INTRODUCTION
The wide spread of mobile communications at 27 MHz acutely raises the question about the antennas for such communications.
This question is complicated by the fact that the use of a quarter-wave antenna, the length of which is for the range of 27 MHz to 2.7 meters, in many cases unacceptable. The use of shortened antennas is associated with a number of specific issues in the popular literature is not considered, but the ignorance of which the effectiveness of SV-communication can significantly deteriorate.
Portable CB radios are primarily used unbalanced whip antenna. This is due to the fact. what other antenna types just almost impossible to use with this type of radio.
1. WORK ELECTRICALLY SHORT ANTENNAS FOR PORTABLE STATIONS
Electrically short antenna consists of the antenna, which vklyuchav radiating element, and the elements of the system of approval and system grounding. In accordance with the total resistance of the antenna Ra consists of a resistor pin (Rш) and resistance to ground (Rз) (Fig.1).
Included in the formula and the environment resistance" RS. which decreases with an increase in the number of counterweights and the length of the antenna.
Ra=Rш+Rз+Rcp
Useful RF energy is dissipated in Rш, so you need to aim to reduce quantities Rз and Rсp. In the General case, using special methods, you can measure the resistance to ground, but in practice it can be assumed that the resistance of the hull CB radios length of 20 to 30 cm, are used as counterweights, the length of this formula is the value of not less than 150...300 Ohm.
Contact with the human hand inconsequential changes well value. But the connection of a resonant quarter-wave counterweight length of 2.7 meters reduces the resistance of the earth Rз. Already one counterweight reduces the resistance Rз approximately to a value of not more than 50...60 Ohms. and if there are three or four counterweights can be considered Rз negligibly small value 5...10 Ohms. The resistance of the medium is determined by the interaction of the probe antenna with its "ground" system. If a full-sized quarter-wave whip antenna this interaction occurs in a large area and consequently has little value, then shortened antennas, the electromagnetic interaction is short antenna with a short counterweight happenin g in a limited volume of space. Moreover, any intervention in this volume significantly changes the resistance of the medium, I. therefore, has a significant influence on the parameters of this antenna system. Moreover, in such an antenna system with a shortened elements significant increase in one of them. for example pin to the size of a quarter wave, or counterweight, does not cause a significant reduction Rcp. And only the increase (i.e. elongation) as pin and counterweight causes a drop in Rcp.
From this we can conclude that the resistance of the short antenna SV-station - value is not a constant but a variable that, in particular, depends on the position of foreign objects (including operator) relative to the antenna.
In the General case, a well-coordinated antenna under the influence of these factors can fully rassoglasovaniya.
From this it follows that the output stage of the transmitter SV-radiostyle must be constructed so that such a mismatch is not significantly influenced his work, and to eliminate the causes of the error output stage continued to function normally. This requires that the output transistor had 3...4x the power reserve. Also needed is a compromise matching circuit of a P-loop. to allow operation on a complex variable load. Should be removed when changing the excitation of the antenna parameters. Already these requirements. presented to the output cascades of SAINT portable stations, show that the approach to their design is extremely serious. For mobile vehicle radios operating on stationary car antenna requirements for RA are much lower. This is due to the use as a counterbalance to the chassis of the vehicle, which is a good ground for a CB antenna. The pin used for car CB antenna. has a length of about one meter, and in many cases longer. This creates the preconditions for the work of a car antenna with a much bigger effect than the antenna of the portable station. It is significant that in the interaction zone displacement currents in the system pin of the antenna - opposed ' no foreign objects, making Rсp for these antennas is more stable than in portable stations.
Of all existing antennas CB portable stations can be divided into two groups - resonant and non-resonant antenna. Among shortened whip antennas from the group can distinguish resonant helical antenna and the whip antenna extended the inductance. Among the non-resonant whip antennas, it is advisable to use only one type - short pin in the composition of the output of the resonant circuit. In this case, the pin is contoured by a capacitor having a distributed capacity.
2. HELICAL ANTENNA
Helical antenna can be considered as an open spiral resonator [1]. In this case, the antenna is a helical resonator, the circuit matching circuit of the transmitter - continuation of the spiral resonator and is included in the circuit of its initiation, and the external space can be regarded as infinitely remote screen (Fig.2).
Fig.2
The validity of these assertions are easily checked in practice. So, when you change the parameters of the matching circuit changes the resonant hour Utah antenna system. Even a very slight change in the terminal capacitance of the antenna strongly changes its resonance frequency [2]. And a helical antenna are greatly affected by foreign objects. Already the approach of the hand at a distance of 20 cm leads to misalignment of the antenna to the transmitter, because due to the change in terminal container changes its resonant frequency. Here it is appropriate to carry out the adjustment according to the method proposed in [3]. It lies in the fact that the helical antenna set up so that the hands on approach (or because of other massagesaudio influence) the field strength of the signal increases and then decreases. In this case, the dish is not exactly in resonance, and a little away from him.
As the field strength measurement, in this case, the field strength is about 85% of the field strength at the exact resonance. But when testing the radio with an antenna tuned to resonance, and with an antenna tuned to the slope characteristics of the antenna, its advantages are obvious. Thus, with the use of the station with a resonant antenna in the process of radio communication when approaching the antenna to the man there were significant fluctuations in the field strength. When using the same radio with an antenna that is configured to slope characteristics, massagesaudio human influences manifested much weaker and fluctuations in the field strength was negligible. On this basis, we recommend to set a helical antenna according to the method proposed in [W]. Only if the helical antenna operates in conditions where it is possible influence of rassipassi factors, you can configure the antenna to the maximum field strength.
When measuring the field strength provided by the helical antenna and a whip antenna with a lengthening coil, it was found that tuned in resonance with the whip antenna length not less than three times greater. than the test helical antenna, provided the same field strength. From this we can conclude that in portable stations are the most optimum choice of antenna is a spiral, which is stronger and simpler in design than the same but the parameters whip antenna. Thus it is necessary to activate that in this case short case radio is the best ground for a helical antenna than for the same parameters whip. But spiral antenna. providing large field strength, creates the preconditions for unstable operation of the transmitter.
Indeed, in the experiments it was found that the same transmitter, steadily working with external antenna cable power supply, by connecting it to the helical antenna was excited. Only a more thorough screening and the tuning of the impedance matching circuit is allowed to operate the transmitter with a spiral antenna without excitation.
Helical antenna, as well as girevoy, you can configure the frequency with shortening capacity and lengthening inductance. The use of a tank increases the resonance frequency of the antenna, and the use of inductance lowers it. In this case, to improve the efficiency of the antenna is required to lengthening the coil was probably smaller inductance, and shortening capacity - the greatest possible value. The use of such setting items allows you to use a helical antenna in a wide range of frequencies, because depending on the performance and the quality of matching bandwidth helical antenna is small and is 200...300 kHz in the range of 27 MHz.
There is another very important point when using helical antennas. When connected like this antenna via coaxial cable resonant frequency due to the introduction of the reactivity of the cable in the complex impedance of the antenna and, consequently change it. changed and needs to be adjusted.
When building a spiral antenna, for that matter, any other shortened antenna, you should pay attention to another feature of this antenna system consists in the fact. connecting a quarter-wave counterweight changes the resonant frequency of this antenna system. This can be explained by the fact that opposed to having their Rз, changes Rсp. There are also changes to the capacity of the "antenna - space". Expanding the bandwidth of a helical antenna about 1.5...2 times by reducing its q-factor and at the same time - due to more efficient radiation. Basically, the experimental study of the resonance frequency of the spiral with quarter-wavelength balances did not exceed the bandwidth limits of the antenna. At the same time, the intensity of the field with a quarter-wave counterweight has increased by at least twice.
Helical antenna should be connected as short as possible conductors to the output matching circuit. This helps secure the necessary bandwidth and minimum spurious emission line connector.
3. PRACTICAL DESIGN OF SPIRAL ANTENNAS
Below describes the practical design of spiral antennas, published in the recent literature. The parameters of the antennas were measured using antennasia.
The spiral antenna, the design of which is shown in Fig.3, was published in [4]. Testing this antenna has shown that this quarter-wave antenna is on the 21 MHz band. Indeed, together with the quarter-wave resonant counterbalance the resistance of the antenna here was of the order of 40 Ohms. with a small reactivity.
When connecting the antenna to the transceiver with a power of 40 W via coaxial cable with a length of about ten meters and the antenna are positioned in the window opening managed to spend a few links to 21 MHz with RST56-58, which further strengthened my opinion of her true resonance. But by tweaking coils and capacitance, as shown in [4]. managed to establish that in the range of 27 MHz is possible resonance corresponding to the equivalent length of the antenna in half the wavelength.
The bandwidth of the antenna on the 21 MHz band was 200 Hz, in the range of 27 MHz to 250 kHz with a quarter-wave counterpoise.
Spiral antenna whose data is shown in Fig.4, refers to a quarter-wave antennas. Using a riser pin it can be rebuild in a wide range from 26 MHz to 35 MHz. The range of 27 MHz her input Impedance of the transceiver was 1300m and bandwidth - 650 kHz. With a quarter-wave counterweight 65 Ohms. Bandwidth was 800 kHz. resonance has shifted to 200 kHz upwards. It should be noted that this method of adjusting the resonant frequency of the antenna though quite good in its simplicity and efficiency, but reduces the quality factor of the spiral resonator and, as a consequence, reduces the efficiency of the antenna. This is reflected in the decrease of the field strength and the extension of the bandwidth of the antenna.
The spiral antenna is shown in Fig.5 [5], when tested on anendoscope showed no resonance in the range of 27 MHz and showed quarter-wave resonance in the 21 MHz band. Together with a quarter-wave counterbalance the resistance here was 25 Ohms bandwidth of 250 kHz. But when using Systemy approval given radio station [5], it was found that in fact in the range of 27 MHz is achievable resonance. Obviously, here the resonance of the antenna is not at the expense of her work as a quarter-wave resonator, and as P-Kongur with distributed capacity. In this case, the helical antenna is equivalent to the system P-circuits included in the output of the transmitter, the capacity is the capacity of the antenna to the ground. The radiation occurs due to the settings in the resonance of the whole system of P-circuits of the transmitter. However, field strength measurements showed that in this case the use of a helical antenna is inefficient. The same field strength can provide tuned in resonance with the assistance of the extension coil whip antenna with a length of only 1.3 times greater than the length of the helical antenna.
Helical antenna shown in Fig.6 [6] showed the input impedance at the resonant frequency range of 27 MHz To 110 Ohms with the shell station and 40 Ohm quarter-wave counterweight. The bandwidth with the building of the station was 300 kHz. the counterweight is 450 kHz. Due to the fact. that its upper portion is wound with the discharge, the effect of human body on the setting of this antenna is not so strong as in the case of a continuous winding. Connecting a quarter-wave counterweight changed the resonance frequency of 200 kHz upwards.
We investigated the antenna used in radio type "Kolibri-M2". Its construction is shown in Fig.7. In the range of 27 MHz this antenna showed a resistance of 100 Ω and a bandwidth of 300 kHz with the building of the station, and the resistance of 47 Ohms and a bandwidth of 200 kHz with a quarter-wave counterpoise. Connecting a quarter-wave counterweight changed the resonance frequency of 120 kHz up. Exactly antenna shown in Fig.5 and 6. provided the field strength. comparable with the field strength, develop whip antenna with a lengthening coil, with the length of the pin, three times the length of such a helical antenna.
A practical view of the frequency response of the last two antennas shown in Fig.8. From this figure it is seen that the frequency response of the antenna is unbalanced. When connecting a quarter-wave counterweight AFC few mixed up about. 100 kHz for a range of 27 MHz, the bandwidth of the antenna allows it to operate in the ne-channels. Knowledge of the frequency response of a spiral antenna allows you to properly configure it - not in the middle of the operating range, and slightly higher.
4. PRODUCTION AND TUNING OF SPIRAL ANTENNAS
In the literature it is recommended to perform a helical antenna on a plastic core of the coaxial cable. Indeed, this is the ideal material for such an antenna. Cable for manufacturing a helical antenna, it is desirable to use 75 Ohm, because it usually contains a single inner conductor, which can easily be pulled out with pliers, holding the cable at the other end in a vise. If you use for the frame antenna 50 Ohm cable, which usually has a center conductor, consisting of several copper wires can be difficult to remove them.
The simplest way is to heat the conductors passing through them a current of 50...100 A with the help of some powerful current source. and then quickly pull.
Plastic frame has after Stripping rough surface, which facilitates the winding of the wire with tension. It should be remembered that the spiral antenna is a high-q system, and if it is done carelessly, under the influence of temperature resonant frequency may go beyond the range for which it is configured. In the study of spiral antennas found that their resonant frequency is shifted by 50...80 kHz up when cooled to a temperature of -15°C. the Antenna should be wrapped tightly with electrical tape to avoid bias coils. and consequently, changes the resonant frequency. Fits flexible PVC electrical tape. Sticky tape "Scotch" is not appropriate for this purpose because of its stiffness.
It should be noted that the helical antenna is unbalanced system. K. the transmitter should be connected to the same end, which is listed in its description. When connecting the antennas shown in Fig.6 and 7, the other end, they will have a completely different resonances, far ecstasie from the range of 27 MHz. Even with a change of end connections like this, it would seem. symmetrical antenna as in Fig.5, there is a shift of its resonance due to some irregularity in the execution of the antenna.
Structurally, it is convenient to perform its end connected to the transmitter, using the connector SR-50, SR-75. by zaplavlenie back of the plastic base of the antenna. From the metal frame of the connector prior to the winding of the spiral must be at least 12 mm In the manufacture of the antenna is not necessarily seek to use the basics of a specified diameter. The retreat of 2...3 mm is acceptable. For example you can use instead of 7-millimeter plastic bases 9 mm, it can also be used instead of 12 mm. Although the parameters of the antenna are thus changed, it can be set in the range of 27 MHz.
Adjust the antenna, as indicated in the description, by unwinding of the turns from the more dense winding. In Sluchak manufacturing of all antennas described here was able to configure the range of 27 MHz by unwinding part turns. i.e. they have been pre-designed for a resonant frequency slightly below 27 MHz. For efficient operation of the antenna should have a good ground station, for example a metal case. If there is none, you need to lay in a convenient location on the entire length of the station copper or wide aluminum foil. Such a counterweight gives an increase of the field strength by about 15...20%, which is about as increases the range of communication. In some cases, it helps to remove the excitation of the transmitter.
The dimensions of the helical antenna can be considered optimal when its length is approximately 20% greater than the length of the hull of counterbalance. If the antenna is smaller than this value. increases the influence of the human body and other foreign objects. A further increase does not cause the same increase of the field strength, it is easier to use a quarter-wave counterweight to increase the communication range.
(Continued)
Author: I. Grigorov (RK3ZK, UA3-113), Belgorod; Publication: N. Bolshakov, rf.atnn.ru