Dual band VHF antenna. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / VHF antennas
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Interest in operating on the 144 and 430 MHz bands in local networks has caused a corresponding interest in multi-band VHF antennas. Installed on one mast and powered by one coaxial cable, they can significantly reduce the cost of their manufacture.
On fig. 1 shows the structure of a simple dual-band VHF antenna proposed by DJ2AZ. It consists of a quarter-wave emitter (GP) for the 144 MHz band, on the lower part of which a quarter-wave "glass" for the 430 MHz band is installed ("bottom" down, electrically connected to the emitter at the bottom).
(click to enlarge)
On the 144 MHz band, this "glass" is equivalent to a small inductance included in a quarter-wave radiator at the level of the upper cut of the "glass". Its influence is small and in the process of tuning is compensated by a change in the length of the emitter. On the 430 MHz band, the length of the upper (above the "glass") part of the radiator will be 5/8L. "Glass" works on this range as a matching transformer, providing normal power to the upper part of the emitter.
A possible variant of the design of such an antenna is shown in Fig. 2. It is designed for direct connection to the radio station through the PL-259 connector (standard for many foreign-made radio stations). For other installation options, its lower part (relating to the connector) is modified accordingly.
The main emitter is composite. Its lower part is made of aluminum (preferably brass or copper) rod with a diameter of 4 mm. A tube with an outer diameter of 6 mm and a wall thickness of 1 mm is pulled over its upper part, by moving which the antenna is tuned. The "glass" is made of a tube with a diameter of 8 mm and a wall thickness of 0,5 mm. In its lower part there is a metal insert through which the position of the "glass" on the emitter is fixed with a locking screw. In the upper part of the "glass" there is an insert made of Teflon or any other high-frequency dielectric, which fixes its position in relation to the main radiator and prevents atmospheric precipitation from entering the "glass".
The radiator diameter of 4 mm in this embodiment of the antenna was chosen for purely design reasons - it corresponds to the diameter of the central contact of the PL-259 connector (the lower end of the radiator performs the functions of this contact). With other versions of the antenna, the diameter of the radiator, of course, may be different, but then it is necessary to proportionally change the diameter of the tube forming the "glass", as well as the upper regulating nozzle on the radiator.
The "ground" of this antenna is ordinary: counterweights for each range (at least three), a metal surface (car roof), etc. By the way, this antenna can also be made as a conventional dipole, consisting of two such emitters.
The antenna is tuned to a minimum SWR on both bands by selecting the length of the radiator and, within small limits, the position of the "glass" on the radiator. You need to start from the lowest position of the "glass". First, by selecting the length of the radiator, the antenna is tuned to the 144 MHz band. Then, by changing the position of the "glass", a minimum SWR is achieved in the 430 MHz band. After that, if necessary, adjust the antenna on the 144 MHz band.
Author: Victor Menzlewski. Mehrbandantennen fur den VHF/UHF Bereich; Publication: cxem.net
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