On Sat, 26 Feb 2000 14:00:10 GMT
"Andy Talbot" wrote:
For an estimate of losses :
Skin depth of copper at 137kHz is approximately 0.18mm From D = 503
SQRT(Resistivity / Freq / uo)
For Cu Resistivity = 1.7E-8 Ohms / m, and uo (magnetic permeability) =
1
Diameter of centre conductor = 2.5mm (near enough anyway)
so cross sectional area of conducting path is
0.18mm * 2.5mm = 0.45E-6 m^2
RF Resist = Resisivity * Length / Area = 1.7E-8 * 370m /
0.45E-6m^2 = 14 ohms.
For a quick estimate assume the braid losses are a lot less than the centre
conductor as they have a much larger surface area, so can be ignored
(that may not necessaily be the case) and we can also ignore
dielectric losses (a reasonable assumption at these freqs) so Q = Xl / R
= 1722 / 14 = 123
Which is about what I got on my 5mH conventional coil of 1.5mm wire,
300mm
diameter and 400mm long.
You forgot a Pi in the calculation of the cross sectional area. The
circumference is Pi D, not just D. Therefore cross sectional area is approx.
1.41E-6 m^2
and RF Resist = 1.7E-8 * 370m / 1.41E-6m^2 = 4.46 ohms.
So Q = 1722/4.46 = 386, which is somewhat better than your conventional
coil.
John, G4CNN
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