Coaxial vertical antennas. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / HF antennas
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This is the name of Groundplane-type antennas with a built-in short-circuited quarter-wave loop of coaxial cable (Fig. 1). The main advantage of this type of antenna is the extended frequency band.
Fig.1. Scheme of a coaxial vertical antenna (Rv is the sum of the loss resistances)
It is formed by a vertically located coaxial cable with an arbitrary wave impedance. The lower end of the central core of the cable is connected to the grounding network, and its upper end is soldered to the cable screen. The length l of the cable is equal to l / 4xVk, where Vk is the value of the shortening factor from the specification (usually 0,66). Thus, we are talking about a coaxial closed quarter-wave line acting as a parallel resonant circuit. Radio waves are emitted only by the cable shield, but due to the small l/d ratio, its velocity factor is close to 0,95 and therefore too short for quarter-wave resonance. In order to form a quarter-wave Ground-plane, it is required to increase l1 with a segment l2 up to the resonant length l/4.
Example
A coaxial cable with Vk=0,66 is used. Geometric quarter of the wavelength l1=0,25lх0,66=0,165l. If for the cable shield, taking into account its l / d ratio, the shortening factor V = 0,95 is taken, then the normal length will be l1 + l2 = 0,25lx0,95 = 0,2376l, and the length of the segment l2 = 0,2376l-0,165l = 0,0725l. At resonance, the built-in quarter-wave stub does not work due to a very high input impedance (parallel resonant circuit). If you increase the frequency of the transmitter, then the segment l1 + l2 will be too long - in other words, an inductive reactive component will appear on it. At the same time, the short-circuited quarter-wave coaxial line (stub) will also become excessively long. A line that exceeds a quarter wavelength has a capacitive effect, and as a result, the inductive component of the radiator segment and the capacitive reactivity of the quarter-wave plume are mutually compensated, and the radiation resistance Rs increases.
With a decrease in the frequency of the transmitter, the opposite happens: the radiator section becomes capacitive, and the loop becomes inductive, which also leads to mutual compensation of reactive components. Due to this ability of a quarter-wave line, the frequency band of the antenna is expanded. From above, it is limited by undesirable changes in the radiation pattern, and from below, by a sharp drop in radiation resistance. Due to this broadband, the length of the antenna elements does not have to be precisely maintained. As with other vertical antennas, a good grounding network is a prerequisite for high efficiency.
DL2FA has described coaxial antennas in detail [1]. A sketch of the simplest of them is shown in Fig. 2a.
Rice. 2. Types of vertical coaxial antennas:
a - shortened Groundplane; b - full-size Groundplane;
c - a greatly shortened Groundplane with an extension coil L and an end capacitance CD
If the coefficient of shortening of the coaxial cable is Vk=0,66, then its geometric length will be 0,25lх0,66=0,165l; it is the length of the emitter, since no methods of lengthening this element were used. Therefore, there is a shortened Groundplane with a length of ~60° (1 l=360°). The radiation resistance Rs of this version of the antenna is approximately 13 ohms. To achieve a high efficiency of the antenna, the loss resistance should be the lower, the lower the radiation resistance. Somewhat more favorable conditions are created by the use of a coaxial cable with a semi-air dielectric and Vk = 0,82. Then the cable length l1=0,25lx0,82=0,205l (about 74°), the value of radiation resistance (20 Ohm). Due to the already described action of the coaxial quarter-wave stub, the input impedance remains active in a wide frequency range, and its value changes along with the radiation resistance. With the help of an omega-matching link (CА-FROMК) is coordinated with the characteristic impedance of any cable.
Fig. 2b basically corresponds to Fig. 1, differing only in the display of antenna parameters and omega-shaped matching. Figure 2c shows a greatly shortened Groundplane with an extension coil and end capacitance.
Coaxial antennas are operated in multi-band mode. In this case, you should be aware of changes in the vertical radiation pattern with the transition from one range to another and radiation resistance, as well as the need to adjust the omega-shaped link when switching ranges. This is not required when operating in a single range.
The significant bandwidth of the frequency range opens up many possibilities for local adaptation. Coaxial cable needs artificial or natural support. The ideal solution would be a fiberglass pipe with a coaxial cable inside. Sometimes it is possible to run a cable between two high-placed reference points (for example, in trees).
Literature
1. Wurtz H. DX-Antennen mit spiegelnden Flachen - Koakxiale Antennen. cq-DL 7/1981, S.330-332
Publication: N. Bolshakov, rf.atnn.ru
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