In modern receiving and transmitting equipment subject to high standards of selectivity, spectral purity of signal transmitters and local oscillators. This is especially beneficial when designing equipment for the microwave. Good results can be achieved only by using in the design process of a range of techniques to improve the quality of the equipment. We list the main ones. This progressive circuitry, the use of modern low-noise components, the rational installation, screening, stabilization of the supply chains and, of course, filtering of RF and microwave signals.
No design УSW equipment can not do without filters. In the design there can be some difficulties. What type and design of the filter is more acceptable? The task of choice.
The main criteria here are:
- center frequency;
- bandwidth;
- the figure of merit;
- the loss in bandwidth;
- matching method;
- overall dimensions;
- cost.
Most often in everyday practice hobbyists use LC filters with wire coils up to 200 MHz, wire and printed a line at frequencies above 200 MHz.
When applying these filters at frequencies above 30 MHz have problems with the quality factor of the coils. So, at a frequency of 30 MHz, while maintaining acceptable dimensions of the coil, you can get a q-factor of about 200. The quality factor of the coils used in serial equipment does not exceed 150. The use of printed lines is limited to the applicable material and dimensions of the lines, depending on the frequency. Excellent results are obtained by using the coaxial quarter-wave resonators. Such resonators provide the quality up to 5000, but in small-sized equipment their use becomes prohibitively large. So the resonator for a frequency of 30 MHz has a length of 2.5 meters and a frequency of 500 MHz-15cm.
In 1950, the American Alexander Horvath posted a message, and in 1956 received a U.S. patent N2.753.530 at HIGH Q FREQUENCY TUNER. The invention revolutionized the theory of filters and resonators. The world learned of a fundamentally new type of resonator is a spiral.
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 the coaxial quarter-wave resonators. However, the length of spiral resonators can be reduced 30 times. Easy configuration, high efficiency, variety of forms of harmonization has opened the road wide practical application of spiral resonators and filters.
Spiral cavity has a round or rectangular screen, which is placed inside a single-layer coil. One end is closed and the second open. The metal core that is input from the open output spiral, changes the capacitance of the resonator, frequency.
Fig.1
In the calculations of spiral resonators should be aware of the physical limitations of the elements, configuration methods, mutual connections between resonators a and with loads. In Fig.1 shows a helical resonator classical form. (D - inner diameter of the screen, d is the average diameter of the spiral, do is the diameter of the wire, S is the pitch of the helix, b - height of the spiral, In - internal height of the screen). The magnitude of these are selected in the following proportions: 0.5<d/D<0.39; 0.55<do/S<0.73; 2<b/d<3.
CALCULATION OF SPIRAL RESONATORS ON THE NOMOGRAM
Theoretical calculations and the derivation of the equations describing the parameters of the spiral resonators is very cumbersome and never in practice not used. The most appropriate method of calculation of spiral resonators-the use of nomograms, where all the theoretical conclusions are placed in 5 lynellnovotny nomograms.
The electrical length, the outer container at the open end of the coil and the length of wire in the coil will be approximately as follows:
the electrical length is 94% from a quarter wavelength, the regional capacity-0.15 PF, the length of the conductor - 28% of the wavelength in free space.
Consider the examples of calculation of spiral resonators. For the calculation we will use the nomogram (Fig.2).
The first example
It is required to calculate the resonator at a frequency of 10 MHz and q-factor without load equal to 1000. Connecting line 1 point on the axis fo=10 MHz with a point on the axis Q= 1000, determine that the inner diameter of the screen D=150mm. Knowing the diameter D, of the connect point fo=10 MHz with a point D=150 mm and, continuing the line to the intersection with the axis N, Z0, get the number of turns N=30. Selecting d/d=0,55, obtain the average diameter of coil d =83,5 mm acceptable values are: S=4.5 turns per cm, Ø=125 mm, H=200mm. As can be seen from the calculation, spiral resonator 10 MHz has a very large size.
The second example
It is required to calculate the resonator for a frequency of 70 MHz.
The unloaded quality factor of the cavity should be at least 850. The resonator is mounted in a screen with a square cross-section. Of nomograms can be seen (line 2) that screen with circular cross section shall have a diameter D=60mm. The inner dimension of the square side of the screen is equal to D/1.2 - 50 mm. the Required number of turns equal to 11. When d/D - 0.55 diameter of the coil will be equal to 33 mm. Length of the coil is equal to 50mm. The length of the screen is equal to 95mm.
The third example
Calculate the resonator for a frequency of 400 MHz q no-load Q - 2000. The nomogram determine that the inner diameter of the screen D - 50mm, and the number of turns n - 2.25 turns. The average diameter of the coil will be equal to 27mm, and the winding step - 19mm. Coil length is 40mm, the length of the screen - 55mm.
When designing a spiral resonators remember the following points: the material from which made the frame of the coil should not make losses. It is recommended to use polystyrene, radioterapia or PTFE. If the coils are thick, hard wire or bus bar, it is better to do without the frame. To ensure good conductivity it is desirable to use silver plated wire and silver-coated inner surface of the screen. At frequencies up to 100 MHz you can use ordinary copper wire (including PSUs), but silver-plated wire provides the increment q by about 3%. Remember that the cleanliness of the internal surface of the screen is much more important than the subsequent plating. The screen should not have seams that are parallel to the axis of the coil, and if any exist, to ensure low resistance contact them to be well propaivat. The lower end of the coil should be wired to the side of the screen as straight as possible and soldered thereto. If the end of the coil is connected to the bottom wall of the screen, the latter to reduce the losses in the joints should be carefully soldered to the screen. 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 will be lowered excessively low to the bottom of the screen, the bottom few coils will be ineffective for energy storage, will make a significant loss that 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 arc discharge in a powerful resonators. You should remember that if the spiral resonator connected to the output УSW transmitter with a power output of 10 watts, then at the end of the spiral, the amplitude of the voltage will be 60-80 kV.
As the setting item, it is advisable to use brass core with a diameter of from 3 to 8 mm. When setting up make sure that the core was not deeper than 5-10% of the length of the coil. Good results are obtained core with a disk on the end with a diameter of 60-80% of the diameter (side) of the screen. On the outer end of the adjusted core make the cut. After configuring the core securely contrat (contragate). Of particular importance is the contact resistance of the core to the screen. It should be as small as possible.
Authors: Sergey Kuznetsov (UC2CAM), Vladimir Chepurenko (RC2CA); Publication: N. Bolshakov, rf.atnn.ru