Electrical components needed, can't find!

Discussion in 'Technical' started by dragonfly, Feb 5, 2010.


  1. dragonfly

    dragonfly Monkey+++

    I am building replica's of the 70's era C.B. type "snoop loops" which are also known as DF or RF directional finders....
    Problem: I cannot locate the "vintage" or so they are called today, electrical components...
    They are:
    "Impedance Matching Capacitors", or also known as:"Tuning capacitors"
    They "were" generally speaking, a small flat square or rectangular block, made of white ceramic material, that had a flat head screw, on top for adjustment. They were about 1 inch or a bit smaller in size. Some also called these tunable resistors, but I cannot find any reference to that name today!
    The only requirement is that they be of 50 ohm (resistance?)
    That's where I get lost!
    The rest was easy, after numerous hours of research and reading articles on google.
    Here's the basics, IF anyone should want to try to build any of these:

    A DF loop is a fairly simple and basic device. A piece of 3/8" aluminum/copper tubing, bent into an approximately 10-12" loop, with an impedance matching capacitor mounted at the top, and a shunt fed coax connection near the bottom of the loop. This assembly is then connected to a mast which can be rotated 360 degrees. (12-16" x 3/4" wooden dowel handle) Determining direction was accomplished by turning the loop until the lowest signal (called a null) is obtained, which occurs when the loop is broadside to the direction of the signal.

    Using an aluminum fish net: about 10-12 inches in outside diameter....
    cut a section out of the "top" of the loop to mount the tuning capacitor.
    attach the coaxial shunt feed, using a hose clamp for the center contact of the coax cable, to one of the side rails.
    Using a low power walkie-talkie, tune the feed point and tuning capacitor for best VSWR.

    I don't know if these can still be made, but there is a lot of info on the types for VHF, and 2 meter "Fox and Hound" games.....Even a diagram for building one into an aircraft!
     
  2. kckndrgn

    kckndrgn Moderator Moderator Founding Member

  3. kckndrgn

    kckndrgn Moderator Moderator Founding Member

    Is this what you're looking for?
    Variable capacitor - Wikipedia, the free encyclopedia@@AMEPARAM@@/wiki/File:Variable_Capacitor.jpg" class="image"><img alt="" src="http://upload.wikimedia.org/wikipedia/commons/thumb/6/6c/Variable_Capacitor.jpg/220px-Variable_Capacitor.jpg"@@AMEPARAM@@commons/thumb/6/6c/Variable_Capacitor.jpg/220px-Variable_Capacitor.jpg

    I've re-read your original post, and I'm confused. You say you are looking for a capacitor, but mention "50 ohm". Capacitor are usually measured in pF, uF, etc (pico Farads, micro Farads, etc.) Resistance is in ohms.

    Adding this link just for S&G cause it looked neato.

    http://books.google.com/books?id=9d...resnum=7&ved=0CBcQ6AEwBg#v=onepage&q=&f=false
     
  4. ghrit

    ghrit Ambulatory anachronism Administrator Founding Member

    Dragonfly is looking for a micro sized variable capacitor. K-drag's link shows a much larger widget than he needs; since there is almost zero power in the RDF application, there's no need to dissipate heat. The ohms mention is for impedance, not the capacitor rating. Could do it with a variable inductance almost as easily. The link shows the device df is looking for, but the one he wants is micro, since there is almost zero power to dissipate.

    Day-um, I wish I could remember the stuff that rolled off my mind 40 years ago. (And there hadn't been the basement flood that took out half my library.)

    Df, have you tried to find a Radio Shack with an OLD counter guy?
     
  5. Brokor

    Brokor Live Free or Cry Moderator Site Supporter+++ Founding Member

  6. dragonfly

    dragonfly Monkey+++

  7. SLugomist

    SLugomist Monkey++

    KC DRAGON, Your last link has surround sound dated 1941 and you plug in the speakers anywhere. WOW we haven't come a long way.

    The directional antenna is great.

    And the EMERGENCY NUT, very neato

    Cool post.
     
  8. dragonfly

    dragonfly Monkey+++

    Looked all over the valley, and the Radio Shacks have people here that are in their 20's! Not one had any idea of what I was talking about...A couple years back I was looking for some silver PL-259's and some SO-239's....everyone thought I had escaped from a lunatic asylum! Can't ask for RG-8 coax either!
    Used to be...(digressing here) you could go to a radio store, or parts store and buy anything....Like the Heathkit stores.....Used to be a lot of CB outlets too...all old hat now with cell phones and such. I can see why they went out of business....I went to several Radio Shacks here, looking for some LASCR's, (light activated silicon controlled rectifiers), but no one even knew what they were...when I explained what they were and what they did, they still had no idea! I'm gettin' too old for this stuff!
     
  9. ghrit

    ghrit Ambulatory anachronism Administrator Founding Member

    hehehe. Tried to find a nixie tube lately? State of the art in the 60s.
     
  10. dragonfly

    dragonfly Monkey+++

    It ain't easy being old...!
    People ask me if we had rocks when I was a kid!
    Or worse, if we had to fight dinosuars to go to school!
    Little brats!
    SNORT!
    I have all the compnents, and working on the prototype, but I have run into a SMALL problem.....The "shunt" is near the bottom of the loop, using the center (solid wire) of the coax...soldered in place...
    So far so good....
    Now where on earth did they attach that braid?
    On the trim cap?
    I have one soldered into place on the loop, but what connects to the opposing leads?
    I was wondering IF the braid is the opposing connection, so that the trim cap actually works with the loop being centered...?
    It's beyond my knowhow at this point.
    One good thing is: If I goof up, no harm!
    Since I am receiving only, not TX'ing, no harm to the finals!
    Worse case scenario, it won't be adjustable or work as it's designed to!
    Then it's re-wire time!
     
  11. vegasrandall

    vegasrandall Monkey+++

    try the ARRL site,ham stuff
     
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