Gamal and all,
I had very good results with my loading coil. The construction
is probably rudimental, but works and may be seen on the web
site of G3YXM (look at pictures).
The coil has about 3,5 mH, and tunes a 13 m vertical wire with
3 top wires, spanning about 23m, spaced 1 m (2 m total width).
The coil is wound on a garbage bag holder (no idea how it is
called in English ...) made of PE (poliethylene). The wire is a 2 mm
solid Cu insulated with silicon plastics, for an outer diameter of 3
mm. Total wire lenght is abt 120 m, and my coil has 100 turns.
The Q at 137 is in the order of 1200 (calculated by G4FGQ sw) or
over 1000 (extimated from real losses). The coil handles 20 kV on
the high side (1 kW, about 6A RF x 3.3 kOhm reactance).
The variometer is made of 10 turns on diameter 30 cm on the
high side of the coil. The total excursion of the vario is more than
enough to tune across the band and to compensate wet/dry soil
conditions.
If anyone is interested I may send detailed pictures. All calculations
were done with G4FGQ SOLENOID.EXE. Initial extimations were from
Bill Bowers' article in the LF Experimenter's Source Book (a very
suggested reading). He tested various coil configurations, and the
conclusions are valid.
73 - Marco IK1ODO
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