Small-sized antennas for portable CB communication stations (Part 1). Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / HF antennas

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INTRODUCTION

The widespread use of mobile communications on 27 MHz raises the issue of antennas for such communications.

This issue is complicated by the fact that the use of quarter-wave antennas - the length of which is 27 meters for the 2,7 MHz band - is in many cases unacceptable. The use of shortened antennas is associated with a number of specific issues that are not considered in the popular literature, but if they are not known, the effectiveness of MW communications can significantly deteriorate.

For portable CB radios, asymmetrical whip antennas are mainly used. It's related to that. that other types of antennas are simply almost impossible to use with this type of radio.

1. OPERATION OF ELECTRICLY SHORT ANTENNAS OF PORTABLE STATIONS

An electrically short antenna consists of both the antenna itself, which includes a radiating element, and elements of its matching system and its grounding system. In accordance with it, the total resistance of the antenna Ra consists of the resistance of the pin (Rsh) and the resistance of its grounding (Rg) (Fig. 1).

Included in the formula and the "resistance of the environment" Rav. which decreases with an increase in the number of counterweights and the length of the antenna.

Ra=Rsh+Rz+Rcp

Useful RF energy is dissipated by Rsh, so you need to strive to reduce the values ​​of Rc and Rcp. In the general case, using special methods, it is possible to measure the resistance of the "earth", but for practice it can be assumed that the resistance of the housing of the CB radio station 20 ... 30 cm long, used as counterweights, for this formula is at least 150 ... 300 Ohm.

Contact with a human hand does not significantly change the value. But connecting a resonant quarter-wave counterweight 2,7 meters long reduces the earth resistance Rz. Already one counterbalance reduces the resistance Rz approximately to a value of not more than 50 ... 60 Ohm. and in the presence of three-four counterweights, Rz can be considered a negligible value of 5 ... 10 ohms. The resistance of the medium is determined by the interaction of the antenna pin with its "earth" system. If in a full-sized quarter-wave whip antenna this interaction occurs in a large space and therefore has an insignificant value, then in shortened antennas, the electromagnetic interaction of a short antenna with a short counterweight occurs in a limited volume of space. Moreover, any intervention in this volume significantly changes the resistance of the medium, and. therefore, has a significant impact on the parameters of such an antenna system. Moreover, in such an antenna system with shortened elements, a significant increase in one of them. for example, a pin to a quarter-wave value, or a counterweight, does not cause a significant decrease in Rcp. And only an increase (i.e. elongation) of both the pin and the counterweight causes Rcp to drop.

Already from this we can conclude that the resistance of the short antenna of the CB-station is not a constant value, but a variable, which, in particular, depends on the position of foreign objects (including the operator) relative to the antenna.

In the general case, a well-matched antenna under the influence of these factors can completely mismatch.

It follows from this that the output stage of the MW radio station transmitter must be designed so that such a mismatch does not significantly affect its operation, and that when the causes of the mismatch are eliminated, the output stage continues to function normally. To do this, it is necessary that the output transistor has a 3 ... 4-fold power margin. A compromise version of the P-loop matching circuit is also needed. allowing work on a complex variable load. It is necessary to eliminate self-excitation when changing the parameters of the antenna. Already these requirements. presented to the output stages of CBs of portable stations show that it is worth taking their design very seriously. For a mobile car radio operating on a stationary car antenna, the requirements for RA are much lower. This is due to the use of the car body as a counterweight, which is a good "ground" for the MW antenna. A pin used for a car CB antenna. has a length of about a meter, and in many cases even longer. This creates the prerequisites for the operation of a car antenna with a much greater effect than a portable station antenna. It is also significant that there are no foreign objects in the zone of interaction of bias currents in the "antenna pin - counterweight" system, which makes Rсp for such antennas more stable than in portable stations.

Of all the existing types of MW antennas for portable stations, two groups can be distinguished - resonant and non-resonant antennas. Among shortened whip antennas from the resonant group, spiral antennas and whip antennas elongated by inductance can be distinguished. Among the non-resonant whip antennas, it is advisable to use only one type - a short pin as part of the output resonant circuit. In this case, the pin is a distributed loop capacitor.

2. SPIRAL ANTENNA

A helical antenna can be considered as an open helical resonator [1]. In this case, the antenna itself is a spiral resonator, the transmitter's matching circuit is a continuation of the spiral resonator and enters its excitation circuit, and the outer space can be considered as an infinitely distant screen (Fig. 2).

Ris.2

The validity of these assertions is easily verified in practice. So, when the parameters of the matching circuit change, the resonant part of the antenna system changes. Even a very slight change in the end capacitance of the antenna greatly changes its resonant frequency [2]. And helical antennas are highly susceptible to the influence of foreign objects. Already approaching a hand at a distance of 20 cm leads to a mismatch between the antenna and the transmitter, because due to a change in the terminal capacitance, its resonant frequency changes. Here it is appropriate to carry out tuning according to the method proposed in [3]. It consists in the fact that the helical antenna is tuned so that when the hand approaches (or due to other mismatching influences), the signal field strength increases and then decreases. In this case, the antenna is tuned not exactly to resonance, but slightly away from it.

Field strength measurements show that in this case the field strength is about 85% of the field strength at exact resonance. But when testing a radio station with an antenna tuned to resonance, and with an antenna tuned to the antenna slope, the advantages of the latter are obvious. So, when using a station with a resonant antenna in the process of radio communication, when the antenna approached a person, significant fluctuations in the field strength occurred. When using a radio station with an antenna tuned to the slope of the characteristic, the mismatching influence of a person manifested itself much weaker and the fluctuation of the field strength was insignificant. Based on this, it can be recommended to tune helical antennas according to the method proposed in [XNUMX]. Only if the helical antenna operates under conditions where the influence of mismatching factors is excluded, it is possible to tune the antenna to the maximum field strength.

When measuring the field strength provided by the helical antenna and the whip antenna with an extension coil, it turned out that the resonant whip antenna was at least three times longer. than the helical antenna under test provided the same field strength. From this we can conclude that in portable stations the most optimal antenna option is a helical one, which is stronger and simpler in design than a whip antenna of the same parameters. In this case, it must be taken into account that in this case the short body of the radio station is a better "ground" for a helical antenna than for a whip antenna of the same parameters. But the spiral anenna. providing a large field strength, creates the preconditions for unstable operation of the transmitter.

Indeed, during the experiments it turned out that the same transmitter, which worked stably with an external cable-fed antenna, was excited when a spiral antenna was connected to it. Only more careful screening and adjustment of the matching circuits allowed the transmitter to work with a helical antenna without self-excitation.

A helical antenna, as well as a whip antenna, can be tuned to an operating frequency using a shortening capacitance and lengthening inductance. The use of capacitance raises the resonant frequency of the antenna, while the use of inductance lowers it. In this case, to increase the efficiency of the antenna, it is necessary that the extension coil be as small as possible inductance, and the shortening capacitance - as large as possible. The use of such tuning elements allows the use of a helical antenna in a wide frequency range, since, depending on the design and quality of matching, the bandwidth of the helical antenna is small and amounts to 200 ... 300 kHz in the 27 MHz range.

There is another very important point when using helical antennas. When such an antenna is connected through a coaxial cable, its resonant frequency is due to the introduction of the reactivity of the cable into the complex impedance of the antenna and, accordingly, its change. changes and needs to be adjusted.

When building a helical antenna, as, indeed, any other shortened antenna, one should pay attention to one more feature of this antenna system, which is that. that when a quarter-wave counterweight is connected, the resonant frequency of this antenna system changes somewhat. This can be explained by the fact that the counterweight, which has its own Rz, changes Rsp. The capacitance "antenna - space" also changes. The bandwidth of the helical antenna is expanded by about 1.5...2 times due to a decrease in its quality factor and, at the same time, due to more efficient radiation. Basically, in the experimental study of the resonance frequency of the helix with quarter-wave balances did not go beyond the bandwidth of the antenna. At the same time, the field strength with a quarter-wave counterbalance increased by at least two times.

The helix antenna should be connected with as short conductors as possible to the output matching circuit. This allows you to provide the necessary bandwidth and minimal spurious radiation of the connecting line.

3. PRACTICAL DESIGNS OF HELICAL ANTENNAS

Below are considered practical designs of helical antennas published in the literature of recent years. Antenna parameters were measured using an antenoscope.

A helical antenna, the design of which is shown in Fig. 3, was published in [4]. Tests of this antenna showed that this antenna is quarter-wave on the 21 MHz band. Indeed, together with a resonant quarter-wave counterbalance, the antenna resistance here was about 40 ohms. with little reactivity.

Connecting such an antenna to a 40 W transceiver via a coaxial cable about ten meters long and placing the antenna in a window opening, I managed to make several connections at 21 MHz with RST56-58, which further strengthened my opinion about its true resonance. But still by adjusting the turns and capacitance, as shown in [4]. it was possible to establish that in the range of 27 MHz its resonance is possible, corresponding to the equivalent antenna length of half a wavelength.

The bandwidth of the antenna on the 21 MHz band was 200 Hz, on the 27 MHz band - 250 kHz with a quarter-wave counterbalance.

The spiral antenna, the data of which is shown in Fig. 4, refers to quarter-wave antennas. With the help of an add-on pin, it can be tuned over a wide range - from 26 MHz to 35 MHz. On the 27 MHz band, its input impedance with the body of the radio station was 1300m and the bandwidth was 650 kHz. With 65 ohm quarter wave counterbalance. The bandwidth was 800 kHz. the resonance shifted 200 kHz up. It should be noted that this method of adjusting the resonant frequency of the antenna, although quite successful in its simplicity and efficiency, still reduces the quality factor of the spiral resonator and, as a result, reduces the efficiency of the antenna. This is reflected in the reduction of the field strength and in the expansion of the antenna bandwidth.

The helical antenna shown in Fig. 5 [5], when tested on an antenoscope, did not show resonance in the 27 MHz band and showed a quarter-wave resonance in the 21 MHz band. Together with a quarter-wave counterbalance, its resistance here was 25 ohms with a bandwidth of 250 kHz. But when using the matching system of the given radio station [5], it was found that, in fact, resonance is achievable in the 27 MHz range. Obviously, here the resonance of the antenna occurs not due to its operation as a quarter-wave resonator, but as a P-congur with a distributed capacitance. In this case, the helical antenna is equivalent to a system of P-circuits connected to the output of the transmitter, the capacitance of which is the capacitance of the antenna to ground. Radiation occurs due to the tuning of the entire system of P-circuits of the transmitter into resonance. However, field strength measurements have shown that in this case the use of a helical antenna is inefficient. The same field strength can be provided by a whip antenna tuned to resonance with an extension coil that is only 1,3 times longer than the length of this helical antenna.

The helical antenna shown in Fig. 6 [6] showed an input impedance at the resonant frequency of the 27 MHz range of 110 ohms with the station body and 40 ohms with a quarter-wave counterbalance. The bandwidth with the station body was 300 kHz. with a counterweight - 450 kHz. Thanks to. that its upper part is wound with discharge, the influence of the human body on the tuning of this antenna is not as strong as in the case of continuous winding. Connecting a quarter-wave counterweight changed the resonance frequency by 200 kHz up.

The antenna used in the Kolibri-M2 type radio station was investigated. Its design is shown in Fig.7. In the 27 MHz band, this antenna showed 100 ohms impedance and 300 kHz bandwidth with the station body, and 47 ohms impedance and 200 kHz bandwidth with a quarter-wave counterbalance. Connecting a quarter-wave counterweight changed the resonance frequency by 120 kHz up. It was the antennas shown in Figs. 5 and 6 that provided the field strength. comparable to the field strength developed by a rod antenna with an extension coil, with a rod length three times the length of such a helical antenna.

A practical view of the frequency response of the last two antennas is shown in Fig. 8. From this figure it can be seen that the frequency response of the antenna is asymmetric. When a quarter-wave counterweight is connected, the frequency response will slightly mix up - by about. 100 kHz for the 27 MHz band, however, the antenna bandwidth allows it to work in the MW channels. Knowing the frequency response of a helical antenna allows you to properly tune it - not in the middle of the operating range, but a little higher.

4. MANUFACTURE AND ADJUSTMENT OF SPIRAL ANTENNAS

In the literature, it is recommended to perform helical antennas on the polyethylene core of the coaxial cable. Indeed, this is the best material option for such an antenna. It is desirable to use a 75-ohm cable for the manufacture of a helix antenna, because it usually contains a single inner conductor, which can be easily pulled out with pliers by holding the cable itself at the other end in a vise. If you use a 50 ohm cable for the antenna frame, which usually has a center conductor consisting of several copper wires, it may be difficult to remove them.

The simplest way out is to heat the conductors by passing a current of 50 ... 100 A through them using some powerful current source. and then quickly pull them out.

The polyethylene frame has a rough surface after removal of the braid, which facilitates winding the wire with tension. It should be remembered that a helical antenna is a high-Q system, and if it is not done carefully, under the influence of temperature, its resonant frequency can go beyond the range for which it is tuned. In the study of helical antennas, it was found that their resonant frequency shifts by 50...80 kHz upwards when they are cooled to a temperature of -15°C. The antenna must be tightly wrapped with electrical tape to prevent coils from shifting. and consequently, changes in the resonant frequency. Flexible PVC tape is suitable for this. Scotch tape is not suitable for this because of its excessive rigidity.

It should be noted that the helical antenna is a non-symmetrical system. It should be connected to the transmitter only with the end indicated in its description. When connecting the antennas shown in Figures 6 and 7 at the other end, they will have completely different resonances, far removed from the 27 MHz band. Even with a change in the end of the connection, it would seem like this. symmetrical antenna as in Fig. 5, its resonance is shifted due to some asymmetry in the execution of the antenna.

Structurally, it is convenient to carry out its end, connected to the transmitter, using the SR-50 or SR-75 connector. by melting the plastic base of the antenna there. There must be at least 12 mm from the metal frame of the connector to the beginning of the winding of the spiral. In the manufacture of the antenna, it is not necessary to strive to use the base of the specified diameter. A deviation of 2 ... 3 mm is quite acceptable. For example, it can be used instead of a 7 mm polyethylene base 9 mm, it can also be used instead of 12 mm. Although the parameters of the antenna change, it is quite possible to tune it to the 27 MHz band.

The antennas are tuned, as indicated in the description, by unwinding the turns from the side of the denser winding. In the case of the manufacture of all the antennas described here, it was possible to tune to the 27 MHz range by unwinding part of the turns. those. they were pre-calculated for a resonant frequency just below 27 MHz. For effective operation of the antenna, it is necessary to have a good station ground, such as a metal case. If there is none, it is necessary to lay copper or aluminum wide foil in a convenient place along the entire length of the station. Such a counterbalance gives an increase in the field strength by about 15 ... 20%, which approximately also increases the communication range. In some cases, it helps to remove the self-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 body-counterweight. If the antenna is less than this value. the influence of the human body and other foreign objects on it increases. A further increase in it does not cause the same increase in field strength, it is easier to use a quarter-wave counterbalance to increase the communication range.

(Continued)

Author: I. Grigorov (RK3ZK, UA3-113), Belgorod; Publication: N. Bolshakov, rf.atnn.ru

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