Conintuing from 4NEC2 plots of STL VSWR II, this article is a tutorial in using 4NEC2 to determine the Half Power Bandwidth of a simple model of the main loop.

The model is drawn from AA5TB’s calculator’s initial values.

The model is in NEC-4.2, and is a 20 segment helix in free space, and tuned for resonance at 7.000MHz. (If you repeat this using NEC-2, you may need fewer segments to avoid violating NEC-2’s segment limits.)

Above, the model was resonated at approximately 7.000MHz, and the feedpoint R noted, it is 0.02976Ω.

Above, this is entered directly into the modified 4NEC2 dialogue (the change does not ‘stick’ in the unmodified version, it quietly ignores Zo<0.1Ω).

Above, the VSWR curve is now based on reference Z=0.02976Ω.

I have used a screen ruler to measure the Half Power Bandwidth (bandwidth between points where ReturnLoss is 6.99dB (or VSWR=2.618, see Antennas and Q).

In this case, Half Power Bandwidth is 1.96kHz. (No, it does not reconcile with the calculator, a defect of the calculator in this case.)

Of course, modelling the loop in the real world, you would include tuning capacitor loss, and add real ground to the model. All of these things influence the results.

If you then build such an STL, the model of the loop should reconcile with real world Half Power Bandwidth measurement… if your model was complete and valid. If you measure wider bandwidth, you might consider that a bonus but it is more likely a warning of unaccounted loss.