Encyclopedia of radio electronics and electrical engineering / Civil radio communications
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It is known that the dimensions of a half-wave dipole can be reduced by bending its ends and running them along the web. Such a shortened dipole, with some complication of the design, will become dual-band. Recall that the 20-meter half-wave dipole arms folded in half have a length of 2,5 m. And this is the resonant length of the 10-meter dipole!
The diagram of a dual-band antenna is shown in the figure. The only difficulty in its manufacture is that when switching to a higher frequency range, it is necessary to close the ends of the dipole to the main canvas. This problem was solved with the help of powerful reed relays (manufactured by the former GDR) purchased at the "collapses" of the radio market. When working on the 20-meter range, the relay contacts K1.1, K2.1 are open and the entire length of the antenna works. When the supply voltage is supplied via the coaxial cable to the windings of the relays K1 and K2, their contacts close the ends of the dipole to the canvas. The antenna becomes resonant on the 10 meter band, matched to the coaxial cable.
The antenna fabric is made of copper wire with a diameter of 2 mm. Fiberglass spacers are installed along the length of the arms every 50 mm. When the antenna was made exactly according to the dimensions indicated in the figure, its tuning on the 10-meter range was not required. For a range of 20 meters, it required adjustment with jumpers. In my case, the jumpers were installed at a distance of 30 mm from the ends of the antenna. Jumpers do not affect the operation of the antenna in the range of 10 meters.
Inductors L1, L2 and L3 - standard DM-0,2 20 μH. The trimmer resistor R1 sets the minimum current of the relay operation. All parts are placed in a box with dimensions of 70x70x20 mm made of foil fiberglass. This box is also the central insulator of the antenna.
The double antenna sheet ensured its normal operation. SWR - within 1,2 in the entire 10-meter range, in the range of 20 m did not exceed 1,3. The antenna is powered by a coaxial cable with a characteristic impedance of 75 ohms.
With a distance between the contacts of the reed switches of 1 mm, power up to 20 watts can be supplied to the antenna in the range of 100 meters. Instead of reed switches, it is permissible to use relays that operate at high frequencies.
Author: I. Grigorov (RK3ZK)
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