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Limitations and features of antenna calculations
The J-antenna belongs to the category of omnidirectional antennas. It is a half-wave pin with a counterpoise of length λ/4 in the form of a short-circuited two-wire transmission line (U-shaped). On this line there is a point with an input impedance of 50 ohms to which the feeder is connected. In fact, this antenna is a relative of a quarter-wave monopole with a gain of about 2.2 dBi and is used in the same conditions. The J-antenna is even somewhat easier to making than the monopole.
It is enough to take a piece of wire (tube) a little more than λ long, bend it accordingly, and now you have an antenna in your hands. The connector for microwave frequency range 1.8 - 2.4 GHz can be soldered, as shown in the figure on the left (SMA connector), and on UHF, where tubes are already used instead of wires, as shown in the figure below. Alternatively, you can do without a connector at all by connecting the feeder directly to the tube. Antenna measurements are calculated taking into account the shortening (velocity) factor vf, which depends on the ratio of the frequency to the diameter of the wires (tube).
The antenna needs a balun. This can be done by passing the feeder inside one of the tubes and leading it through the zero potential point at the bottom of the U-bend. At VHF frequencies, a coaxial cable coil as a rejector filter can be used.
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