5. Resonant whip antenna, elongated inductance
In portable and mobile CB-radio stations use antenna length of 30... 100 cm for portable and up to 1.5 meters for mobile radio stations. Calculating the input impedance of the active parts of these short pins for frequency of 27 MHz, the resulting values from 0.5 Ohm to 30 cm to 10 Ohm to 1.5 m. of Course, to connect such short pins to the output stage of the transmitter without the corresponding permission is unwise. Firstly, the low efficiency of the this pin as antennas, secondly, matching the low impedance of the pin with the output stage of the transmitter is very difficult. The most rational decision, which came solving this problem, was that the pin is part of a complex system, which is a shortened antenna. Next, it examines the efficiency of the pin in such a system.
Classic whip antenna is a vibrator length of a quarter wavelength and the ground system under it. In the simplest case, the grounding system is a system of quarter-wave balances. Naturally, such a system to use for a portable station is difficult. Therefore, try to shorten the antenna and balances. The simplest in this case to include lengthening the coil in the antenna. But here is the question, in what point of the antenna to enable the extension coil for maximum effect. The role of the system of checks plays a casing of the station.
Should pay attention to the most effective way of lengthening short antenna - enable lengthening of the coil in its base (Fig. 9). The maximum current flowing through the antenna at its base. From antenna theory it is known that to obtain the maximum radiation of the antenna and, consequently, its maximum efficiency, it is necessary to provide the maximum current in a radiating antenna element and the maximum voltage at its emitting end. Here the maximum current flows through the coil, therefore, the maximum interaction with the environment occurs through the coil.
The advantages of the antenna with a lengthening coil at the base is only that due to the large capacitance of the pin such antennas have a relatively large bandwidth, allowing them to work in all CB or Amateur bands.
Another type of antenna is antenna, an elongated coil in its middle (Fig. 10). Here is already achieved significant current in the antenna socket, the upper part of the pin plays the role of a capacitive load. Due to the increase in terminal capacity increases the bandwidth of the antenna to a value that enables you to work throughout the ST range, significantly increases its efficiency.
Pin to the coil is the main radiating element, it should be made as fat, especially because he is holding the lengthening coil. The pin after the coil represents the capacitive load. It can be made thinner. Placing on the end of the antenna is even a small capacitive load increases its efficiency, but reduces the mechanical strength.
You should still pay attention to the fact that, in principle, in bad ground, taking place in portable radios, all types of short antennas work equally bad, and there is no significant difference when using them. But connecting a quarter-wave counterweight indicates the difference in effectiveness of different types of antennas. Also there is the effect in mobile car radio, where the vehicle body is an effective grounding.
The resistance of an ideal quarter-wave vertical antenna - pin on a perfect conducting surface is 36 Ohms. Resistance ideal shortened antenna CB range, depending on the degree of shortening is 10...20 Ohms. Given that the real "earth," these antennas are far from perfect, in the General case, the antenna can be coordinated with coaxial and power cable antenna in a mobile station (here typically use 50-Ohm cable), and with the output stage portable radio, bad "earth," which increases the resistance of a short antenna to 50...100 Ohms.
6.Practical design whip antennas, elongated inductance
Basically, all shortened antenna portable radios have the form shown Fig. 11. Coil inductance of about 2 mH and a pin length of about 120 cm represent the antenna system operating in the range of 27 MHz. And various versions of the coil and pin depend on the efficiency of the antenna and the bandwidth of its transmission. The antenna is depicted in Fig. 7, is shown in many other, earlier sources [7, 8,9, 10].
When testing antennas from [7, 8] used a identical lengthening coil 2 µh, and the following results were obtained.
The input impedance of the quarter-wave counterweight - 35 Ohms, with the housing of the radio - 80 Ohms. The bandwidth at the level of half-power (-3 dB) - 600 kHz with a counterweight, 750 kHz transceiver. Human influences exerted on the antenna is small and its low reactivity. The frequency offset when connecting a quarter-wave counterweight has reached 700 kHz.
When testing antennas from [9], where the pin length was 80 cm, lengthening the coil represented the 18 turns of wire PEL 0,55 wound on the frame with a diameter of 4 mm round, the following results were obtained.
The input impedance of the quarter-wave counterweight 60 Ohm, with the counterweight-body radio -1100m.
Bandwidth with quarter-wave counterweight - 800 kHz, the station - 900 kHz. The shift of the resonant frequency when connecting the counterweight to almost 1 MHz.
When testing antennas from [10] with the length of the pin 0,8... 1,2 m extension coil consisted of 25 turns of wire PEL 0,35 wound on the frame with a diameter of 5 mm round results have been obtained, similar to the antenna [9].
Of interest and short antenna - up to 50 cm moreover, these antennas are not as significantly lose in the long range antennas - length of about 1m.
Antenna from [11] is a pin with a length of 45 cm with a lengthening coil containing 60 turns of wire PEL of 0.5 on the frame with a diameter of 5 mm, is wound round. When testing such antennas the following results were obtained.
With a quarter-wave counterbalance input impedance - 75 Ohms, bandwidth - 700 kHz. With the building of the station in the role of a counterweight to the input impedance of 120 Ohms, bandwidth - 900 kHz. The shift of the resonant frequency when connecting a quarter-wave counterweight amounted to 1.2 MHz. The human influence on the antenna higher than in long antennas.
The increase in the input resistance and the extension of the bandwidth of a short antenna (45 cm) than long (1 m) suggests that lengthening coil short antenna nizkodobrotnoi. But the increase of the quality factor of the extension coil has little effect on the efficiency of these short antennas. Connecting the counterweight shifts the resonant frequency of the antenna up. For efficient operation of radio stations when you connect the counterweight in this case it is necessary to provide for prompt adjustment of the inductance of the extension reel.
Preferably the transceiver when the switch pin of the antenna use a different extension of the inductance to the receiver and transmitter. So you can optimally adjust the pin on both reception and transmission. Of course, if the impedance of the input receiver and output transmitter differ only slightly, you can do one of the extension coil, as in this case, the displacement of the resonant frequency of the system when switching the RX/TX is small. But here already it is necessary to solve practical terms, it is easier to switch lengthening coil or lead to the inputs of the transmitter and receiver to the same value. In "branded" equipment tend to last, and although there are options with the tuning of the receiver input when switching antennas. In a self-made instrument range of 27 MHz to the issue of matching antennas in receive mode and transfer often do not pay enough attention, which leads to lower efficiency portable radios.
In [12] described antenna with shoulder length 110 mm and lengthening coil in the center with 130 turns of wire PEL 0,15 wound coil to a coil on the frame having the diameter of 6 mm. When tested this antenna showed the following results. With a quarter-wave counterbalance the input impedance of 90 Ohms, bandwidth - 400 kHz, with a counterweight to the transceiver input impedance of 140 Ohms, bandwidth - 600 kHz. Offset bandwidth when connecting a quarter-wave counterweight was 900 kHz. Adding a capacitive load shown in Fig. 13, has reduced the frequency offset when connecting the counterweight to 600 kHz. Bandwidth is increased to 50 kHz in both cases. The input resistance decreased with the counterweight was 75 Ohms, with the building of the station is 90 Ohms. The field strength is increased 1.3 times. All this speaks about the advantages of capacitive load for these types of antennas. It should be noted that more effective capacitive load as shown in Fig. 12, but unfortunately, it is more complicated in practical implementation than the load in Fig. 13.
Comparison of the values of the field strength generated by the antenna with inductance and Central lengthening inductance at the base, showed that in practice the antenna with the Central inductance, equal to the height of the antenna with inductance at the base, creates a field strength of approximately 1.4... 1.6 times greater. Adding a capacitive load the advantages of such antennas increase even more. The measurements were carried out at the quarter-wave balances. When using the chassis of the radio station as a counterbalance to the advantage of the antenna with the Central inductance was weaker field strength was only 1.2 times more than that generated by the antenna with inductance at the base. This suggests that for mobile stations is not much difference in the type of whip antennas for mobile stations, it is better to use the antenna with a Central load inductance. In any case, it is desirable to use a capacitive load, even in the form of a ball with a diameter of 5 to 20 mm. Capacitive load gives the effect and when used with an antenna with a lengthening inductance at the base.
Almost for portable stations may use antennas of thick copper wire with a diameter of 2...2.5 mm Antenna of a smaller diameter is less durable and has less mechanical efficiency. For the manufacture of antennas for mobile stations can use a short "Kulikovka" or suitable antenna military radio stations from the corresponding length and, most importantly, strength.
7. Non-resonant whip antenna
Non-resonant whip antennas are the most inefficient of all existing short whip antennas. They lose on the field strength in 2...3 times as long whip antennas with a lengthening inductance, the antennas are much more sensitive to human impact. But they are still in use, however, mainly only two types of transmitters.
The use of such non-resonant antennas justified only in simple toys, a range which is not higher than 50... 100 m. For more effective communication it is necessary to use a resonant antenna, although in front of her and need to put an isolation cascades for the simplest schemes. As experience shows, simple Western radio stations, which consume more power than domestic "Hummingbird", but working on non-resonant antenna, provide a much lower DALnet connection.
The third use case of short non-resonant antennas is a wrong construction of the output stage of the transmitter with its chains of matching with the antenna. As a result, by connecting it to the normal resonance of the antenna, whether full-length or shortened, it is self-excitation. Although these transmitters often and have a P-loop output, its action is ineffective.
8. Magnetic loop antenna for portable CB radios
Magnetic loop antenna, I have not met in any of portable CB radios. But this does not mean that their use in this type of radio stations is impractical. I have made a magnetic loop antenna for the range of 27 MHz with the dimensions shown in Fig. 14.
The antenna showed the following results. Input impedance - 75 Ohms with very low reactivity. Bandwidth - 600 kHz. The antenna was made from two millimeter insulated copper wire type PEL, air variable capacitor was mounted on glass fibre laminate base. The antenna proved to be very insensitive to the influence of man and balances.
Since this antenna mainly radiates a magnetic component of an electromagnetic wave, it cannot strictly be compared indicators such as level of field strength with whip antenna, because the latter emits mostly electric component of the electromagnetic wave, and the measurements for the pin, should be provided on the electric component of emws, and the frame on the magnetic component of emws.
Two antennas shown in Fig. 14, was connected to the radio type "Kolibri-M" and has been tested communication range compared to the regular helical antenna. It turned out that, ceteris paribus, the transmission distance when using the magnetic antenna was not less than 1.5 times in an open area, and in 2...3 times more in the city. This was largely affected by the orientation of the magnetic antenna.
Author: I. Grigorov (RK3ZK, UA3-113); Publication: www.cxem.net