Compact antenna range of ST
The antenna farm is a necessary component of any radio station. For CB radios that can be used as mobile versions - from the car, and stationary - from home, there should be a permanent "home" antenna. One of the simplest and most effective variants of this antenna was presented in , but this antenna works effectively only on the upper floors of buildings. When the location on the lower floors of its effectiveness decreases significantly due to the strong absorption of the vertical component of radiation standing next to houses that are quite effective screen.
In such cases it is more efficient loop antenna works. One of the options framework car antennas was considered in . Framework indoor antenna may be located on the perimeter of the window (Fig. 1). Capacitor, tuning the antenna to resonance, you'll be on the bar that divides the window into two parts.
Two options were tested antenna. The dimensions of the first - A=140 cm, B=140 cm, H=40 cm; D=70 cm Condenser was located in a box of foiled fiberglass, its final capacity was 3.5 pF. At smaller sizes the perimeter of the window the capacitance of the capacitor increases. The fact that the condenser is not symmetrical, and somewhat lateral to the geometrical, and in this case, the electrical center of the antenna, does not interfere with its normal operation.
The second variant of the antenna has dimensions of A=140 cm, B=210 cm, H=40 cm. In this case, the capacitor is not needed, adjusting the antenna in resonance was carried out by guides G, which in this case were 60 cm long.
Both antennas were made from flexible copper wire of diameter 1 mm and are arranged on the inner side of the window. It should be noted that for the antenna makes no difference where it is installed from inside or outside the window, only determines the ease of its installation. The distance can also be changed in various embodiments of the window.
In case of poor antenna location is on the lower floors, and hence if necessary "go beyond the screen", the greatest effect is achieved by washing the antenna or one of its lower corners or in the center of its bottom side. But in practice the implementation of such power, on the contrary, reduces the efficiency of the antenna in comparison with food in the center of the vertical sides. This is because under the window is usually a metal sill and radiator, which strongly absorb RF energy.
If bedrooms are on different sides of the house, it makes sense to install two antennas, which allows you to work in two directions. The antenna tuning is not difficult and can be done in several ways. The simplest of these is using the indicator of the field strength. Changing the capacitance of the capacitor or the length of the elements of G, it is necessary to achieve the maximum field strength. But a more thorough configuration is possible only by using an SWR meter or an RF-bridge, for example given in [W]. Indeed, tuning the antenna on the tension indicator and using industrial CB radio with automatic power adjustment, which is provided with the majority of stations, it is difficult to achieve correct adjustment of the antenna. Using the same bridge whose input is consistent with the output of a transmitter, you can spend a very precise adjustment of the antenna into resonance and to determine its real resistance. As it turned out, the resistance of the first embodiment of the antenna was about 35 Ohms, the second option is about 55 Ohms, in both cases with a small reactivity. This shows that most effectively both antennas, you can power cable of 50 Ohms, which is very convenient, because imported radios are designed to a resistance of the antenna.
But the greatest effect in the application of this antenna is the use of matching device (Fig. 2). The device is made asymmetric, because the loop though, and refers to symmetric, but in this case the impact of foreign objects assymmetrical her. Coil L2 is made of copper wire 1.5 mm thick, frameless. It contains 6.5 coils, diameter 25 mm, length of winding - Cold 40 mm. end soldered to the bottom of the box, hot - rotor S1. Coil L1 contains two turns of the same wire, is located on top of L2 and is in the bottom third. This matching device allows you to bring the resistance indoor loop antenna from the input matching unit (SU) resistance to any standard cable - 50 or 75 Ohms, the reactance is almost not there. Matching device operates with great efficiency - not less than 90% (measured practically), so almost all the power from the transmitter is fed to the antenna. Being narrowband circuit, SU effectively eliminates TVI, which is especially important when using the radio with the amplifier. The specified variant of the SU can be configured so that it will work all the ne with a small range of the CWS on its edges.
Matching device was performed in a box of foiled fiberglass dimensions HH cm and placed in the vicinity of the antenna. The rotors C1 and C2 for the adjustment were brought out. It should be noted that the use of SU that allows you to reach the CWS in the cable feeding the antenna, almost 1:1, gives you the opportunity to safely apply different noise filters, repeatedly cited in the literature (e.g. [4,5]), which allow to reduce the level of TVI considerably. The filter should be installed immediately at the exit of the station.
In a comparative test of the loop antenna described in this article, with the antenna in , revealed their distinct advantage. Loop antenna provided greater communication range and a much lower level of TVI and RFI. The latter is particularly noticeable when using a matching device. What's more important - these loop antennas can be placed almost imperceptibly, that does not spoil the interior of the room.
The antenna trimmer is in good agreement in the frequency range from 21 to 30 MHz, which gives her the opportunity to work not only in the ST range, but in several Amateur LW bands. A matching unit to these values of the components are in good agreement only from 30 to 24 MHz. For work on 21 MHz the capacitance of the capacitors C1 and C2 should be increased to 50 pF or the number of turns of the coil L2 to increase to 8.5, while maintaining the length of the winding. In this case, the upper frequency of operation of the SU is limited to 29 MHz. The second antenna when using it as a variable capacitor overlaps the range of 14 to 24 MHz. Coil L2 matching device for this range must contain 11.5 turns when the winding length of 45 mm. Coil connection in all cases contains 2.5 turns. It can be moved around the contour coil for finding optimal communication and optimal SWR.
In all cases, when you configure the SU should strive to end the capacitance C2 was possible. The minimum value of C2 indicates a misconfigured system cable - SU - antenna. When the output of over 10 watts on the capacitor of the antenna can be high RF voltage, so you should take measures, its electrical insulation.
Author: I. Grigorov (RK3ZK, UA3-113); Publication: www.cxem.net