Antenna Extended pole. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / HF antennas
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It is well known that the "quarter-wave pin" vertical antenna, which is widely used by radio amateurs, has an input impedance of approximately 37 ohms. Since most of the coaxial cables used to power the antenna have a wave impedance of 50 or 75 ohms, shortwavers sometimes have problems matching the feeder with the radiator. There is a simple and very convenient solution to these problems from a constructive point of view.
By increasing the length of the emitter, you can bring the value of the active component of the input resistance to the required value - 50 or 75 ohms. Such an elongated antenna has a noticeable reactive component of the input impedance, which is inductive in nature. It is not difficult to eliminate it - it is enough to connect capacitor C in series with the emitter (see figure). Its capacitance is chosen such that, together with the inductive component of the input resistance, it forms a series oscillatory circuit, the resonant frequency of which corresponds to the operating frequency of the antenna.
If the feeder has a wave impedance of 50 ohms, then the length of the radiator of such an antenna L should be approximately 0,28 / l (A is the wavelength corresponding to the middle frequency of the amateur band). With a wave impedance of 75 ohms, L=0,31l. Note that in order to minimize losses due to the imperfection of the "ground" - the system of counterweights, it is better to use a 75-ohm coaxial cable for the feeder. Each of the elements of the counterweight system has a length A=0,25/l. To achieve a sufficiently high efficiency of the antenna, their number must be at least 15 (the efficiency will be more than 50 percent).
Here is the initial data for the manufacture of an "extended pin" for various amateur bands. For each range, the following are given: the length of the emitter in meters for a 50-ohm feeder and in fractions for a 75-ohm feeder, the length of the counterbalance system element in meters and the maximum capacitance of the coupling capacitor in picofarads.
7 MHz band: 11.64/ 12,86 - 10,4 - 250.
14 MHz band: 5,82/ 6,46 - 5,2 - 150.
21 MHz band: 3,85/ 4,27 - 3,49 - 130.
28 MHz band: 2,86/ 3,16 - 2,62 - 100.
Antenna tuning is carried out by adjusting the coupling capacitor according to the minimum SWR or according to the maximum readings of the field strength indicator.
Author: V. Uzun (ex UB5MCI); Publication: cxem.net
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