Veľa rádioamatérov používa na spodné pásma dipólové drôtové antény pomerne nízko nad zemou. Tým sa výrazne degradujú vlastnosti týchto antén, nakoľko väčšina energie vyžaruje kolmo horeThe solution would be to raise the antenna to a greater height or use a different antenna. From the perspective of a low radiation angle, vertical antennas are interesting, but building at least a 20m high vertical (lambda/4 on 80m) is almost an unsolvable riddle.
ARRL Antenna Handbook hovorí o dvojelementový anténach „veľa muziky za málo peňazí“. By adding one element to a dipole low to the ground, we achieve a significant reduction in the radiation angle, thus significantly improving communication efficiency. Nezanedbateľnou výhodou je aj smerovosť antény, výhodné impedančné zakončenie a prekvapivo aj jednoduchosť.
Inšpiráciou môže byť článok “Antény yagi for the 3.5 MHz band' published in the Radio Amateur Newsletter 1/1986. I simulated this antenna in the program MMANA, where I wanted to verify how the antenna will behave at different heights above the ground and whether it would be possible to achieve an impedance of 50 ohms at the feed point.
V rozmedzí výšky 10 až 20m nad zemou sa rezonančná frekvencia mení len minimálne, so zvyšujúcou sa výškou klesá impedancia. Ako optimum sa javí výška 12 až 15m nad zemou, kde anténa má impedanciu 50ohm, šírku pásma pre PSV<2 about 100kHz. The front-to-back ratio is low, about 7 - 12dB, and gain (according to MMANA) about 9.1dBi. The last piece of information needs to be taken with caution, more than any 'mathematical' gain, what is interesting is that the energy no longer radiates vertically upwards, but much more towards the ground.
Poďme však k samotnej anténe, ktorú som spočítal na SSB segment 80m pásma. Zvolená bola sústava žiarič – direktor s rozostupom prvkov 7,74mThis distance is not ideal for achieving the best parameters, but it allows for easy antenna implementation. Žiarič má 2x 20,03m (celkovo 40,06m) a direktor 38,07m. Hrúbka vodičov je 2mm. Napájanie antény je cez balun 1:1.
The construction of the antenna is simple. We will need about 80m of wire, a 1:1 balun, four insulators, and thin steel ropes (e.g. 3mm), the length of which we choose according to the placement of the suspension points. These ropes are light and have excellent strength.
According to the drawing, we will make the radiator and director and attach climbing ropes to the insulators to define the spacing between the elements and hangers. We need four points at the same height for hanging. These can be trees above a meadow or adjacent apartment buildings. Due to the small spacing, it is possible to mount the bracket on a family house.
Tuning should not be necessary. At a height of 10 - 15m above the ground, the minimum SWR should be below 1.3 (even 1.0). At a greater height, we will increase the spacing between elements, otherwise the impedance will be too low.