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Making the car-radio whip serve as a single-wire transmission line.
After reading Tilton's article on v.h.f. mobile antennas in December QST, I wanted to try a halo antenna on 2 meters. I had been having many fine mobile contacts on 2 using a Conset Communicator, on the car-radio whip adjusted to a length of nineteen inches. However, it seemed that, where horizontal antennas were in use at the home stations, the mobiles using various horizontal systems were covering greater distances.
One of the problems of the ham who wants to operate mobile is how to install an effective antenna without disturbing the car's normal furnishings. I wanted to use the existing feed-through arrangements for the broadcast antenna, and it occurred to me that perhaps the whip could serve as a combined support and feedline.
If a half-wave resonant antenna is fed at a point fourteen per cent from its midpoint, a single-wire feed may be used, as shown in Fig. 1A.
Fig. 1. The principle of the single-wire feed system is shown at A. The feeder should run perpendicular to the antenna for at least a quarter wave length. The dipole is folded around to make a halo, B, and the car broadcast whip is used as a combined support and feedline, mounting the halo on the whip at fige inches off center.
The characteristic impedances of the antenna and transmission are approximately equal at this point, so the feedline will not be the principal radiator. The line must run at right angles to the antenna for about a quarter wave length or more, so that the field from the antenna will not couple energy back into the feeder and disturb the impedance relationship.
To use the broadcast whip as part of the feedline, a resonant antenna was attached to the top of the whip and bent in a circle, as shown in Fig. 1B. The halo was made of 3/32 inch brass tubing, though stiff wire or 1/8 inch brass rod will do. The dipole was 34 inches long for resonance in the 2-meter band, and the optimum point for feeding was found to be five inches from the center. A heavy lug was soldered to the halo at this point. The ends of the dipole were inserted in an insulating sleeve and cemented for rigidity. Care should be taken to see that the ends do not touch each other.
The bead at the top of the whip was drilled out and a 4-40 brass bolt soldered into it. The head of the bolt was cut off, so that the halo lug could be slipped over it, and tightened in place with a nut. The proper length for the dipole can be determined with a grid-dip meter, and it will depend on the spacing between the element ends. The length of 34 inches is about right if the ends are close together. If the ends are well apart, and not held together by an insulator (as in the W1HDQ halo, page 11 of December QST), the length will be more nearly that of a normal half-wave dipole. The larger the diameter of the halo, the less critical will he the element length.
In the absence of a grid-dipper, the optimum element length and tap position may be found by experiment, juggling these until a combination is found that allows the length of the line (telescoping whip) to be varied without affecting the transmitter loading appreciably. At the proper tap position a neon lamp will glow at either element end and will extinguish at the middle of the dipole.
Obviously, there will be an appreciable mismatch between the coaxial line and the whip portion of the feed system, but very little power is lost in so short a line as the few feet necessary to run from the rig to the whip. Also, the impedance of the whip (serving as the transmission line) will he considerably lower than the nominal value of 500 to 600 ohms for the usual single-wire transmission line. Thus the s.w.r. may not be as high as expected, though the ultimate value will vary considerably with differing installations.
The main objective is to get as good a match as possible between the line (whip) and the antenna, so that the vertical portion of the system does not radiate too greatly. Some radiation from the whip may not be too harmful, as the polarization of the signals encountered in v.h.f. mobile work is likely to be far from pure horizontal.
Many tests under varying terrain conditions have demonstrated the value of this antenna, particularly in work with horizontally-polarized fixed stations. A contact was made by W3KDZ/8, Youngstown, with W8WEN, Alliance, Ohio, over about a thirty-mile distance. Here the halo showed about 8 db. gain over the quarter-wave whip, vertical. Later, the author had a QSO with W8BTK, Hubbard, Ohio, over forty miles of the Ohio and Pennsylvania Turnpike. Farther east, W3BNC, Hagerstown, Md., was worked as far south as Gaithersburg, Md., about fifty miles over mountainous terrain. For the balance of the trip home contact was maintained with W3GKP, Spencerville, Md.
Louis D. Breetz, W3KDZ/W8QLP.