Off Center Fed Dipoles fed with Ladder-Line

by Cecil Moore, W5DXP, Rev. 1.0, 10/01/17

With the availability of balanced antenna tuners, more hams may be tempted to feed their Off Center Fed Dipoles (hereafter called OCFDs) with ladder-line or open-wire feedline to lower feedline losses. Unlike a coax-fed OCFD, the inability to install a common-mode choke at the antenna feedpoint may result in extremes of common-mode current depending on the length of the feedline. This article presents some data on such systems, compares them to Center Fed Dipoles (CFDs), and discusses the potential problems of feedline radiation and RF-in-the-shack. We will not worry about the radiation patterns of the antennas and will concentrate on the common-mode characteristics.

The examples chosen for this article are a typical 80m (test frequency equals 3.69 MHz) OCFD compared to the same length CFD fed with the same length of ladder-line. The following convention written 44/88/65 means an OCFD with one 44-foot leg, one 88-foot leg, and 65 feet of ladder-line for the feedline. A similar CFD would be written 66/66/65. These are typical values for an 80m ladder-line fed OCFD or CFD. Let's look at the current envelope distribution of the two antennas according to EZNEC.

The CFD exhibits a familiar antenna current envelope where the out-of-phase transmission line currents result in in-phase radiating currents on the antenna wire, just as it should be. However, the OCFD exhibits phasing problems. We do not want out-of-phase currents on the antenna at the feedpoint because the two antenna elements are fighting each other and reducing the radiation magnitude in the far field. The only thing that can cause the antenna currents to be out of phase is for the transmission line currents to be in phase. That's the definition of common mode currents, i.e. currents that are in phase in the transmission line. The thing that causes the antenna radiation to be reduced is the same thing that causes the feedline to radiate, i.e. common mode currents are a double whammy taking radiation away from the antenna and giving it to the feedline. Fig. 2 shows the magnitudes of the common mode vs transmission line mode currents for the above OCFD and the CFD.

The reason for the out-of-phase currents on the OCFD antenna become clear. The (bad) common-mode currents on the transmission line at the OCFD feedpoint are almost five times the magnitude of the (good) transmission line currents. Yet the CFD has close to zero common mode current at its feedpoint simply because the impedance looking into each leg of an ideal CFD is the same value whereas the OCFD impedances are wildly different. It seems obvious that the OCFD needs some changes if it is going to be a trouble- free performer on 80m.

One thing we can do is change the length of the ladder-line. 65 feet was a convenient length for us but is a worst-case length for common mode problems because it is 1/4 wavelength on 80m. A high impedance at the source is seen by the common mode signals as a low impedance at the feedpoint. We can reduce the common mode problems by making the feedline 1/2 wavelength instead of 1/4 wavelength as seen in Fig. 3.

Now we are getting somewhere. The common mode current is still there on the OCFD but now its amplitude is 1/10 of the transmission line current amplitude at the antenna feedpoint. That will put the antenna currents in phase as seen in Fig. 4 and result in acceptable radiation from the antenna. In fact, it is now hard to tell the difference between the OCFD and the CFD antenna current distribution. There will still be feedline radiation because of the common-mode standing wave but its radiation level may be tolerable. It may even improve the low angle radiation ala Carolina Windom style.

The fact that the common mode current on the feedline of the 44/88/130 OCFD is low at both the source and the feedpoint is a distinct advantage in keeping RF out of the shack with a good common-mode choke-balun. The 1/2 wavelength of feedline keeps both the OCFD and OCF impedances at the source at a reasonable value.

Conclusion: If one feeds an 80m OCFD with ladder-line, the results are best when the ladder-line is an integer number of 1/2 wavelengths long. With any luck, the feedline will be one wavelength long on 40m, two wavelengths long on 20m, and four wavelengths long on 10m.