WHY HAM RADIO - WHY NOT CB?
YOUR FIRST RIG
HF AMPLIFIER BASICS
GROUNDED GRID BASICS
POWER SUPPLY BASICS
WHAT NOT TO DO AROUND HV!
RF Chokes for the plate circuit of power amplifiers may be very easily constructed. Finding a sufficient form will in every event represent the biggest obstacle. Amplifiers in the 1.5 KW range, will almost universally require a 32 uH choke, resonant at 43 MHZ. To accomplish these values, start with a 6 inch by .75 inch ceramic form. A phenolic form may be substituted, however, the use of PVC or other plastic materials that become unstable when heated should be avoided at all cost. If ceramic or phenolic cannot be located, consider a glass dowel the same dimensions. If a glass dowel is employed, frosted glass would be far preferable to polished glass, because the porosity of the frosted surface will aid in retaining the windings.
Once the form is in hand, begin by close winding it with 89 turns of #18 Formex, or equivalent. The completed length of the coil will be around 4 1/8 inches long. Secure the ends carefully. A nylon screw type conduit clamp may then be employed to secure the form to the chassis.
Venturing above the 1.5 KW range will require about 78 uH, series resonant at 28 MHZ. The form required for this choke will need to be 1 inch in diameter and about the same length as stated before.
The 78 uH choke will initially require a 110 turn, bifilar winding (two wires wound right next to each other in parallel) of #26 enameled wire, which should span about 4 inches of the 6 inch form. Once wound, tightly secure the wire ends and use fingernail polish to hold the turns in place. When the polish dries, remove one of the wires and unwind it from the form, being a careful as possible not to disturb the second winding that's been left in place. Reapply the fingernail polish to seal the turns to the form. Utilize the mounting hardware discussed above to attach the choke to the chassis.
Locate a 4.5 - 6 inch length of 3/8 to 1/2 inch ferrite rod. If you don't already have one in your junk box, stop by your neighborhood Good Will, or Salavation Army store, or hit the garage sales on Saturday and spend some time peering into the backs of old clock radios that may be offered for sale. You should be able to spot a rod if the manufacture utilized one as part of the internal antenna. While you're at it, see if you can find a radio with not only a ferrite rod, as described above, but a large three section tuning capacitor as well. You can salvage the tuning capacitor as well.
Once you've secured a ferrite rod, as described, pickup about a 10 foot stretch of 3 conductor, 12 Ga. interior electrical wire (if you're planning to use the choke for anything above 15 amps, use 10 Ga.). Strip off the plastic jacket, and you should have a black and a white insulated lead, as well as a bare copper wire the same length. Put the bare copper wire in your junk box for some other project.
Next, locate a wooden dowel the same diameter as the ferrite rod. Straighten and place the black and white wires next to each other and tape one end together. Leaving about two inches, begin winding the wires together, as tightly as possible, onto the wooden dowel. Try to wrap the turns as perpendicular to the dowel as possible - this will permit as many turns per inch as will be possible. Wrap 14 turns. When you are done winding, it should look like a candy-cane stick - black, white, black, white, etc. Now carefully slip the coil off the wooden dowel and slip it onto the ferrite rod. Once on the rod, twist the turns carefully to tighten and take up the slack. While applying tension to tighten the coil, wrap silk tape, available
at most drug stores, over the coils to ensure that they remain in place. No mount will be needed - once soldered in place, the wire leads will very capably support the choke.
SELECTING A TUBE
RF INPUT CIRCUIT
RF TANK CIRCUIT
TRANSFORMER POWER CAPABILITIES
PARTS IS PARTS
ROLLING YOUR OWN - TRANSFORMERS AND CHOKES
POWER SUPPLY PROJECTS
LEGAL LIMIT AMP PROJECT