Dear LF Group,
last saturday morning we had the first hoar-frost
this year at -6 degree centigrade. All the trees and bushes in the backyard
looked beautiful and so did my 13-m-Marconi-antenna. Feeder and 33-m
4-wire-topload were coated by fragile white needle crystals rising the wire
diameter from normally 1 mm to 4 or 5 mm.
Switching on the TX with te usual settings the
antenna current had decreased from the normally 4.7 A (at frost) to 2.75 A! The
antenna capacity remained nearly unaffected (about 660 pF), but tuning the
loading coil showed a much broader maximum than without hoar-frost.
Some calculations showed the following values for
the total loss resistance of the antenna system:
- no frost, normal conditions: R = 24
- frost: R = 18 Ohms
- hoar-frost: R = 31 Ohms
According to some other measurements and
observations these values should be rather realistic. The difference between
no-frost and frost is well known and should mainly arise from the reduced
losses in the greens - but where do the additional losses at hoar-frost
come from? Could the hoar-frost be a lossy dielectric? There were neither visual
corona effects nor seemed the hoar-frost be melted by the antenna current.
Sunday morning it disappeared on itself and the antenna current rose again to
the usual 4.7 A at -3 degree centigrade.
Anyone with a good idea?