The so-called End fed Zepp (EFZ) is often cited as the basis for many more recent antenna designs, and is leveraged to provide and explanation… though few hams understand how the EFZ actually works.
End fed Zepp
Above is a diagram from the ARRL Antenna Handbook (Silver 2011).
Though a short conductor is shown to the right of the right hand feed line wire, the length is not specified or discussed in the accompanying text. It is popularly held that this is a “counterpoise” that provides a path for current equal to that flowing left into the main horizontal wire.
Let us explore the EFZ using NEC. The models are a reflection left to right of the above diagram, ie the feed is on the left hand end.
Fed with ideal current balun
Above is the model topology. The ‘counterpoise' wire of indefinite size in (Silver 2011) is zero length in the model. It is fed by an trivially small transmitter at the base of the vertical section using an ideal current balun.
Above is a close up view of the current amplitude and phase distribution at the left hand side of the antenna system. It can be seen that the currents on the two vertical wires are almost equal in amplitude and opposite in phase, ie there is both common mode and differential mode components that are significant.
The differential components give rise to impedance transformation between the low impedance feed point and the end of the horizontal section.
The common mode components of the vertical pair and the horizontal wire give rise the radiation.
Above is a plot of the common mode current vs distance from the feed point with 1W input. The first 10m is the vertical pair, and the next 20m is the horizontal wire.
The current at the left end of the horizontal wire is 0.026A, which for 1W implies Req=1480Ω, and the action of the differential current is to offer a quite low feed point impedance at the bottom end. So, this is the essence of the EFZ, the quarter wave line transforms the high impedance seen at the junction of the half wave horizontal section and quarter wave section to a lower impedance at the lower end of the quarter wave section. Note though that there is common mode current on the quarter wave section, and it contributes to radiation, albeit a small contribution.
Fed with an ideal voltage balun
Above is a plot of the common mode current when fed at ground level with an ideal voltage balun. The common mode profile on the vertical quarter wave is very different.
In these simple configurations, the feed point has essentially been at ground level and two types of balanced feed were explored, one which drove the feed point with equal but opposite currents and the other which drove the feed point with equal but opposite voltages.
The use of a non-trivial length of transmission line to the transmitter introduces a potentially common mode current path.
The EFZ antenna system comprises much more that depicted the ARRL's diagram above. The feed line and its capacity to carry common mode current are relevant.
J Pole variant
The J Pole is a variation on the EFZ where the nominal half wave radiator is rotated to vertical to achieve vertical polarisation.
Above, the J Pole uses a vertical half wave conductor over a quarter wave transformer as used in the basic EFZ. Let's model the system in NEC.
Above is a plot of current distribution of the J Pole system where the base of the feed section is three half waves above real ground, and the conductive support pole is connected to ground.
Note the common mode current on the support pole.
In fact the total Ampere-metres of current moment on the three half waves support pole is about the same as that of the top half wave, ie about half the radiation is from the support pole.
Above is the current distribution with the support pole lengthed a quarter wave (7/4 wavelengths in total).
Note the greatly reduced common mode current on the support pole.
Again, there is more to the J Pole antenna system than shown in the diagram, the feed line / support pole may carry significant common mode current.
The feed line is an important part of an antenna system, including its common mode operation that is almost always ignored.
The common mode current distribution on feed line and transformation sections of end fed antennas is highly configuration dependent which makes the term “End Fed Halfwave” somewhat nondescript, rather it is an invitation to ask for sufficient detail.
Link / References
- Duffy, O. Oct 2010. Counterpoise. https://owenduffy.net/files/Counterpoise.pdf.
- Silver, H Ward ed. 2011. The ARRL handbook for radio communications. 2011 ed. Newington: ARRL.