J-pole antenna
[2] 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.
[1] 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.
A half-wave antenna fed at one end has a current node at its feedpoint, giving it a very high input impedance of around 1 000–4 000 ohms.
The input impedance seen at a point along the stub varies continuously, decreasing monotonically from this high value to zero at the shorted end.
[7] Primarily a dipole, the J-pole antenna exhibits a mostly omnidirectional pattern in the horizontal (H) plane with an average free-space gain near 2.2 dBi (0.1 dBd).
[8] The slight increase over a dipole's 2.15 dBi (0 dBd) gain represents the small contribution to the pattern made by the current imbalance on the matching section.
[citation needed] Typical construction materials include metal tubing,[1] ladder line, or twin-lead.
[1][3] The J-pole design functions well when fed with a balanced feed (via balun, transformer or choke) and no electrical connection exists between its conductors and surrounding supports.
[19] 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.
[9][24] Slim Jim antennas made from ladder transmission line use the existing parallel conductor for the folded dipole element,[9] but in the copper pipe variation, the Slim Jim requires almost twice as much material, for which it returns no performance benefit.
[28] 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.
[29] The collinear J antenna improves the Super-J by separating the two radiating half-wave sections to optimize gain using a phasing coil.
[31] 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.
