq and power range.

    Another suggestion I have is to forget about stripline and
    use microstrip using G-10 glass epoxy. 50 ohms is about
    ..1 inch in width. Real easy to do with an exacto knife.

    Trying to do it using coax might work, but is a lot more
    trouble than microstrip..... stripline isn't that much harder,
    but microstrip will work just fine, and you can see stuff......

    Good luck. It's been done before by people with less
    brains and you should have no trouble if you just look around
    first...

    Andy W4OAH in Eureka, Texas
     
  9. tk5ep

    tk5ep Guest

    Hi All,

    Perhaps did i use the wrong term stripline instead of microstrip ...

    Google gave me more results using microstrip and i've found some infos.
    Btw, i have a 300W amplifier which seems to use ordinary FR4 PCB, so
    i'm quite confident that i will not cook my board !
    The only problem is to find the right dimension to have a 50 Ohm line
    and the right coupling.
    Thanks for the answers.

    Greetings,
     
  10. Upon thinking a bit longer about the subject,
    I think the stripline coupler should have dimensions
    longer than the wavelength. Even with an epsilon of
    4, it still will be large.

    But I remember an application note at minicircuits
    about doing yourself coupler.

    Rene
     
  11. I didn't really have a look at their bigger parts.

    Rene
     
  12. Wes Stewart

    Wes Stewart Guest

    The "right coupling" is not particularly important. There is no right
    coupling, you are measuring ratios, not absolutes. Good directivity
    is more important.
     
  13. tk5ep

    tk5ep Guest

  14. vk5ajl

    vk5ajl

    8
    0
    Feb 1, 2010
    Have a look at http://vk5ajl.com/projects/swrmeter.php

    For 1.6 mm thick PCB meterial (standard) centre micro-stripline needs to be 6mm wide to maintain 50 ohm impedance through the meter. At 144MHz it needs to be only 50mm long to get plenty of signal at 1/2 watt. (SWR is independant of power). If you leave it in circuit at 300w, you will have to attenuate the signal heaps. At 1.5 mm spacing between centre conductor and pickup strips it will handle 2 kilovolts, plenty enough for your 300w.
     
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