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Shoning Plug

Qialeclric B&sds

Met AI Plug

Met AI Plug

Figure 5. Basic construction, plug is inserted which shorts the outer conductor of the coax to the inside of the lower radiator, thereby making AB and DC a pair of quarter-wave (not quite) coaxial chokes* The coax used is 0.140" o.d. semirigid 50-ohm cable, if the shorting plug is made of brass it may be soldered directly to the copper tubing outer jacket of the coax. If the plug is made of aluminum, fashion a brass or copper sleeve which can be soldered to the coax and connected to the aluminum with screw pressure. One source for the miniature semirigid coax is Precision Tube Co. Inc., Microwave Division, Church Road and Wis-sihickon Ave,, North Wales PA 19454,

The two little capacitors used are Johan-son air variable Q,7-to-l0.5 pF units. 1 picked these up at a hamfest Tor a few cents. A pair of piston caps mighi be better since they can handle more power and arc easier to tunc. The inductor consists of three turns of #16 tinned wire wound on a 3/4" form. We will discuss Eune-up later.

The insulators for this unit ucre made from white PVC water pipe. The construction is shown in Figure 6. The construction technique used on this model is best suited to those who have a lalhe available, but other constructions not requiring a lathe are also feasible.

The radiators are, as noted earlier, 7/8" o.d. aluminum tubing. In order to get a good fit, the nominal I" pipe, which is actually L063" i.d., must be shimmed. To do this T cut two 1,25" lengths of the pipe. I then cut out a portion of the circumferencc so that I couid compress the remaining segment into a 1" diameter cylinder I then slathered some PVC pipe glue on the center piece and the insert and drove the insert in with a soft mallet. Immediately upon insertion, clamp the insulator in a vise to round up the egg shape thai develops because of the uneven push of the insert.

The PVC glue dries very fast. When the first end of the center insulator is done, do the other end. After the glue has dried for about a half hour the insulators may be bored out in a lathe to give a good fit for the upper and lower radiators. After the boring operation, the ccnter insulator should have a center window about 1/2" x 1" opened to permit connection to the coax.

The base insulator is made similar to the center insulator except that only one end is equipped with an insert. The base flange is made of any convenient material, It may be either metal or plastic. It must, of course, be strong enough to hold the overturning mo ment due lo wind loads on the radiators. In my case, I made it by laminating two pieces of 3/8" XXXP plastic (a paper-base melamine) and boring the assembly to accept the base insulator outside diameter.

Assembly

To begin with, prepare the ends of the miniature semirigid coax. At each end mark

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