The G0KSC hybrid balun

*** DRAFT ***

At (Johnson 2009) the author describes the [t]he G0KSC Hybrid - 'Antenna Balun' as follows:

It behaves in the same way as a Pawsey stub. However, there is no velocity factor to take into account or calculate. You simple cut a 1/4 wave length piece of rod, tube or wire. Connect one end at the feedpoint (inner core) and the other further along the boom making sure it stands clear of the other elements. That's it! It really is that simple.

This article includes some pictures of constructions by others who have been seduced by the concept.

Fig 1:

Fig 0 is an implementation of the concept

Johnson states that it works in the same way as a Pawsey stub, implying:

that it is effective in reducing common mode current on the feedline; and
that the mechanism is like that of the Pawsey stub.

An NEC model was constructed of a Yagi boom, the driven element (DE), and coaxial feedline with the shield connected to one half of the driven element and the DE driven by a central source (the latter is equivalent to connecting the coax inner to the other half of the DE).

The model is intended to demonstrate the workings or not, and different scenarios might respond differently but an effective balun will reduce the common mode current almost completely and ensure balanced current feed to the DE for any practical configuration.

Variants of the model were then constructed with:

the G0KSC hybrid antenna balun; and
a modified Pawsey stub.

Fig 1:

Fig 1 above shows the G0KSC hybrid antenna balun structure in the NEC model.

The ONLY change needed to switch configurations is to move the connection of the end of G0KSC's quarter wave line from the boom to the coax shield at the same point.

G0KSC hybrid antenna balun

Fig 2:

Fig 2 above shows the structure and in green, the magnitude of current in the conductors.

Some key issues to note:

there is relatively large current flowing on the boom and feedline shield beyond the balun (ie to the upper left) and although the phase is not shown, they are each different in magnitude and they will not cancel under any circumstance, not nearly, and so these conductors contribute current moment to the radiation field which will skew the antenna pattern and probably degrade its key performance parameters; and
as a result, the current in each side of the dipole is significantly different in magnitude which again will skew the antenna pattern and probably degrade its key performance parameters.

This is exactly the problem that baluns are supposed to alleviate, but the G0KSC hybrid antenna balun is not effective in this scenario.

Some claim that the device does create a DC path at the end of the coax, as if that is an important and redeeming feature. The G0KSC device does bond the inner conductor of the coax to the boom at DC, and there MAY exist a DC path to the coax shield somewhat indirectly, possibly by a route via the transmitter. This indirect DC loop may in fact risk equipment damage, particularly receiver input circuits.

Modified Pawsey stub

Fig 3:

Fig 3 above shows same structure and currents with the link moved to make this a modified Pawsey stub.

Some key issues to note:

there is almost zero current flowing on the boom and feedline shield beyond the balun (ie to the upper left);
the currents in the elements of the Pawsey stub are equal in magnitude and opposite in phase so they cancel out; and
as a result, the current in each side of the dipole is almost equal in magnitude (and phase though it is not shown) which allows the Yagi symmetric drive and a good chance it will achieve its key performance parameters.

This is exactly what baluns are supposed to do.

Fixing your Yagi

The good news is that the G0KSC hybrid antenna balun is fixable to converting it to a modified Pawsey stub.

There is no disadvantage in this situation to having the link of the Pawsey stub bonded to the boom, so the simple fix for the defective G0KSC hybrid antenna balun is to bond the coax shield to the same point on the boom at the existing rod or whatever.

There is some small issue about the difference velocity factors of the un-insulated rod and the outer surface of the jacketed cable, but though non-ideal, for most scenarios it will be close enough and in any event a whole lot better than the existing non-balun balun.

At the end of the day, the effectiveness can be measured by measuring common mode current.

Conclusions

The G0KSC hybrid antenna balun is described without any explanation, just claims of its method of operation and no modeling or measurement to validate those claims.

The NEC models presented above are a reasonable model of the scenario, and valid. They show that the G0KSC hybrid antenna balun:

is not effective in reducing common mode current to ensuring balanced DE drive.

It is not unusual that antenna designers and manufacturers make unsubstantiated claims for their designs, and it is not unusual that those claims are shown to be untrue.

If you have used an optimised design (if the designer can be trusted), why compromise it with lack of certainly of balanced DE currents?

The problem on an existing antenna is easily fixed.

Changes