Compact antenna. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / HF antennas
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This antenna can be used in cases where there is not enough space to install a full-size dipole on a range of 160 meters. It should be noted that, using the calculated ratios given in this article, similar shortened antennas can be made for other amateur bands.
The antenna is (see figure) a radiator of length A with an extension coil L1. This coil "extends" the radiator to the electrical length L / 4, and as the "earth" they use the mast B and the grounded fixture C of the building. To increase the efficiency of the antenna, if possible, it would be good to install a shortened counterweight D with an extension coil L2. It is better if there are several counterweights.
The calculation of the antenna is carried out in the following sequence. Having determined the length of the radiator A (meters), choose the resonant frequency of the antenna f (megahertz) and the diameter d (meters) of the wire from which the radiator will be made. In the calculation example below, the following values of these parameters will be used: A=29 m, f=1,86 MHz, d=0,0015m(1,5mm).
First, determine the wavelength L (meters) for the selected resonant frequency of the antenna, its operating angle φ (degrees) and the intermediate parameter S:
For our example, L = 161,3 m, f = 64,7 "and S = 19333. Then they find the characteristic resistance Z (ohms) of the antenna conductor and the corresponding reactance of the antenna Xc at the point of connection of the inductor L1 to the emitter sheet:
For our example, Z = 600,6 ohms and Xc = 283,8 ohms. Note that the reactance of a shortened radiator is capacitive. Therefore, an inductor L1 is used to tune the antenna to resonance. Its reactance Xl must be numerically equal to the reactance of the antenna Xc. Coil inductance L (microhenry) is calculated by the formula
For our example, L=24,3 μH. The braid of the supply cable is connected to the left (according to the figure) end of the L1 coil, and its central conductor is connected to the outlet from this coil. The connection point (n1 turns, counting from the left end of the coil) depends on the impedance of the supply cable R, the inductive reactance of the extension coil and the number of its turns n. They are related as follows:
If, for example, the extension coil L1 has 28 turns, and the characteristic impedance of the cable is 50 ohms, then its center conductor must be connected to approximately the 12th turn. More precisely, the connection point is determined experimentally - by the minimum SWR in the supply feeder.
The extension coil is calculated using standard formulas. Since a high RF voltage arises during operation, it is best to make the coil single-layer with a forced winding pitch equal to the diameter of the wire used to make it. This wire must have a diameter of at least 1 mm.
In the original article (K. Bottcher. Endgespeiste 160-m-Antenne fur ungun-stige Lagen. - Funkamateur, 1997, N11, s. 1314-1315) it is noted that an extension coil can also be made on a carbonyl iron ring magnetic circuit if the transmitter has a small power, and the diameter of the magnetic circuit will be several centimeters.
Literature
- Radio 5-99, p.61
Publication: N. Bolshakov, rf.atnn.ru
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