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LW ANTENNA DETAILS



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Fig. 1
In fig. 1 is depicted the story of my antenna. The first picture shows the originary look (a dipole for 160 m), the second refers to the first modification of the top hat, the third shows a further enlargement of the 'balloon' and the fourth refers to the present situation with a new coil located outdoor and the radiator interrupted by a TEFLON rod to allows the connection in series of the coil.
Fig. 2 shows three different layouts, the first circuitry has been used till end October 2002, the second shows the present situation and the third refers to the circuitry used to operate on 160 m band.

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Fig. 2
L = top hat (100 pF)
A = top coil (flat spiral 80 µH)
B = new coil (5 mH, shunted in 160 m) (see fig. 3)
C = bottom capacitive load (55 pF)
D = the point where are connected the nearby metallic structures (not connected in 160 m)
E = link (used in 160 m)
F = bottom coil (55 µH used in 160 m)
G = 50 ohm coax cable to the RTX
H = tuning unit with variometer (8 mH in the first case and 4.5 + 6 mH in the second case) (see fig. 4)
I = variable ratio matching transformer (50 > 40/130 ohm).

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Fig. 3
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Fig. 4
I had noted a small increase in efficiency after the last enlargement of the 'balloon', being the current, with the same power, about 10% stronger, while the tension on top was slightly decreased, considering the minor inductance required for tuning; I had measured 8135 µH instead of 9360 µH required before. The input impedance was about 55 ohms with dry weather, while with wet weather I've measured 80 ohms and even more. With the last configuration, I've noted a further increase and, moreover, the variation of impedance with dry or wet weather results very little, around 50 ohm with dry weather and not more than 60 ohm with wet weather. In any case, with raining weather the antenna becomes unusable because of lack of insulation in the supports. The current flowing in the ground lead is smaller in respect to that flowing in the ATU, since a noticeable amount of current reaches directly the bottom of the coil from the other nearby structures connected in D. With the last circuitry a moderate current comes also from C. The ground lead, although outdoor, being near to the wall, contributes moderately to the radiation, moreover, also the current flowing in it is not completely useful for this purpose, a certain amount comes from the capacitive coupling between the antenna and the building itself, being this last a reinforced beton structure, with a quantity of iron. It's very hard to calculate precisely the actual efficiency of such an antenna, nevertheless I have made contacts with farther stations, improving my previous record in visual CW from 1155 km to over 2100 km; for this reason I've attempt to calculate with reasonable accuracy its efficiency. In Antenna calculation is detailed the method used.

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