Loading Coil Calculator
WORK IN PROGRESS
This is a tool to calcuate inductive reanctance\(X_L\) for shortened dipoles. This is one method that can be used to determine the inductance needed to shorten an antenna by a specific amount. These loading coils can be used for single-band dipoles and also for multiband trapped dipoles, in the LC circuit that makes the traps.
This calculator uses the method described by Luiz Duarte Lopes, CT1EOJ, which was published in the October 2003 edition of QST Magazine.
Frequency (MHz)
Normal dipole length (meters):
Coil distance from feedpoint relative to normal length (decimal percentage - 0.50 to place loads at pre-shortened halfway point):
Amount to shorten (decimal percentage):
Wire diameter (in mm, see awg chart):
Electrical height from ground (in meters):
Load reactance, \(X_L\):
Coil inductance, \(L_{coil}\):
Notes
As explained by CT1EOJ, the electrical height is difficult to measure. For a 17m trap functioning as a load on 20m, this method gave me a inductive reactance, \(X-L\), of 63Ω (14 awg and electrical height of 20ft). After building this calculator and trying different electrical heights, the coil inductance doesn’t seem to be too sensitive, even with wild swings in electrical height.
When I built my trapped 17m/20m trapped dipole with \(X-L = 200Ω\), the 20m section was less than half the length I expected it to be. That means my inductance, and therefore my inductive reactance, was too high and should have been lower. However this calculator gives a \(X-L\) of over 200Ω for almost all combinations of wire diameter and electrical height, when I should have expected less.
Turns out my reactance was too high because I overshot the coil distance from feedpoint. I calculated
\(17m/20m = 85\%\)
However, the proper calculation would have been
\( \frac {143} {18.118MHz} ÷ \frac {143} {14.055MHz} = 77.58\%\)
The takeaway is that it is very important to compute the amount to shorten and coil distance and as accurately as possible, since these inputs produce wide swings in inductive reactance, \(X-L\). Do not just use the band meters! Instead, calculate precise lengths from precise frequencies.
Electrical height and wire diameter are inputs to the antenna wire impedance, \(Z_0\). My intuition is that the Nano VNA can take an accurate measurement of the antenna wire’s characteristic impedance. This would allow for more accuracy using this calculator. I will update this page if I find out how to do that. However, neither wire diameter, nor electrical height seem to have too much impact on the final coil inductance.
TODO
On an upcoming blog post, I will expand this calculator to compute trap inductance and capacitance for a two band trapped dipole.