The following is presented for informational purposes and is not intended as a construction project of it's own. This information is a supplement to RadCom Project, Feb 99. For a copy of the original article, click here.
The bracketed letters in the following text refer to the corresponding letters in the sketch.
From bottom to top, there is a PVC base rod (A), a short aluminium tube (B) which serves as a housing for the motor (C) and the matching transformer (D) and on which the connectors for the counterpoise, coax feeder and motor wires are mounted. (E) is the PVC base insulator, which carries the lower radiator (F), an aluminium tube 900 L x 54 OD x 51 ID.
Within this tube, there is a stainless steel rod (G), threaded M12, which is keyed to the motor shaft. The coil carrier (H), a PVC tube, has a Delrin  M12 nut fixed in its lower end, which makes the coil carrier, coil and upper radiator go up and down as the motor shaft turns one way or the other. The lowest 25mm of the M12 rod are undercut to 10mm to disengage the nut from the thread on the rod when the nut comes to its lowest position. A foam rubber bumper on top of the base insulator prevents the nut with all that it carries from dropping down further. The M12 rod is too short to push the coil completely out of the lower radiator. The coil carrier is kept centered in the lower radiator by two PVC rings (J).
The coil (K) is the most difficult part to make. A lathe and good craftsmanship are essential. One-millimeter-deep grooves in the coil former are needed for 150 turns of 1.5mm ø silver-plated copper wire. Other constructors have used PVC tubing for the coil former but we thought that too flimsy, especially after cutting grooves into it. Also, PVC has rather high RF losses; we therefore used a solid polyethylene core, in spite of the extra weight. For best Q, the turn spacing should be roughly equal to the wire gauge. We used a pitch of 2.54mm. The estimated Q of this coil is 275 when clean and dry.
The exact diameter of the coil former can be determined only after the fingerstock (L) has been fitted into the upper rim of the lower radiator. Our beryllium-copper fingerstock, the kind used to make the doors of shielding cabinets RF-proof, was ordered from the US. To fit the 51mm ID of the lower radiator, we used a 158mm long strip with 34 fingers, each 7Lx4W. The strip is fastened with five stainless M2 bolts, heads inside, with washers and nuts on the outside. A plastic ring (M) protects the fingers from external damage. The coil diameter now must be chosen so that both the outside of the windings as well as the spaces between turns are within the spring travel of the fingers. There also must be clearance between the five M2 bolt heads and the coil wire. Having tried with a sample first, we ended up with a former of 46 ø and wire grooves 1mm deep for an over-the-turns diameter of 47mm.
One end of the coil wire is fixed in a small hole drilled into the former at the low end of the wire groove; the wire then is wound into the groove under tension (from a gloved hand!) on the lathe; the top end of the wire runs straight up through a slot in the former to a brass ring (N) which is held in place on top of the coil by a Hustler QD-2 quick-disconnect coupling (P); the latter provides for easy installation and removal of a selection of upper radiators (R). The coupling is firmly threaded into the coil former; this connection must withstand the considerable bending moment exerted on it by the upper radiator (Q) in strong winds. A plexiglass cover (R) keeps rain, dirt and insects off the coil and finger stock.
If an N-type coax connector is used on the antenna, it may be considered weatherproof.