# Discussion of WA7ARK’s contribution to a QRZ thread on an End Fed Dipole

In another long running discussion on QRZ about End Fed Antennas, WA7ARK offered a contribution:

(1) Back in post #30 I showed that with a halfwave wire fed close to its end works just like the same wire fed in the center; the only difference being the feed point impedance. I let EzNec figure this out; I didn't have to explain it with any mysterious “displacement” currents. Shown as (1) in the attached.

Since, in the model, the source is a constant current source, that forces the current on either side of the source to be equal, and the radiation pattern predicted by EzNec reflects that, because the patterns for the end-fed and center-fed match… (go back and look at post #30)

His post #30 is of a 67′ dipole at 66′ above poor ground @ 7.18MHz, fed at one end.

Above is the current distribution of my approximate re-creation of his model in NEC-4.2. It reconciles with his published graphs.

I should note that the current graph is an interpolation from stepwise segment currents, and as such it is potentially misleading. In this case the current in the driven segment (#2) is 1A and the magnitude of current in the end segment is 0.375A.

The issue is his reasoning:

Since, in the model, the source is a constant current source, that forces the current on either side of the source to be equal, and the radiation pattern predicted by EzNec reflects that, because the patterns for the end-fed and center-fed match… (go back and look at post #30)

In this scenario, it does not matter whether a current or voltage source is used.

Above is the same scenario but using a voltage source. The current distribution is identical, the type of source is not the reason for the distribution, the current distribution is determined by the model geometry.

WA7ARK later gives a diagram where he develops the idea of some feed configurations.

Above, (2) depicts an ideal transformer coupling a coaxial feed line to the dipole and shows Icm=0. In fact, the presence of the common mode conductor (the outer surface of the coax shield) in the E and H fields caused by dipole current makes Icm likely… simple linear circuit analysis of the ideal transformer ignores that coupling.

Lets just add the common mode conductor to the NEC model. In this case, a 6mm wire is configured 0.1m from the dipole end, from 2m below ground to 0.02m below the dipole wire. (Note: it is actually 2 GW elements and NEC-4 models buried wires better than NEC-2.)

Above, it should be no surprise that there is current flowing on the common mode conductor, and in this model it is not by virtue of transformer imperfections (the capacitances shown in WA7ARK's (3)), it is because the dipole and common mode conductors are inextricably E and H field coupled.

Additionally, practical transformers incorporate coupling elements as shown in WA7ARK's (3), and autotransformers as shown in (4) are often described and have another coupling element.

The scenario of (4) is rather simple to model.

Above, the current on the feed line shield is higher than for scenario (2) shown above. That is not surprising, and it is the reason why that type of feed configuration is used.

WA7ARK's diagram shows Icm=I2, and it is often argued that since I2 is very small (it is at the end of the dipole, and as everyone ‘knows' that is almost zero), Icm is insignificant.

Firstly, Icm=I2 is true only at a point. As can be seen, both the dipole current and the feed line common mode current are standing waves. Secondly, the connection of the feed line common mode path modifies the current distribution and the current at the dipole end is small, but not insignificant. In this particular scenario, Icm near ground is significant (and greater than I2).

Most explanations of OCF dipoles ignore the existence of the feed line common mode current path and these various couplings.

This is not to represent that these effects are only present with OCF dipoles, they exist also with symmetric CF dipoles, but the key issue is the magnitude of coupling, of common mode current, and the effects on radiation pattern on transmit, and local noise pickup on receive.

Physical symmetry of dipole AND feed line helps to reduce coupling from dipole to common mode conductor.

OCF dipole antenna systems are quite complex to analyse, very sensitive to implementation detail, and it seems that in the wider world, the hams with the strongest views for and against rely upon the simplest (inadequate?) understandings.