a "funny" ATU  tuner
its a "FRI-MATCH!"

my implementation by  RALPH KLIMEK VK3ZZC OCT-2013

Antenna matching units  all require a variable inductor, either a switched tapped inductor or the nearly impossible to obtain roller inductor.  For the best match, the switched inductor never has the taps in exactly the right spot. Maybe we need to think about an "intepolation" arrangement with a secondary switched inductor of finer resolution.  But I digress. A search of the interwebs reveals some interesting and off beat designs for continuously variable ATUs.  Here is one I found that does not require a roller inductor or even a switched inductor.  It uses a pair of old style egg-slicer broadcast tuning gangs and it appears to create the variable inductance by shunting out turns of the fixed inductor.  It appears to work.  This implementation of mine matches  random bits of wire and other assorted metallic fixtures and turns it in to 50+j0 ohms  as measured by my little return loss bridge.  It has a usefull operational frequency range of 5 to 28 Mhz.  It is not my design, I will look at my notes so I can attribute  ( ie  pin-the-blame ) it to the ACTUAL  designer !  I do not actually understand how this unit works.  It is reproducible.  The coil is wound from thin gauge copper "thermostat pipe"  with approx od of 1/8 inch and coil inner diameter of about 12mm.
You can find this material from discarded gas ovens

the coil is designed to have taps  at n/5  , 2n/5,  3n/5  and 5n/5  which limits somewhat the options with the coil. I have chosen 15 turns and this inductance permits good range up to the 12 meter 25.890Mhz band  and we run out of tuning at about 28Mhz.  The coil is mounted on plastic insulating columns

As it is very difficult to make good RF ground bonds to aluminium, all RF grounds run through heavy strapping to a copper flashing to which the external connectors are mounted.  The heavy braided strapping comes from coaxial cable outers.

The old broadcast gangs are now available only from the flea markets at hamfests.  Many older hams will have spare ones in their junk boxes. Just ask.   The gangs were originally assembled by swaging  and will perform erratically at higher frequencies.  I have soldered all fixed metal junctions. This ensures the absence of strange intermittant behaviour.  If  your gangs are heavily contaminated by gunge and dust (remember that the last ones were made in the early sixties !)  boil them in dilute dishwashing green detergent.  An acid flux can be used to assist soldering the frames. Reboil in dilute borax solution to remove all acid traces and to passivate the metal.  The gangs you see here  were absolutely disgusting and completely covered in aged dust and 60 years worth of grime.  My cleanup results was very pleasing.  Borax solution is remarkably effective in cleaning up acid flux residues which cannot be removed with simple rinsing.  You might also try very dilute  2% hydrochloric acid  to brighten the metal after the degreasing. If you do, you MUST passivate the metal with borax after a thorough rinse and soldering.  This project's purpose was just as much an experiment to see how well I could clean up old broadcast gangs.

The original inspiration for this ATU design  came from this extremely interesting webpage

conductors are made from silver plated teflon coax outers off-cuts for the last word in low loss.
see also the insulating shaft coupler
bnc and so-259 mounted to copper flashing for a positive galvanic RF grade groundthe coil
the rotors of my tuning gangs are positively grounded to frame by rotary straps. the insulator block adapts the 3/8 inch rotor shaft to 1/4 inch for front panel knob.gang mounted on perspex plateoriginal brass wiper was just riveted to frame
connect rotors to the capacitor frame for trouble free tuning. The brass block is used to contain  and terminate the shaft grounding strap. I do not rely on the old wiper arrangement. You will have erratic tuning if you do. positve grounding ensures
trouble free operation
I can never find a BNC-PL259 adapter when I need one, so I provision  BOTH  ! Always use silver plated connectors for RF if available.
(it does make a differance )


mod-record.jpgWed Nov 26 14:25:11 EST 2014,Fri Dec  5 18:17:15 EST 2014