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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING The use of spiral resonators in amateur VHF equipment. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Nodes of amateur radio equipment In modern transceiver equipment, high requirements are imposed on selectivity, spectral purity of transmitter and local oscillator signals. This is especially true in the design of microwave equipment. Good results can be achieved only when using in the design process a set of techniques to improve the quality of equipment. Let's list the main ones. These are advanced circuitry, the use of modern low-noise components, rational installation, shielding, stabilization of supply circuits and, of course, filtering of RF and microwave signals. No design of VHF equipment can do without filters. When designing, difficulties often arise. What type and design of the filter is more acceptable? The task of choice is set. The main criteria here are:
Most often in everyday practice, radio amateurs use LC filters with wire coils up to 200 MHz, wire and printed lines at frequencies above 200 MHz. When using such filters at frequencies above 30 MHz, there are problems with the quality factor of the coils. So, at a frequency of 30 MHz, while maintaining an acceptable coil size, you can get a quality factor of about 200. The quality factor of coils used in serial equipment does not exceed 150. The use of printed lines is limited by the material used and the size of the lines, depending on the frequency. Excellent results are obtained when using coaxial quarter-wave resonators. Such resonators provide a quality factor of up to 5000, but their use in small-sized equipment becomes unacceptable due to their large dimensions. So the resonator at a frequency of 30 MHz has a length of 2.5 meters, and at a frequency of 500 MHz it is 15 cm. In 1950, the American Alexander Horvath published a message, and in 1956 received a US patent N2.753.530 for HIGH Q FREQUENCY TUNER. The invention revolutionized the field of filter and resonator theory. The world learned about a fundamentally new type of resonator - a spiral one. GENERAL INFORMATION The quality factor of spiral resonators, depending on the design and frequency, is in the range of 200...5000 and reaches 85% of the quality factor of coaxial quarter-wave resonators. On the other hand, the length of spiral resonators can be reduced by a factor of 30. Ease of tuning, high efficiency, a variety of forms of matching opened a wide way for the practical application of spiral resonators and filters. The spiral cavity resonator has a round or rectangular screen inside which a single-layer coil is placed. One of its ends is closed to the screen, and the other is open. The metal core, introduced from the side of the open output of the spiral, changes the capacitance of the resonator - this is how frequency tuning occurs.
When calculating spiral resonators, one should keep in mind the physical limitations imposed on the elements, the tuning methods, the mutual connections of the resonators with each other and with loads. Figure 1 shows a classically shaped spiral resonator. (D is the inner diameter of the screen, d is the average diameter of the helix, do is the diameter of the wire, S is the pitch of the helix, b is the height of the helix, B is the inner height of the screen). These values are chosen in the following ratios: 0.5 CALCULATION OF SPIRAL RESONATORS FROM NOMOGRAMS Theoretical calculations and derivation of equations describing the parameters of spiral resonators are very cumbersome and are never used in practice. The most acceptable method for calculating spiral resonators is the use of nomograms, where all theoretical conclusions fit into 5 linearly connected nomograms. The electrical length, edge capacitance at the open end of the coil, and the length of the wire in the winding will be something like this:
Let us consider examples of calculation of spiral resonators. For the calculation, we will use the nomogram (Fig. 2). First example It is required to calculate the resonator for a frequency of 10 MHz and a Q-factor without load equal to 1000. By connecting a line 1 point on the fo=10 MHz axis with a point on the Q= 1000 axis, we determine that the inner diameter of the screen is D=150mm. Knowing the diameter D, we connect the point fo=10 MHz with the point D=150 mm and, continuing the line until it intersects with the axis N, Z0, we get the number of turns N=30. By choosing d/D=0,55, we get the average coil diameter d=83,5 mm. In this case, acceptable values will be: S = 4.5 turns per cm, b = 125 mm, B = 200 mm. As can be seen from the calculation, the 10 MHz helical resonator has very large dimensions. Second example It is required to calculate the resonator for a frequency of 70 MHz. The quality factor of an unloaded resonator must be at least 850. The resonator is mounted in a screen with a square cross section. It can be seen from the nomogram (line 2) that a screen with a circular cross section should have a diameter D=60mm. The internal dimension of the side of the square screen is D/1.2 - 50 mm. The required number of turns is 11. With d / D - 0.55, the coil diameter will be 33 mm. The coil length is 50mm. Screen length is 95mm. Third example We calculate the resonator for a frequency of 400 MHz with a quality factor without load Q - 2000. From the nomogram, we determine that the inner diameter of the screen D is 50 mm, and the number of turns n is 2.25 turns. The average diameter of the coil will be 27mm, and the winding pitch will be 19mm. Coil length - 40mm, screen length - 55mm. When designing spiral resonators, it is necessary to remember the following: the material from which the coil frame is made must not introduce losses. It is recommended to use polystyrene, radioceramics or fluoroplast. If the coils are made with a thick hard wire or bus, it is better to do without a frame at all. To ensure good conductivity, it is desirable to use a silver-plated wire and a silver-plated inner surface of the shield. At frequencies up to 100 MHz, you can also use a conventional copper wire (including SEW), however, a silver-plated wire gives an increase in the quality factor by about 3%. Remember that the cleanliness of the processing of the inner surface of the screen is much more important than the subsequent silvering. The screen should not have seams parallel to the coil axis, and if there are any, then they must be well soldered to ensure low contact resistance. The lower end of the coil should be brought to the side wall of the screen as straight as possible and soldered to it. If the end of the coil is brought to the bottom wall of the screen, the latter must be carefully soldered to the screen to reduce losses in the joints. The coil should reach the edge of the screen at a distance no closer than a quarter of the diameter of the screen. If the coil is lowered too low to the bottom of the screen, then the lower few turns will be inefficient for energy storage, introduce significant losses, which will adversely affect the quality factor of the resonator. The gap at the top of the screen serves to reduce parasitic capacitance and to avoid arcing in powerful resonators. It should be remembered that if the spiral resonator is turned on at the output of a VHF transmitter with an output power of 10 W, then at the end of the spiral the voltage amplitude will be 60-80 kV. As a tuning element, it is advisable to use a brass core with a diameter of 3 to 8 mm. When setting up, make sure that the core does not go deeper than 5-10% of the length of the coil. Good results are obtained by a core with a disk at the end with a diameter of 60-80% of the diameter (side) of the screen. A slot is made at the outer end of the rigged core. After adjustment, the core is securely locked (you can use a locknut). Of particular importance is the resistance of the contact of the core with the screen. It should be as small as possible. Authors: Sergey Kuznetsov (UC2CAM), Vladimir Chepyzhenko (RC2CA); Publication: N. Bolshakov, rf.atnn.ru
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