Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Compact broadband directional coupler / circulator / SWR meter / designs & equations?

  1. Jul 1, 2007 #1


    User Avatar

    Hi all,

    Has anyone got any good tips on broadband (HF->UHF)
    somewhat physically compact directional coupler
    SWR / directional watt-meter designs?

    I'm also looking for equations or quantitative
    design parameters relating to their design and operation.

    Two use cases are of interest:

    1) Specifically for weakly coupled power / SWR
    probe operation, I'd like somewhere very generally
    in the region of 1% to 0.001% coupled power.
    The probe coupler should not to impose much load or
    power limitation on the primary line which may be
    operating with 0.001W to 1000W kinds of powers, though
    0.1 to 10W would be most commonly of use / interest.

    I'd like to cover all of HF, VHF, and UHF with one
    simple design *if* that's practical, though having one for
    each band wouldn't be a bad compromise if it greatly
    simplifies or improves the design.

    2) Low insertion loss, highly directive circulator / isolator
    functions at modestly high power levels (e.g. 0 to 1kW),
    and good operational characteristics over a single
    specific narrow band of operation. The cases of HF,
    VHF, and UHF are most relevant since other techniques
    are applicable above or below those ranges.

    For (1), the weakly coupled power probe, I've seen
    designs using just a ~1/100th wavelength probe
    alongside a transmission line. I haven't references
    for the coupling ratios achieved with such a small probe
    line length, but I'd guess it's in the 0.1% range in
    order for it to be useful for its purpose. I question the
    directivity these are achieving, and would welcome some
    quantitative information on the subject.
    I wonder how much smaller than 1/100th of a wave long
    one could make the coupling probe line and still get
    good directivity and achieve around 0.1% coupling ratio?
    Has anyone any references relating to design types or
    design performance parameters?
    I understand that 1/4 wavelength is the optimum length
    for a secondary directionally coupled line, though in
    that case I'd expect there'd be a very high percentage
    of directionally coupled power, well in excess of what
    I'd need for a measurement.

    For (1), the weakly coupled power probe, I've seen
    designs using ferrite coupled transformers for the HF
    frqeuencies that are surely much smaller than
    1/100th wavelength in physical size,
    though I am not aware of specific details of their performances or maximum achievable bandwidths.
    I'm inclined to be skeptical as to whether such
    a ferrite transformer design could work well for
    HF + VHF + UHF, or which could easily work with
    higher power levels at any frequency.
  2. jcsd
  3. Jul 2, 2007 #2


    User Avatar

    Looks like I found a few related references to the
    questions I had:

    http://www.arrl.org/tis/info/pdf/5904024.pdf [Broken]

    It seems like most broadband designs fall off
    either on the lower end of HF and/or the upper end
    of UHF, but they seem to be able to function decently
    well in simple / compact forms given how broadband
    and tolerant of power levels they actually can be.
    Last edited by a moderator: May 2, 2017
  4. Jul 2, 2007 #3


    User Avatar

    Staff: Mentor

    I did a search at the ARRL.org website, and got some good hits, but the better ones are in the "members only" area -- sorry about that. Have you looked in the ARRL Handbook? It's got some reasonable circuits for directional couplers and SWR measurements. Your local technical library should have copies of the ARRL Handbook.

  5. Jul 2, 2007 #4


    User Avatar

    Thanks for the help.

    I've got an old copy of the book which is
    (surprisingly) mostly useless on the topic,
    but I'm glad to hear that the new ones are better.

    I'll look through a newer one and see what they have to

    The links I turned up show that most of the topologies
    in common use since ~ 1959 are either the
    coupled line probe ones or the ferrite transformer
    ones I was somewhat familiar with, though they've
    done some impressive jobs at making the ferrite
    transformer ones more broadband in a couple of cases.

    Given the broadband 'black box' module units I've seen
    I thought perhaps I was missing some key topology that
    might be totally different and better than the others.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook