Dvojelementová yagi na 80m
Many radio amateurs use dipole wire antennas relatively low above the ground for the lower bands. This significantly degrades the properties of these antennas, as most of the energy radiates vertically upwards. The solution would be to raise the antenna to a higher height or use a different antenna. From the point of view of low radiation angle, vertical antennas are interesting, but build at least a 20m high vertical (lambda/4 of 80m) is an almost unsolvable puzzle.
The ARRL Antenna Handbook calls two-element antennas "a lot of music for a little money". By adding one element to the dipole low above the ground, we achieve a significant reduction in the radiation angle, which will clearly improve communication efficiency. The directivity of the antenna is also a significant advantage, favorable impedance termination and surprisingly also simplicity.
The article "Yagi antennas for band 3,5 MHz" published in Rádioamatérský zpravodáj 1/1986. I simulated the mentioned antenna in the MMANA program, while I wanted to check how the antenna would behave at different heights above the ground and whether it would be possible to achieve impedance at the power point 50 ohm.
Within the height range 10 up to 20 m above the ground, the resonance frequency changes only minimally, impedance decreases with increasing height. Height appears to be the optimum 12 up to 15m above the ground, where the antenna has an impedance of 50ohm, bandwidth for PSV<2 asi 100kHz. The front-to-back ratio is low, about 7 – 12dB and gain (according to MMANA) about 9.1dBi. The last figure should be taken with a grain of salt, more than some "mathematical" gain is interesting, that the energy no longer radiates vertically upwards, but much more down to earth.
But let's go to the antenna itself, which I counted on the SSB segment of the 80m band. The system was chosen emitter - director with an element spacing of 7.74 m. This distance is not ideal for achieving the best parameters, however, it allows easy implementation of the antenna. Žarič has 2x 20.03m (a total of 40.06m) and the director 38.07m. The thickness of the conductors is 2 mm. The antenna is powered via Wonderful article 1:1.
The construction of the antenna is simple. We will need about 80m of wire, Wonderful article 1:1, four insulators and thin climbing ropes (primer 3mm), whose length we choose according to the location of the suspension points. These cables are light and have excellent strength.
According to the drawing, we will make the radiator and the director, and we will also tie climbing ropes to the insulators to define the distance between the elements and the curtains. For hanging, we need four points at the same height. It can be trees above the meadow or opposite apartment buildings. Due to the small distance, it is possible to attach the console to the family house.
Tuning should not be necessary. At Hight 10 – 15 m above the ground, at least the PSV should be below 1,3 (aj 1,0). At a greater height, we increase the spacing of the elements, otherwise the impedance will be too low.
