This is a calculator to assist with designing a two band shortened trap dipole. Prefilled values are for a 10/20m trapped dipole, with loading removing 20% of antenna length.

Inner Frequency in MHz (trapped, higher frequency)

Outer Frequency in MHz (untrapped, lower frequency)

Normal dipole length in meters (determined by outer frequency):

Coil distance from feedpoint relative to normal length (determined by inner frequency):

\(d_{fromfeedpt} = \)

Amount to shorten (decimal percentage):

Shortened length of dipole:

\(l_{shortened} = \)

Length removed from shortened dipole:

\(l_{removed} = \)

Wire diameter (in mm, see awg chart):

Electrical height from ground (in meters):

Load reactance, \(X_{load} = \)

Load inductance, \(L_{load} = \)

Trap inductance, \(L_{trap} = \)

Trap capacitance, \(C_{trap} = \)

Trap reactance, \(X_{trap} = \)

Caveats

This is an experimental tool that has not been thorougly vetted. I have not yet built an antenna using this tool, however these values are close to alternative calculations I’ve done while designing and building my first trap dipole (which was successful).

Parasitic capacitance

I’ve noticed that the practical resonant frequency is generally lower than the theoretical provided here. My guess is that the inductors have a significant amount of parasitic capacitance (more at higher frequencies). Resonant frequency is given by this formula:

\(f = \frac 1 {2π \sqrt{LC}} \)

Parasitic capacitance would have the effect of lowering the resonant frequency. To compensate, you’d need to lower inductance, which would then reduce loading. Or, to maintain the same loading, you could compensate for parasitic capacitance by reducing your capacitor value.

For example, on a 10m/20m trap/load I assembled recently (based on the default parameters for this tool), the inductance had to be dropped from 6.28μH to 4.1μH. This drops the reactance from 555Ω to 362Ω. Theoretical loading was 20%, but practical loading turned out to be 15% due to parasitic capacitance.

Notes

When I built my trapped 17m/20m trapped dipole with \(X-L = 200Ω\), the 20m section was much shorter than I expected. That means my inductance, and therefore my inductive reactance, was too high and should have been lower.

It 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

\( \frac {143} {18.118MHz} ÷ \frac {143} {14.055MHz} = 77.58\%\)

It is very important to compute the amount to shorten and coil distance and as accurately as possible, since these inputs can produce wide swings in inductive reactance, \(X_L\), which will produce wide swings in your antenna dimenions. Do not just go by band meters! For this tool, that means entering precise frequencies to calculate accurate values.

As explained by CT1EOJ, the electrical height is difficult to measure. Electrical height and wire diameter are inputs to the antenna wire characteristic impedance, \(Z_0\). My intuition is that the Nano VNA can accurately measure \(Z_0\). This would allow for more accuracy using this calculator. I will update this page if I find out how to do that. Regardless, neither wire diameter, nor electrical height seem to have much impact on the final coil inductance.

Further information