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A Stationary Reversible Beam
March 1940 QST Article

March 1940 QST

March 1940 QST Cover - RF CafeTable of Contents

Wax nostalgic about and learn from the history of early electronics. See articles from ARRL's QST, published December 1915 - present. All copyrights hereby acknowledged.

A vertical antenna can have a significant advantage over a horizontal antenna from a maintenance perspective, since, depending on how high the antenna is mounted off the ground, the 'business end' where electrical connections are made are more accessible. The configuration shown here would be difficult to implement if a mast rotator is to be used because of the stabilizing guy wires on the lower frame. Although it should be possible to achieve the necessary rigidity without guys by using an aluminum or fiberglass tubing frame rather than wood, preventing weathervaning in strong winds could prove difficult. A nifty feature of this 'reversible beam' antenna is that reciprocal directivity is implemented simply by swapping out a short length of wire between the director and reflector elements.

A Stationary Reversible Beam

Two Directions with a Fixed Three-Element Vertical Array

By Walter J. Stiles, Jr.,* W8DPY

The beam antenna pictured in Fig. 1 consists of a vertical dipole with reflector and director. The gain in anyone preferred direction is about 4.5 db, and the lobe has a vertical angle under 25 degrees. The operator has the choice of either of two directions by simply pulling up the desired director and reflector. Further, it can be used as a simple dipole, a great help for local contacts in the evening.

A Stationary Reversible Beam, March 1940 QST - RF Cafe

Fig. 1 - A simple uni-directional beam that can be used for either of two directions. One of the outside wires is changed from director to reflector by lowering the wire and clipping on an additional length. Lowering of the wire is facilitated by supporting the elements with heavy twine run through screw-eyes and counter-balanced by suitable weights. The antenna and feeder remain fixed under all conditions.

The array is supported on an inverted Y mast consisting of three 18-foot 2 by 4's bolted together with the bottom spread out as a support. Only two guy wires are necessary if this spread is at least 6 feet, but they can be eliminated if property conditions allow the mast to be leaned against a garage and bolted tight. A 16-foot 2 by 2 is bolted on top of the tower so that it points along the desired line of transmission. A small shelf bracket is attached to the top of the mast, to support the antenna about 6 inches away from the pole.

The operating frequency in the 28-megacycle band is selected, and the length of the antenna and director wires is computed by the following formulas:

Antenna length =  - RF Cafe

Director length =  - RF Cafe

One antenna wire and two director wires are cut from No. 14 enameled wire.

A matching section can be constructed from two 8-foot 6-inch pieces of the No. 14 wire spaced by 6-inch spreaders. This section is connected to one end of the antenna wire as shown in Fig. 1, and the antenna is fastened in place on the shelf bracket. A 60-foot piece of heavy binding twine is fastened to one end of each of the directors and this twine is placed through the "eye" hooks as shown. A sash weight or other object weighing about three to five pounds is then attached to the end of the twine, and the directors can be pulled in place by simply letting out twine. You now have a vertical dipole antenna with two directors spaced 1/4 wave from it.

Next compute the length of the reflector from the formula:

Reflector length =  - RF Cafe

Subtracting the director length from the reflector length will give the length of wire to be added to either director to make it serve as a reflector. This done, cut such a wire and fasten a small battery clip to one end. By lowering one of the director wires and clipping on the short piece of wire at the lower end, it will act as a reflector when raised again. If transmission is desired in the opposite direction, simply lower the reflector wire, remove the lengthening piece, add it to the other director, and raise the two elements back into place.

Let's consider a specific example: Assuming our operating frequency to be 30 Mc. we find, from the above formulas, the director length to be 14 feet 3.3 inches and the reflector length 16 feet 2.6 inches. By subtracting the director length from the reflector length we find the length of the clipping wire to be 1 foot 11.3 inches.

The array also works well on 20 meters and is still well within the space limits of the average backyard. When used on 20 meters it is sometimes difficult to get enough height, but the array seems to work well with the bottom only 2 feet from the ground. Naturally this type of operation is not ideal if the antenna is screened by trees or houses, but it still out-performs a simple half-wave "Q" strung high in the air. The same formulas are used to compute element lengths on 14 Mc. - the spacing is double that of the 28-Mc. array.

The array has also been used in four directions. This was accomplished by using two 2 by 2 poles at the top of the mast, placed at right angles to each other, and four director elements instead of two.

One final word to those who might avoid this antenna because they consider inefficient the short wire used to change a director to a reflector. This wire represents such a small fraction of the entire length of the element that it can introduce but little, if any, loss.



Posted November 9, 2015

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