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Thread: Ham Radio Project - Slotted Line

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  1. #1
    Join Date
    Nov 2009

    Default Ham Radio Project - Slotted Line

    There seems to be quite a bit of cross-over on this board between ham radio and HSM enthusiasts. Here's a 'not-quite-finished' project that has been keeping me busy, and may be of interest to those getting into microwave experimenting...

    It's a slotted line, for measuring impedance (including SWR) from about 400mhz up to about 4ghz.

    In a nutshell, it's like a piece of coaxial cable that has been opened up and flattened so a small antenna (barely visible in the third photo) can travel along down inside the gap between the two slab sides.

    By connecting a level detector (a glorified crystal set for those who built them as kids) to the antenna through the fitting on the top side of the carriage, the rf level on the copper center conductor can be measured at any point along the line by sliding the carriage back and forth.

    It's sensitive enough to measure input impedances on things like amplifier circuits, and robust enough to measure SWR on antennas and feed lines - very handy for all sorts of things.

    Spacing and sizing of the center conductor and side slabs is critical, as is keeping the little antenna centered in the slot.

    To keep the impedance constant between the air insulated part (between the slabs), and the teflon insulated cables/connectors on the ends, it was necessary to fabricate a tapered cone affair (brass cone inside a teflon 'funnel'), which also serves to hold the whole thing straight, while making the electrical connection to the input/output connectors on the ends.

    Its been a fiddly job getting the tapers right, and getting the carriage to slide easily without slop - still a few high spots to polish down, and some 'prettying up' to do. That said, initial testing indicates it's close to a perfect match - once a series of tests are done, its intrinsic swr can be worked out, and a required 'adjustment' value calculated. From that point, very accurate measurements are possible at remarkably high frequencies.

    While the technology has been overtaken in the commercial world by computer controlled vector analyzers and such, Hewlett Packard did well over the years selling a very similar device which is still an economical solution for those without kilo-bucks to spend on exotic test gear. I borrowed heavily from their (50 year old) technical journals to figure out how to build this one.


  2. #2
    Join Date
    May 2002
    SE Texas



    I'm a TV engineer and can appreciate nice test equipment. Please put me in your will.
    Paul A.

    Make it fit.
    You can't win and there is a penalty for trying!

  3. #3
    Join Date
    Mar 2005
    Palmer Alaska


    Yup good old slotted line

    Where is the slide rule for the math???

    Simple tools used properly, can do wonders...

    DE WL7WW
    Last edited by Bguns; 12-29-2009 at 01:56 AM.

  4. #4
    Join Date
    Jun 2009
    Western NC

    Default May be a good post to see how many hams

    It would be interesting to see how many there are on this site. I often use the hobby machine shop to make 33 & 13cm antennas and related parts for LF thru
    VHf but never thought of making test equipment. Chuck/N4aeq

  5. #5
    Join Date
    Dec 2007
    South Wales


    beautifully made, well done you can be proud of your efforts as they are brilliant
    merry Christmas

  6. #6
    Join Date
    Dec 2008
    Waukesha WI USA (20 miles W of Milwaukee)

    Default Measures frequency also

    Quote Originally Posted by YukonHam
    It's a slotted line, for measuring impedance (including SWR) from about 400mhz up to about 4ghz.
    Looks nice. I used to use a commercially manufactured HP one to measure frequencies around 10 GHz.

    Once you know the velocity of propagation of the slotted line, you can intentionally misterminate it, directly measure the wavelength between SWR peaks, apply correction for Vp, and convert that wavelength to frequency.

    You can figure out the Vp, by hooking up a UHF HT on low power on 446.000, measuring it, and comparing the measured result with the calculated wavelength.

    73 de N9NFB

  7. #7
    Join Date
    Aug 2009
    Southern Indiana, USA


    Awesome! The best part of both worlds (ham radio and machining) is building stuff!

    73 de WB9DL

  8. #8
    Join Date
    Nov 2005


    That's very cool!

    I was a ham long ago in a galaxy far away while I was growing up. Microwaves like that were almost inaccessible to hams. We were just starting to get things going like 2 meter repeaters.


    Try G-Wizard Machinist's Calculator for free:

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