A Two-Band Piece of Wire |
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Reading this article from a 1950 issue of Radio & Television News magazine reminds me of a basic truth - at least for receiving, it is usually possible to realize obtain useful operation with just about any reasonable length of straight wire for an antenna. Transmitting is a different story since poor VSWR conditions on the antenna connection can damage or cause to shut down the output stage of an RF power amplifier. Still, if your transmitter can survive a high VSWR, then chances are you can send out a useful signal. You won't be breaking any DX records or winning any contests, but you won't be dead in the water. Surely anyone over 40 years old has jury-rigged an FM radio antenna out of a length of straight (or bent) wire and/or fashioned a sheet of aluminum foil around a set-top television antenna to pick up an acceptable signal. In many cases where the 300 Ω twin-lead cable was improperly snaked along an aluminum gutter or aluminum siding on a house on its way to the rooftop antenna or had a long length of excess cable coiled up on the roof or behind the TV set, the horrendously mismatched connection caused the cable to serve more as the actual antenna rather than the one on the roof. A Two-Band Piece of Wire
An easy-to-build, economical, highly efficient, multi-band antenna for the amateur. In this era of multi-element rotary beams, stacked arrays, squashed cubicle quads, and other catch-named antennas, it may be reassuring to realize anew that a single-wire antenna properly connected to a receiver or transmitter still works well. The degree of its performance depends greatly upon how efficiently it is connected. Being a more than one-band operator, the author some months ago dreamed and figured at considerable length on just how to feed a long, high-wire antenna so that it could be used on two or more harmonically-related bands, using a flat or untuned feed line, and still maintain efficient matching of feed line to both antenna and transmitter or receiver. One cold, early dawn the subconscious mind awakened the outer man, and several curves were hastily drawn, resulting in a very simple yet conclusive answer to how the problem could easily be resolved.
Fig. 1 - Wavelengths on a wire.
Fig. 2 - Chart of inductive reactance of long wire.
Fig. 3 - Method of lacing wire through connector and attaching 300 ohm line. Perhaps at your amateur station location, you have some means of supporting in the clear a single piece of wire 68 feet long and would be interested in erecting in this available space a highly-efficient yet very simple antenna at but little cost. Or maybe you are starting from scratch and desire an effective sky-wire whose erection requires very little technical and structural knowhow and one that is inexpensive to build. If you fall in either of these categories or simply would like to try something a little different in the way of an antenna, then the one described herein should interest you. Theory A piece of wire 68 feet long and in the clear will resonate at approximately two half-wavelengths on 14.25 mc. and four half-wavelengths on 28.50 mc., according to the accepted antenna formula. The characteristic cloverleaf horizontal pattern of radiation makes such a piece of wire desirable as a general coverage radiator. If such an antenna were fed with a non-resonant line, and no electrical adjustments to it or the antenna were required when tuning up or changing operating bands, then you would have the most simple and convenient antenna conceivable. Such an arrangement becomes a reality when thoughtful consideration is given to well-known fundamental electrical characteristics of long-wire antennas and non-resonant feed lines. Fig. 1A depicts the current distribution on a 68-foot piece of open end wire operating at 14.25 mc., and Fig. 1B shows the current distribution at 28.50 mc. In any antenna a multiple of a half-wave in length, as in Figs. 1A and 1B, the impedance of the wire reaches its lowest value at each current loop (point L) and its highest where the current almost equals zero (point H). Theory and practice establish the fact that at the end, or high impedance point, the value is approximately 3600 ohms, and at the center, or low impedance point, approximately 72 ohms. Fig. 2 shows the values of impedance of a resonant half-wavelength of wire plotted against physical length. It is to be noted that the curves cross one another at a common point which corresponds to 265 ohms impedance at a distance of 11 feet, 4 inches from one end. Accordingly, the conclusion is reached that a single piece of antenna wire 68 feet long can be fed 11 feet, 4 inches from one end by a 265 ohm, non-resonant feed line and operated on both 14.25 mc. and 28.50 mc. with perfect transfer of energy to the system. Because height above the ground and proximity of nearby objects affect the electrical characteristics of any antenna to some degree, the common impedance value of 265 ohms in the described system can be considered as 300 ohms for all practical purposes, and the 11 foot, 4 inch dimension shall be considered as 11 feet, even. Construction On that basis, the author cut a 68-foot piece of antenna wire 11 feet from one end and connected in series thereto a 300 ohm, twin-lead ribbon of random length, long enough to reach the operating position in the station. To provide a structurally and electrically sound means of making the connection of the feed line to the antenna, a simple connector made out of Lucite was devised and used as shown in Fig. 3. The antenna was thus assembled, hoisted in the clear about 30 feet above ground and placed in use. The same principle is applied if the antenna is being constructed for 7.13 mc. and 14.25 mc. operation. For such, however, the "300 ohm common point" is to be noted in Fig. 2 as 22 feet from one end, and the feed line must be connected accordingly. Ten and twenty meters at 11 feet, or twenty and forty meters at 22 feet - take your choice; it can't be both combinations with a single connection. Upon installation of this antenna at the writer's station, it was noted that regardless of which of the two bands the antenna was used on, the final stage of the transmitter did not detune from minimum plate current state when the antenna was connected to it, thus indicating an excellent impedance match throughout the entire antenna system. In addition, the system loaded very readily as demonstrated by the required loose coupling of the two-turn pickup loop at the final stage of the transmitter. With a Class B modulated phone transmitter operating at not more than 150 watts input, no trouble has been experienced by the writer and others in working out satisfactorily on the 7, 14, and 28 mc. bands, even during the most congested hours. Surprisingly enough, the antenna seems to be not at all critical as to tuning anywhere within these bands. With the use of a non-resonant feed line as indicated and the absence of standing waves as borne out by test, maximum efficiency in the transfer of energy from the transmitter to the antenna is achieved, and broadcast interference is kept to a minimum. These desirable factors alone should appeal to any amateur, new or experienced, and make construction of this simple antenna system a next-weekend must. The author's address is 2062 Eudora Street, Denver 7, Colorado.
Posted February 24, 2020 |
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By Karl Dreher, W0WO
