Having spent more than 30 years the foreman of the radio workshop started operations involved in the repair Svyaznoy radio with itinerant nature of the work, I came face to face with a situation when needed a compact antenna that can easily to transport and install, for example, on the balcony, loggia, in the window opening or add-on baggage car. In particular, this antenna was needed for essential checks of the civil radio CB range of 27 MHz.
The most optimal variant of this antenna, from a constructive point of view, I seemed shorter vertical radiator with a lengthening coil.
Vibrator my antenna (Fig. 1), more specifically, the element "a" made of the telescopic tube from the old indoor TV antenna "CTA" (as called "whiskers"). This makes it possible, by changing the length of the vibrator, quickly to rebuild the antenna over a range that is significantly due to its narrow band transmission.
Fig. 1
Lengthening coil L1 (the element "0") copper wire wound diameter 1.2 mm PVC insulation (external insulation diameter - 4 mm) on the frame from plastic pipe with a diameter of 25 mm (photo in Fig. 2).
Fig. 2
Winding turn to turn, on the length of 195 mm. Element "C" of the antenna is formed by the lower the output of the coil.
Approval of high resistance antenna with low impedance feeder (and all modern industrial communication equipment is designed for working on a load with a characteristic impedance of 50 Ohms) performs the P-loop (Fig. 3).
Fig. 3
It is mounted in a metal housing from the antenna filter УSW radio "Granite" (Fig. 4).
Fig. 4
Trimmer capacitors C1 and C4 with an air dielectric also on this radio station. The capacitor C2 composed of three capacitors KT-2 capacitance 30 pF+82 pF+82 pF connected in parallel. The capacitor C3 is composed of three capacitors KT-2 with a capacity of 300 pF, also connected in parallel. When the choice of capacitors should be considered the maximum capacity of the instrument going to work with. At a power of 50 W working voltage should be not less than 500 V.
Coil And matching device contains 8 turns of silver coated wire with a diameter of 1.5 mm, is wound round the mandrel with a diameter of 9 mm and then stretched to a length of 20 mm.
To prevent the antenna effect of the feeder between the radio and the matching the device is in stopped the throttle. He represents several turns the same cable, wound on a magnetic core is a ferrite ring suitable sizes with an initial magnetic permeability of 400 or more. The number turns should be determined experimentally by the SWR changes, caused, for example, the movement of the feeder, touching a hand to him or to the body SWR meter. With good coordination and optimally designed number of turns of these the changes are minor.
This operation must be performed after setting the elements P-outline at a minimum the CWS, for example, by using an antenna analyzer MFJ-259. When this should be the selected area of the CB range, which at this point are marked the smallest unit with the analyzer.
The test results showed that the bandwidth of the antenna when the SWR is not more than two approximately equal to 120 kHz. Narrow band is often considered a disadvantage of the antenna. But in this case, due to the fact that the antenna can be modified by range, a narrow strip is rather an advantage, because it reduces out-of-band interference from close working stations at the reception, and reduces radiation side when you transfer.
The capacitor C1 can slightly change the resonance frequency of the entire system, without changing the geometric dimensions of the vibrator. Such antenna can to construct and SW range 10 meters
Author: V. Efremov (UA6HGW), Yessentuki (Stavropol Krai