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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Antenna on the windowsill. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / VHF antennas Almost any owner of a CB radio station eventually comes to the need to install a good stationary antenna. However, in urban conditions this is not always easy, because. At least three problems have to be solved: legal (in terms of authorization and access to the roof), mechanical (providing the necessary wind resistance) and anti-vandal (struggle for the safety and serviceability of the antenna and cable). Obviously, in some cases, these difficulties can be avoided by abandoning the antenna on the roof and using variants of balcony or even window sill antennas. Of course, their effectiveness largely depends on the height of the suspension above ground level, however, with proper construction, you can achieve good results and get an antenna with very good parameters that can work well even at the level of the second floor. It should be noted that in this case we are not talking about balcony antennas of the "BOOMERANG" class, which are not widely used due to their low parameters (band in terms of SWR = 2.0 is not more than 30 ... 35 channels, the minimum SWR value is very difficult lower it below 1.3), as well as some design flaws - a weak attachment point and a relatively large length of the pin (its upper part practically looks into the neighbor's window from above). Readers are offered a description of a simple antenna, which is easily mounted on a windowsill, balcony, etc. and has relatively good parameters. It is based on the idea of a conventional car antenna - a shortened pin with an extension coil and an autotransformer matching device. There is a widespread belief that car antennas with a magnetic mount cannot be set up without a solid counterweight, which is the car body. Actually it is not. Difficulties, as a rule, are associated with the fact that the resonant frequency of the antenna shifts upward with a decrease in the counterweight and goes beyond the MW range. At the same time, due to a large mismatch, the antenna cable also begins to radiate, which makes measurements extremely difficult and leads to contradictory results when trying to tune the entire system. It is not difficult to make sure that by placing the antenna on the windowsill and extending it in some way by 25-30 cm, it is possible to achieve a resonance shift back, however, the minimum SWR value will most likely turn out to be unpleasantly high. Indeed, a longer antenna has a higher input impedance and requires additional matching with the supply cable. Thus, it is possible to use a standard car antenna as a stationary one. In this case, the necessary alterations are reduced to lengthening the pin (in the simplest case, using the usual "crocodile" and a piece of elastic wire) and increasing the lower part of the matching autotransformer by 0.5 ... 1.5 turns. To fine-tune the antenna, you can use the technique below. At the same time, such a path can hardly be considered expedient. Firstly, it's a pity to spoil a good thing and, secondly, not all antennas can be painlessly disassembled to gain access to the matching device. Therefore, below is a description of the design of a simple homemade antenna, which does not contain any expensive components and allows known deviations in the dimensions of minor parts. The design of the antenna, indicating the main dimensions, is shown in Figures 1 and 2. As can be seen from the figures, the antenna consists of the following blocks: a composite pin ( 1,2 ), a block of counterweights ( 3,4 ), a matching device ( 9 ) and a rod with fasteners (5,6,7,8,10).
The composite pin is a metal tube 2 with a diameter of 7 ... 9 mm (preferably made of copper or duralumin), into which a steel wire with a diameter of 2.5 ... 4 mm is inserted. This design makes it possible to obtain high mechanical strength and, if necessary, to easily change the length of the antenna. The block of counterweights is made of foil fiberglass (thickness 3 .. 4 mm) in the form of a circle 4, to which wire beams 3 are soldered. For their manufacture, copper wire with a diameter of 2 .. 3 mm can be used. To mount the antenna, a T-shaped rod (6,7) was used, made of three metal corners screwed together. Corner 7 serves to fasten the rod to the bottom board of the window frame. The block of counterweights is screwed to the rod through the bushings 8 with long screws or studs 5. To make the whole structure more rigid, it is desirable to unfasten the antenna pin with wooden rails 10, as shown in fig. one. In more detail, one should dwell on the design of the matching device, which is shown in Figure 3. Autotransformer coils are wound on the central insulator (textolite, ebonite, etc.) with a diameter of 18 ... 22 mm. Two threaded studs are inserted along its axis, which are fixed with brass screws, which simultaneously serve as platforms for soldering wires and an antenna cable. The antenna pin is screwed onto the upper stud either directly or through a threaded bushing, and by means of the lower stud the matching device is screwed to the counterweight block. It is advisable to close the matching device from above with some kind of non-conductive casing - for example, a plastic cup of suitable size.
For autotransformer coils, copper wire with a diameter d of at least 1.5 mm should be used. The location of the soldering of the central core of the cable may vary for each specific antenna, so you should strip the insulation at several points. When assembling the antenna, special attention should be paid to the reliability of all electrical contacts and their moisture resistance.
The following table shows the design parameters of the matching device, which ensured the fine tuning of the described antenna:
Antenna tuning is carried out in two stages. The first of them is training and can take place directly in the room. In houses with reinforced concrete floors, the antenna should be raised above the floor to a height of 30 ... 40 cm. To reduce the effect of reinforcement on the properties of the counterweight. In this case, it is desirable that the upper end of the pin be as far as possible from the network wires and the walls of the room. The actual tuning of the antenna consists in changing the length of the pin and measuring its SWR in the widest possible frequency band. Here one should not be lazy and limit oneself to two or three dimensions, because at this stage, it is not the absolute values of the SWR that are important, but the clarification of the trend of their change in the frequency band. It is advisable to record the received data, because the difference in values may be very small. The purpose of these measurements is to find the resonance and shift it to the middle of the desired frequency grid by changing the length of the pin. It should be remembered that a decrease in the SWR value with increasing frequency (in the direction of the grid F) requires an extension of the pin and vice versa. With a successful outcome of this operation, it is necessary to select the point of connection of the antenna cable to the autotransformer of the matching device to achieve the minimum SWR value at resonance. An additional sign of successful tuning is the stability of the SWR values with different cable positions and touching the cases of the radio station and the SWR meter with your hands. It should be borne in mind that when installing the antenna outside the window, its parameters will certainly change. Therefore, even at the first stage, it is desirable to provide for the possibility of adjusting the matching device. For this purpose, we can recommend a tuning method using a short-circuited loop, as is done, for example, in some stationary antennas. At the same time, one should not seriously fear that the efficiency of the antenna from this, allegedly, will sharply decrease. A well-made short-circuited coil - from a strip of thick copper foil 15 ... 30 mm wide, carefully soldered at the junction, has very low resistance and slightly reduces the quality factor of the system. In practical terms, care must be taken to ensure that this ring is located as close as possible to the turns of the coil, is well isolated from it and can move along its axis with little effort. The process of tuning the antenna in this way will be illustrated below. After fixing the antenna outside the house, you need to repeat the tuning manipulations. In the case of a particularly incorrect setting inside the building, it may be necessary to both change the length of the pin and solder the tap of the matching device, however, this should not cause difficulties, taking into account the experience gained at the first stage. It is recommended to tune the antenna to resonance in grid A and using a short-circuited loop shift it up in frequency to the desired channel. After completing the tuning process, you need to close the matching device with a casing and make sure that the suspension and struts are mechanically strong. The frequency response of the described antenna is shown in Figure 4. The measurements were carried out under the following conditions: power level - 10 W (r/st. Dragon SY101 + PANDA 10ST), SWR-430 SWR meter. The same figure shows the characteristics of the "BOOMERANG-SYRIO -27A" antenna (type - GP, total length 3.2 m). Their comparison convinces of certain advantages of a homemade antenna. During practical tests, this antenna, being installed at the level of the twentieth floor, provided an increase in the reception of long-range (55 ..65 km) correspondents up to 2 points compared to an antenna similar to the "BOOMERANG-SYRIO" antenna. And, in conclusion, once again about tuning the antenna with a short-circuited loop. Figure 5 shows the corresponding change in the frequency response of the described antenna for a fixed pin length. It can be seen from the figure that when the coil is pushed onto the matching device, the resonance shifts in the desired direction and the working band of the antenna expands somewhat.
Publication: krasnodar.online.ru/hamradio
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