I guess my 35 years at the hobby are showing... Perhaps I am an old
curmudgeon, but, I simply do not understand the spending of hard earned,
dollars on a commercial atu , that usually doesn't do what they claim, when
a heavy atu is so simple to make... An L-Match will match any reasonable
antenna (and most that aren't reasonable), not arc over (assuming a good
cap), handle double the power of the low end "1500 watt" units, and cost a
fraction of the commercial unit... 30 feet of #10 solid wire (wound over a
pvc pipe to get it round) - two 1/4" x 4" x 5" plexiglass plates for a coil
form - a $90 capacitor (or less, from a swap) - one alligator clip - a
shiny, new cabinet - an inexpensive swr bridge - and two SO239's...
"Circuit details playing now in an antenna handbook near you!"
Hints on the coil...
make your plexiglass X form, 4 inches in diameter by 5 inches long... (I
like the larger diameter coil, even though it takes a bit more wire... form
the coil by close winding the wire on a pvc pipe that is 1 inch smaller in
diameter than your plexiglass form.... use a hack saw or file to cut notches
in the edges of the plexiglass X form that approximate a spiral thread with
spacing equal to one wire diameter...
(you can put masking tape, or double sided tape, lengthwise along the edges
of the form, measure and mark carefully, take your time and be proud of the
results... better yet, you can wind two wires along the form in parallel
(ala. balun style), then unwrap one wire and mark the tape between the turns
of the remaining wire... works great)
to measure your final wire, use scrap wire to wind over the form ( or one of
your double windings ) then unwind and measure for length allowing 6 inches
extra on each end... cut your working wire to length, don't kink it while
getting it off the roll... clamp one end in a heavy vice, or clamp to the
bumper of your truck, use vice grips on the other end, straighten and pull
until you feel the wire 'just stretch' slightly... keeping tension on the
freshly straightened wire, close wind onto the pvc pipe, keeping the windings
tight... then release the ends and slip off the pvc, yes it will 'unwind'
slightly... carefully "thread" the coil over the X form (be sure you wound
the coil the same 'hand' that you used for the spiral notches!!)...
If you couldn't find bare wire, then use enameled... get some fine grit,
polishing cloth, usually found in hardware stores in 25 foot rolls, about an
inch wide....cut a foot long strip to 1/8" wide and use that to remove a
narrow band of the enamel on each turn, where the alligator clip will go...
Now, isn't that pretty?....
Hints on the capacitor:
depending on the style, you may need to put it up on plexiglass to insulate
the stator... strap the rotor to ground, and you won't need to spend money on
insulated shaft, couplers, etc. , and the knob won't 'bite' you... get the
widest plate spacing you can afford if you intend to run power into strange
loads... maximum capacity of the variable depends on the bands you run... see
the antenna handbook... 250 mmF for 80-10, and double that for 160, would be
a good start... parallel caps can be switched in and out, as needed.. a
little ingenuity goes a long ways... for more bucks a 5000 volt vacuum cap
will make a "kilowatt killer" tuner...
Denny
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