Lots of rain and strong winds all weekend, so my antenna was is 'save
position' (all ropes loose) all weekend except for friday evening when I
heard ON6ND with his new TX (now 599+), some G's and OH1TN with a very
strong signal (579).
I took some time for reading and found a couple of interesting formulas for
calculating the 'free space' capasitance of wires :
1. Horizontal wire :
C(pF) = 0.24 x Er x L(cm) / log(4 x H / D)
where Er = relative diëlectric constant (1.0006 for air), L = wire length,
H = wire height and D = wire diameter
2. Vertical wire high above ground :
C(pF) = 0.24 x Er x L(cm) / log(L / D)
3. Vertical wire close to ground (H << L/4)
C(pF) = 0.24 x Er x L(cm) / log(1.15 x L / D)
I made some calculations for a 3mm thick wire and the results are very
acceptable :
For a horizontal wire I got a capasitance of 7.7pF/m at 1m height,
decreasing to 6.3pF/ at 5m height, 5.8pF/m at 10m height and 5.2pF/m at 30m
height.
For horizontal wires starting at ground level I got 9.3pF for a 1m heigh
wire (average 9.3pF/m), 36.6pF for a 5m heigh wire (average 7.3pF/m),
67.0pF for a 10m heigh wire (average 6.7pF/m) and 177.3pF for a 30m heigh
wire (average 5.9pF/m)
73, Rik ON7YD
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