1 6' x 3/4" aluminum tube
Table I. Construction materials for the MicroVert antenna.
in the tank circuit from escaping and interfering with my mother's soap operas.
The lesson was learned and remembered. The MicroVert is simply the recreation of the tunable output circuit of that little 40-meter transmitter, enhanced in such a way that it radiates as much RF as possible.
The MicroVert is, in fact, nothing more than a series-resonant circuit with a Q low enough to cover all of the 20-mcter band.
Building a MicroVert
The MicroVert is easy to build. All of the parts you need can be obtained from your local home improvement center. Table 1 provides a parts list for your shopping convenience (see also Photo B).
The plastic pipe used is chlorinated polyvinyl chloride (CPVC), which is normally used in place of copper pipes for household plumbing. 1 chose this material because its dimensions were ideal for this project. It is not the best material for this use because its insulating properties leave a little to be desired. but it is readily available and easy to work with, and mates perfectly with the copper pipe that forms part of the tubular capacitor providing the "C" in the series LC circuit. In my final version of the MicroVert, I substituted a section of Fibcrglas™ tubing for the CPVC coil form. This change greatly improved power-handling capability and is a recommended upgrade for those moving beyond the experimenting stage.
I found the nylon bushing in the specialty hardware section. This particular item is meant to be used on furniture as a washer placed beneath a large screw head to protect the wood. A suitable replacement can be made from any sort of plastic material.
The MicroVert consists of two main components:
1) An inductor, made up of common solid I2-gauge electrical wire wrapped on a 12-inch section of 3/4-inch CPVC pipe; and
2) A capacitor, made up of a 10-inch piece of copper pipe held in place over another 12-inch piece of CPVC, with a threaded brass rod mounted in such a way that it can be threaded into and out of the copper pipe—raising and lowering the "C" value respectively.
Let's take a tour of the MicroVert, starting and ending with the feedpoint— your coax connection.
A small rectangular piece of acrylic plastic or Plexiglas™ supports the SO-239 coax connector. I recommend using the type of SO-239 that is held in place with a single large nut and washer, rather than the more common type that requires at least two additional small screw holes to be drilled. Either will work, but the plastic is brittle, and the fewer holes that need to be drilled, the better. The washer supplied with the nut-mounted SO-239 has a convenient solder lab, which simplifies attaching the jumper (described below). The plastic rectangle is then attached to the bottom of the antenna with self-tapping screws or is simply glued to the assembly.
The ccnter of the SO-239 is connected to the bottom of the inductor. The inductor, consisting of 20 feet, nine inches of solid 12-sau2c insulated wire, occupies 10 inches of the 12-inch CPVC pipe used as the coil form. The remaining area of the pipe is used to slip into the two elbows that hold the inductor in place. The top of the inductor is connected with a jumper wire to the threaded brass rod. The jumper wire has a large plated lug soldered to it, which facilitates a good electrical connection to the threaded brass rod.
Winding 20 feet of 12-gauge solid copper wire onto a one-inch diameter form is a challenge. The method 1 used was to drill two 3/16-inch holes one inch in from each end of the form. I clamped one end of the coiled wire into a bench vise and unrolled the wire onto my garage floor as I backed away from the vise. With the wire fully unrolled, I threaded the loose end of the wire through one of the holes in the form, leaving about six inches free. Pulling the wire tiizht, I then walked toward the vise, simultaneously pulling the wire taut and rolling it slowly onto the form. When the wire was fully wound on the CPVC pipe, I took a firm grip on the wire at the top of the form to keep it tightly wound and then worked the free end through the re-mainins hole.
The capacitor consists of two parts: a 10-inch copper pipe and a 12-inch threaded brass rod. The threaded brass rod is held in place by two brass nuts supported by brass washers. The lower nut is glued into the inner surface of a three-quarter-inch CPVC end cap along with a washer placed underneath it against the cap's surface. With the rod threaded into the fixed nut, another nut is threaded down to compress two additional washers that secure the jumper wire from the inductor.
On the tip of the brass rod that extends into the copper pipe is a three-quarter-inch nylon bushing held in place with two brass nuts. The nylon bushing provides a reasonably tight fit inside the CPVC pipe, providing precise alignment of the brass rod. The 10-inch section of copper pipe provides the surface area for the capacitor. A substantial portion of the capacitance required for the "C" component is actually derived from the pipe's close proximity to the inductor. The spacing between these two components is critical and should not be altered without due consideration.
The copper pipe used is a special type available in most plumbing departments. It is normally used for splicing two sections of regular copper pipe together. As such, its inner diameter is just large enough to slip over the ends of the pipes to be joined. Since the CPVC pipe is exactly the same outer diameter as the copper pipe, the larger joiner pipe fits over it perfectly as well. The copper pipe is cut to a length of 10 inches, positioned directly opposite the inductor, and secured in place with a self-tapping sheet metal screw positioned half an inch from the bottom.
The sheet metal screw is also used to provide a connection point for a short jumper wire that goes from the bottom of the copper pipe to the shield side of the SO-239. The jumper is made of the same 12-gauge wire as the inductor and has a good-quality plated lug soldered to it. The area of the pipe in contact with the lug is burnished with steel wool or light sandpaper to improve the connection quality. As a final assembly step, consider soldering the jumper's lug directly to the copper pipe.
While two elbows support the inductor's coil form, the capacitor's section of pipe is supported with two
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