Theme simple vertical emitter operating at short ranges, continues to be interested in ham radio. Below is described an interesting variant this antenna. Its highlight that as the emitter used a grounded vertical pipe. This antenna works on all five Amateur bands from 14 to 28 MHz.
From the detailed description of gamma-matching [1], it follows that it can be used not only for resonant emitters, but for vibrators almost any length (except for very short). But there is another interesting property, which allows to make multi-band antenna with such harmonisation.
A careful study of Fig. 3.5.8 in [1, p. 159] you may find that the physical length of the tube gamma-Aligner almost does not change with changes height grounded vibrator from 0.15 lambda. to 0.2 lambda and from 0.28 to lambda 0.6 lambda
It follows from this very surprising and non-obvious conclusion: if you pick the size of the grounded pipe falling within the limits above in all required ranges, it is possible to obtain a good agreement with unchanged physical the sizes of both the GP and the tube alignment.
Of course, the reactive part of the input impedance jXa will depend on the frequency, and the capacitor approval will have to adjust when you change the range. But in this if we agree to this: difficult to adjust the capacitor variable capacity (even if it has to be done remotely), and facilities great. The resulting antenna is shown (not to scale).
This grounded vertical radiator with a height of 4.4 m (diameter 30 mm) with tube approval of 1.8 m length (diameter 15 mm). Using the variable capacitor capacity 10...50 PF antenna may be tuned to resonance at Amateur the bands 14. 18. 21. 24 and 28 MHz with VSWR < 1,5. Moreover, in the strip 18...29 MHz the antenna may be configured such SWR on any frequency. Zapityvat antenna coaxial cable with a characteristic impedance of 50 Ohms. On a plot of 15... 17 MHz to get a low SWR is not possible, because the height of the antenna close to a quarter length waves, i.e. does not fall in the above-mentioned the size of radiator required for such alignment. But in the 14 MHz band the GP height is reduced to 0.2)., and configure again. The model file antennas (2) allows to study the properties described above.
Practical test confirmed the validity of the calculations.
...that won't ever write that sentence. Will suffice. In the XXI century.. And there are funny every time, for the sake of doubting computers retrograde, to explain that, in the absence of errors in the model calculation only slightly differs from reality. That doubt in the simulation results, it about the same that he did not believe the calculations of the resonant frequency of the circuit by the formula Thompson or current in the resistor according to Ohm's law: "And you never know what's designed! But I does not have a frequency meter or ammeter and measure, not believe it, suddenly your formulas are wrong!". Modelirovanie antennas work with formulas, although much more complex than Ohm's law. but these formulas refer to the basic physical and immutable much more than a hundred years. What practical the performance of the antenna need not to clarify an entirely unreasonable question, "how does the calculation with reality?" (much better to ask "isn't there in the model errors and limitations?"), and in order for the antenna to work on the air...
But back to our grounded, multi-band vertical antenna. Worked I have it in two versions.
In the first embodiment, the antenna was installed on the ground near the car. Note by the way, that as a reason to autovariance very convenient to use flat metal plate of about 30 x 30 cm with a small peg on the edge for the subsequent fastening of the pipe vibrator. The plate is placed on the ground, on her edge bump the wheel of a car for mechanical fixation. The peg is put and stretched the mast, attached to the plate counterweights. I used 8 wires 4 m, lying on the ground.
In the second embodiment, the same antenna was installed on the roof of the building height 4 M. balances quarter-wave (two for each band) were lying on the roofing material.
In both cases ranges 18,21 and 24 MHz VSWR at resonance does not exceed 1.2, and in ranges 14 and 28 MHz to 1.5.
Adjusting the antenna when changing range is very simple: rotate the KPE to the minimum SWR. I did it by hand, but nothing prevents you from using KPE without limiting device angle of rotation and a small gear motor (for example, from an old floppy drive for rotation.
If the vibrator uses a smaller diameter pipe, it is necessary to lengthen. Thus, the vibrator 10 mm diameter shall have a height of 4.6 m.
Literature
Author: I. Goncharenko (DL2KQ), Bonn, Germany