5. RESONANT WHIP ANTENNA, ELONGATED INDUCTANCE
In portable and mobile CB radio using the 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 Ω to 1.5 m. of Course, to connect such a short pins to the output stage of the transmitter without the Express approval of unreasonable. First, small the efficiency of such a pin as antennas, secondly, matching the low impedance of the pin with the output stage of the transmitter is very difficult. The most rational solution to which came solving this problem, was that the pin is part of a complex system, which is the 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 earthing system under it. In the simplest case, the grounding system is a system of quarter-wave balances. Naturally, such a system be used for mobile stations is difficult. Therefore, trying to shorten the antenna and balances. The most simple in this case - to include lengthening the coil in the antenna. But here is the question, at what point of the antenna to enable the extension coil for maximum effect. System role plays balances the hull of the station.
It should pay attention to the most effective way of lengthening a short antenna - the inclusion of the extension 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 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 only in the fact 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 the antenna, elongated coil in the middle (Fig.10). Here already achieved a significant strength of the 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 allows you to work all the ST range, significantly increases its efficiency.
Pin to the coil is the main radiating element, it should be made as thick, especially since he still is holding the extension coil. The pin after the coil represents the capacitive load. It can be made thinner. Placement at 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, with a bad ground in the portable radio stations, all types of short antennas are equally bad, and there is no significant difference when using them. But the connection of a quarter-wave counterweight shows the difference in the 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" of such antennas is far from perfect, in the General case, such antennas can be coordinated and co-axial 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 the short antenna to 50...100 Ohms.
6.PRACTICAL DESIGN OF WHIP ANTENNAS, ELONGATED INDUCTANCE
Basically, all shortened antenna portable radios have the form shown Fig. 11. Coil inductance of about 2 μh 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 strip her passing. The antenna depicted in Fig.7, shows and many other, earlier sources [7, 8,9, 10].
When testing antennas [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. The human influence exerted on this antenna is small and its low reactivity. The frequency offset when connecting a quarter-wave counterweight has reached 700 kHz.
During the test antenna from [9], where the length of the pin was 80 cm, lengthening the coil consisted of 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 a counterweight-body radio-1100m.
Bandwidth with a quarter-wave counterweight - 800 kHz, the station - 900 kHz. The displacement of the resonant frequency when connecting the counterweight is almost 1 MHz.
During the test antenna from [10] with the length of the pin 0,8... 1,2 m of the extension coil consisted of 25 turns of wire PEL 0,35 wound on the frame 5 mm diameter round results have been obtained, similar to the antenna [9].
Of interest and short antenna - up to 50 cm moreover, these antennas are not 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 this antenna the following results were obtained.
With a quarter-wave counterweight input impedance - 75 Ohm bandwidth of 700 kHz. With the building of the station in the role of a counterweight to the input impedance of 120 Ohms, bandwidth of 900 kHz. The displacement 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) compared to 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 effective operation of the radio station when connecting the counterweight in this case it is necessary to provide for rapid adjustment of the inductance of the extension coil.
Preferably in the transceivers when switching pin of the antenna use a different extension inductance for the receiver and transmitter. This allows optimally pin on both reception and transmission. Of course, if the impedance of the receiver input and transmitter output 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 RX/TX small. But here one needs to solve practical terms, it is easier to switch of the extension coil or lead to the inputs of the transmitter and receiver to the same value. The "brand" of equipment strive for the latter, although there are options to tune the receiver input when switching antennas. In a self-made instrument range of 27 MHz to the issue of matching antennas in the receive mode and transmit often do not pay enough attention, which leads to lower efficiency portable radios.
In [12] described antenna with shoulder length 110 mm and lengthening the 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 testing this antenna showed the following results. With a quarter-wave counterbalance the input impedance of 90 Ohms, bandwidth . - 400 kHz, with a counterweight-body radio input resistance of 140 Ohms, bandwidth of 600 kHz. The offset bandwidth when connecting a quarter-wave counterweight amounted to 900 kHz. Adding a capacitive load as shown in Fig.13, reduced the frequency offset when connecting the counterweight to 600 kHz. Bandwidth is increased by 50 kHz in both cases. Input resistance decreased with the counterweight was 75 Ohms, with the building of the station is 90 Ohms. The field strength is increased in 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 in the height of the antenna with inductance at the base, creates a field strength of around 1.4... 1.6 times greater. When adding a capacitive load of the benefits of such antennas increase even more. The measurements were carried out at the quarter-wave balances. When using the housing 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 portable stations there are not large differences in the type of whip antennas, but 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 produces an effect and when used with an antenna with a lengthening inductance at the base.
Almost for portable stations can be used antenna of thick copper wire with a diameter of 2...2.5 mm Antenna smaller diameter is less durable and has less mechanical efficiency. For the manufacture of antennas of mobile stations can use a short Kulikovka" or suitable antennas from military radio stations of appropriate 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 out on the field strength in 2...3 times as long whip antennas with a lengthening inductance, these antennas are much more insensitive to the influence of man. But they are still in use, however, basically only two types of transmitters.
The use of such non-resonant antennas justified only in the simple toys, the range of which is not higher than 50... 100 m. For more efficient communication, you must use a resonant antenna, although in front of her and need to put decoupling of the cascades for the simplest schemes. Experience shows that simple Western radio stations that consume more power than domestic "Hummingbird", but operating at a non-resonant antenna, provide a much lower DALnet connection.
The third use case of short non-resonant antennas is wrong construction of the output stage of the transmitter with its chains of matching with an 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 PORTABLE CB Radio
Magnetic loop antenna, I have not met in any portable CB radios. But this does not mean that their use in this type of radio is impractical. I have made 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 Ohm, with a very low reactivity. The bandwidth of 600 kHz. The antenna was made of two millimeter insulated copper wire type PEL, air variable capacitor was mounted on glass fibre laminate base. The antenna was very insensitive to the influence of man and balances. Since this antenna is mainly emits the magnetic component of the electromagnetic wave, it cannot strictly compare indicators such as the level of field strength with whip antenna, because the latter emits mostly electric component of the electromagnetic wave, and measurements for pin should be carried out by the electric component of emws and frame - magnetic component emws. Two antennas shown in Fig.14 were 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 more in the open countryside, and in 2...3 times in the city. This greatly affected by the orientation of the magnetic antenna.
Author: I. Grigorov (RK3ZK, UA3-113); Publication: N. Bolshakov, rf.atnn.ru