Distribution in areas remote from telecentres, primitive active antennas without any filters leads to an exacerbation of the manifestations of interference from CB radios and, as a consequence, conflicts with neighbors.
At good relations limited to verbal comments, but sometimes you can expect and sabotage against antenna-feeder devices. One solution is to filter your signal all accessible ways. If you design and configure an efficient filters low frequencies owner ST-station not on the shoulder, then the simplest way to reduce out-of-band radiation is the use of loop antennas, which, unlike linear, much worse radiate harmonics of the signal. These are various forms framework antenna with perimeter equal to the wavelength; frames and quarter-wave stub antenna.
A special case of the loop antenna are of coaxial cable . And a quarter-wave short-circuited coaxial cables, due to their high q, are effective filtering circuits.
To make such antennas have wised up view of the greenhouse, the arc of which were made from lengths of coaxial cable 7/8 inch firm NOKIA cost $ 10 per meter (Hi). Such cables are used in paging, cellular radiotelephony and radio-relay communications, and cutting cable several meters during installation - routine. In principle, the cable can be of any type, but has a 7/8 inch rigid structure, and the antenna is obtained freestanding when attaching its lower end. The center conductor of this cable is a copper tube with a diameter of 9 mm, and a braid of thin-walled corrugated copper tube.
The design described in , for better processability was somewhat changed. As the coefficient of shortening the applied cable - 0,66, the length of the loop portion is not equal to 0.25, and and 0,165 lambda. At the upper end of the Central conductor peremejaetsya with braid a wire jumper, then the cable is cleaned from the braid and the insulation to length usual GP (depending on the diameter of the Central conductor, the total length antenna adjusted during the calculation or in the configuration process). If you are using conventional thin cable, it can be fastened to a wooden mast with electrical tape, or to put in vinyl plastic water pipe. You need to consider the velocity factor, made by a vinyl plastic (0.83 to 0.84). When the diameter of the cable 22 mm cable length antenna grid is Central 1800 mm. the conductor of the supply feeder is connected to the external antenna conductor, braid - on the internal. The counterweight may be one, but, as always for vertical antennas,more than counterbalances the better.
The model for such antenna two-meter band showed good electrical parameters:
in calculating the average the frequency of 150 MHz in the band 140...160 MHz, its VSWR is not exceeded with a minimum of 1.4 1,05. The efficacy was compared with industrial dipole AQIS and, if more than half length, coaxial antenna was not inferior ASCI up to 170 MHz, and in the frequency band 140...147 MHz and exceeded the dipole 3 dB (operating range ASCI - 148...175 MHz).
Several such antennas with successfully used at this time in our company to dispatch connection with stationary objects. The range of 27 MHz on the effectiveness of coaxial corresponds to the usual GP. Antenna VSWR within one grid is not exceeds 1.5 (at the resonance frequency is about 1.1), the mesh down increases to 2, and mesh up to 3. the Higher harmonics of the signal are suppressed to a considerable degree.
In practice, this has resulted in the following. When using conventional GP, on channel TV picture and sound just disappeared. The noise was strong and the other meter channels. After install coaxial antenna when the transmitter power of 10 watts in mesh With an obstacle was expressed only in the breakdown of the color burst on the 1st TV channel, on the other the channel interference is not detected at all, and the transition to other grid disturbance he was lost, and on channel TV. These data refer to a collective system TV using the channel amplifiers.
Another positive the quality of this antenna is that it is closed DC, ie it is not induced static electricity.
Literature1. Rothammel. Antennenbuch, 1989.
Author: Vladimir Pankov (RA3141); Publication: N. Bolshakov, rf.atnn.ru