Antenna book by balanis pdf behind the airship, it consisted of a single element, one half wavelength long radiator with a quarter wave parallel feedline tuning stub. Matching to the feed-line is achieved by sliding the connection of the feedline back and forth along the stub until an impedance match is obtained.
Being a half-wave antenna, it provides a small gain over a quarter-wave ground-plane antenna. E-plane gain measurements of J antenna with respect to reference dipole. Measurements and simulation confirm the quarter-wave stub modifies the circular H-plane pattern shape increasing the gain slightly on the side of the J stub element and reducing the gain slightly on the side opposite the J stub element. At right angles to the J-stub, the gain is closer to the overall average: about 2. The slight increase over a dipole’s 2. J element while the pattern opposite the J element is mostly broadside. The net effect of the perturbation caused by quarter-wave stub is an H-plane approximate gain from 1.
The quarter wave parallel transmission line stub has an external electromagnetic field with strength and size proportional to the spacing between the parallel conductors. The parallel conductors must be kept free of moisture, snow, ice and should be kept away from other conductors including downspouts, metal window frames, flashing, etc. The J-Pole is very sensitive to conductive support structures and will achieve best performance with no electrical bonding between antenna conductors and the mounting structure. The J-pole antenna and its variations may be fed with balanced line.
The feed-point of the J-pole is somewhere between the closed low-impedance bottom and open high-impedance top of the J stub. Between these two extremes a match to any impedance between the low to high impedance points is available. Historical documentation of the J antenna suggests the lower end of the matching stub is at zero potential with respect to earth and can connect to a grounding wire or mast with no effect on the antenna’s operation. Later research confirms the tendency of the mast or grounding wire to draw current from the antenna potentially spoiling the antenna pattern. A common approach extends the conductor below the bottom of the J-pole resulting in additional and undesirable RF currents flowing over every part of the mounting structure.