# Pawsey Balun on an asymmetric load – bench load simulation

The Pawsey Balun (or Pawsey Stub) is described as a device for connecting an unbalanced feed to a balanced antenna.

Pawsey Balun on an asymmetric load reported model results in an asymetric dipole antenna, and showed very high common mode feed line current.

This article looks at two test bench configurations modelled in NEC.

The configurations are of a horizontal Pawsey balun for 7MHz constructed 0.1m over a perfect ground plane. The ‘balanced' terminals are attached to the ground plan by two short 0.1m vertical conductors which are loaded with 33 and 66Ω resistances. At the other end, the horizontal transmission line is extended by two different lengths and connected to the ground plane using a 0.1m vertical conductor. The two extension lengths are almost zero and a quarter wavelength.

## Zero extension

The total horizontal length from the ‘balanced terminals' to the grounded end of the transmission line is a quarter wavelength for the Pawsey balun and a further 20mm making approximately a quarter wavelength in total.

Above is a plot of current magnitude and phase from 4NEC2. The current on the two vertical conductors containing the 33 and 66Ω loads is quite different, and the product gives load voltages that are approximately equal in magnitude and opposite in phase.

You could be forgiven for thinking that the Pawsey stub itself is a good voltage balun, but in fact the voltage balun behaviour is due to the fact that the transmission line and Pawsey stub conductors in common mode are approximately a half wave electrical length, and being grounded at the far end, the common mode impedance looking into the ‘balanced' terminals is very low… a hallmark of a good voltage balun.

## Quarter wavelength extension

The total horizontal length from the ‘balanced terminals' to the grounded end of the transmission line is a quarter wavelength for the Pawsey balun and a further quarter wave making a half wavelength in total.

Above is a plot of current magnitude and phase from 4NEC2. The current on the two vertical conductors containing the 33 and 66Ω loads is approximately equal in magnitude and opposite in phase.

You could be forgiven for thinking that the Pawsey stub itself is a good current balun, but in fact the current balun behaviour is due to the fact that the transmission line and Pawsey stub conductors in common mode are approximately a quarter wave electrical length, and being grounded at the far end, the common mode impedance looking into the ‘balanced' terminals is very high… a hallmark of a good current balun.

## Intermediate length

At intermediate lengths, the common mode impedance will range from one extreme to the other, and for the most part, it will be neither a good current balun nor a good voltage balun.

## Conclusions

The Pawsey stub or balun is not of itself a good current balun or a good voltage balun, but can be used as part of a more complete solution to act as either a good current balun or a good voltage balun.

Creating that context may be impractical for many antenna topologies.

Without careful implementation of the context, the Pawsey balun or stub is anyone's guess. Nevertheless they are written up this way in textbooks and find practical application, even though their performance is likely to be unpredictable and unmeasured.