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J-pole antenna

The J-pole antenna, more properly known as the J antenna, is a vertical omnidirectional transmitting antenna used in the shortwave frequency bands. It was invented by Hans Beggerow in 1909 for use in Zeppelin airships. Trailed behind the airship, it consisted of a single one half wavelength long wire radiator, in series with a quarter-wave parallel transmission line tuning stub that matches the antenna impedance to the feedline. By 1936 this antenna began to be used for land-based transmitters with the radiating element and the matching section mounted vertically, giving it the shape of the letter "J", and by 1943 it was named the J antenna. When the radiating half-wave section is mounted horizontally, at right-angles to the quarter-wave matching stub, the variation is usually called a Zepp antenna.

How it works
The J-pole antenna is an end-fed omnidirectional half-wave antenna that is matched to the feedline by a shorted quarter-wave parallel transmission line stub. For a transmitting antenna to operate efficiently, absorbing all the power provided by its feedline, the antenna must be impedance matched to the line; it must have a resistance equal to the feedline's characteristic impedance. A half-wave antenna fed at one end has a current node at its feedpoint, giving it a very high input impedance of around This is much higher than the characteristic impedance of transmission lines, so it requires an impedance matching circuit between the antenna and the feedline. A shorted quarter-wave stub, a transmission line one quarter of the wavelength long with its conductors shorted together at one end, has a similar high impedance node at its open end, making a good match to the antenna. The input impedance seen at a point along the stub varies continuously, decreasing monotonically from this high value to zero at the shorted end. So any value of input impedance can be obtained by connecting the feedline to the proper point along the stub. One arm of the stub is extended a half wavelength to make the antenna. By attaching the antenna's feedline to the proper point along the transmission line, the stub will transform this impedance down to match the lower feedline impedance, allowing the antenna to be fed power efficiently. 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 2.2 dBi (0.1 dBd). (aka reactive near-field region) and should maintain sufficient separation to minimize these near field interactions as part of typical system installation considerations. 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. by a distance of two to three times the spacing between the parallel stub conductors. 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. ==Construction==
Construction
The antenna consists of two parallel straight metal conductors, one of a wavelength and the other of a wavelength long at the operating frequency, shorted together at the bottom. Typical construction materials include metal tubing, Since the matching section must act as a transmission line, the parallel conductors should be no more than .02 wavelength apart. 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. 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. ==Variations==
Variations
Slim Jim antenna A variation of the J-pole is the Slim Jim antenna, also known as 2BCX Slim Jim, that is related to the J-pole in a similar way to how a folded dipole is related to a dipole. The Slim Jim is one of many ways to form a J-Pole. The phasing stub between the two half-wave sections is often of the Franklin style. The Super-J antenna compresses the vertical beamwidth and has more gain than the conventional J-pole design. Both radiating sections have insufficient separation to realize the maximum benefits of collinear arrays, resulting in slightly less than the optimal 3 dB over a conventional J-pole or halfwave antenna. The approximate gain in the H-plane of the Super-J antenna is from 4.6 to 5.2 dBi (2.4 dBd to 3.1 dBd). Operating a wavelengths this way produces an antenna pattern unfavorable for terrestrial operation. To address the pattern change a variety of techniques exist to allegedly constrain a J antenna operating at or near the third harmonic so only one half-wave is active in the radiator above the stub. All involve the use of a high impedance choke at the first voltage loop. ==References==
Source: Wikipedia ↗
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