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Versione italiana

ACTIVITY ON 137 KHZ



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Fig. 1
Here is a brief description of the equipment that I have realized to operate on the long-wave amateur band (135.7 to 137.8 kHz). Since I'm living just in the town where no spaces are available to erect a suitable "Marconi" antenna, that has been the greatest problem. The only practical solution was to use my vertical dipole for 1.8 kHz, described in HF antennas (see fig. 1), to wich I have introduced some modifications described in LW antenna details.
The efficiency of such an antenna is of course very low, a loss around 38 or 40 dB has been at first estimated in respect to an isotropic radiator, but after the last modifications introduced, the loss seems to be reduced to 34 or 35 dB; so, for a radiated power of 60 - 80 milliwatts, an input of 200 watts is required. This power is supplied by an amplifier, solid state, on purpose built, preceded by a preamplifier working, at first, with the small signal outputted during transmission by my RTX for HF described in HF sets (see fig. 2), when operated in general coverage mode; later, having noticed insufficient stability in working QRSS, I have built a new RTX covering only this band and at last I have built recently a new amplifier with an output power of about 400 watts. These devices are described in LW set details.
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Fig. 2
To increase the overall performance, several wires are arranged horizontally on the roof, covering an area of around 30 square meters. These wires as well as the other metallic structures in the vicinity of the antenna, are connected to the cold side of the circuit, together with the ground lead, so acting like a counterpoise. In this way the current induced in these structures returns directly to the circuit, without dissipate power in the walls.
To achieve better results, a loop antenna is used for receiving; its output, opportunely mixed with signals coming from vertical antenna, generates a "Cardioid" effect, useful for reduction of noise, always present on this band; subsequently I have built a new loop, installed outdoor, so becoming less sensitive to the local noise. These loops are described in LW receiving loop.
The activity on 137 kHz is also discussed in two articles on RadioRivista, published in n. 5 of 2000 and in n. 7 of 2001 (see also Articles).

PERFORMANCE

The efficiency of this equipment, even with limitations due to the fact that the station lies in the town, that is strong noise level and impossibility to erect a good antenna, seems to be satisfactory; see also Activity details. With the estimated radiated power of about 60 - 80 milliwatts the greatest distance reachable in normal CW seems to be around 700 km, while in visual CW (QRSS,DFCW) using Spectrogram, the distance may extend well over 1000 km; I have contacted more than once a finnish station, 2131 km away. Also with JASON or HELL the distance may be noticeable, certainly greater than in normal CW. On the other hand, is quite difficult to increase the power, being the antenna too short and not enough loaded on top, so the RF voltage, with greater power, becomes too high causing insulation problems, nevertheless, the antenna seems to be able to carry the greater power outputted from the new amplifier without problems. In spite of the strong noise usually present on band, several stations were received from Switzerland, France, Portugal, Germany, Netherlands, Belgium, England, Scotland, Sweden, Finland, Russia, Czek Republic, Slovakia, Slovenia, Yugoslavia, some with very good signals; obviously a large number of these stations radiate a power quite stronger than mine, that's why is not as much easy that they can receive my signals. Unfortunately, the small number of stations active in Italy prevents from doing a middle-range experimentation, very useful to upgrade the station.

To everybody who whishes further informations about the activity on this band, I suggest visiting the site Long Wave Radio.

Go to Activity on 472 kHz

© Cesare Tagliabue I5TGC