L. B. Cebik (W4RNL, SK) wrote an article (last updated 04/03/2006) describing an experimental 6 meter omnidirectional antenna. He referred to the antenna as an “interrupted loop – impedance transforming” antenna abbreviated IL-ZX. The antenna is unique because it produces a nearly omnidirectional horizontally polarized radiation pattern with good gain and take-off angle at relatively low heights.
Figure-1 is a diagram of the IL-ZX antenna. Notice the gap located in the center of the right end of the diagram. The gap represents the interrupted loop portion of the antenna’s name. The feed point is located at the center of the lower loop opposite the end of the antenna with the gap. The impedance transformation characteristic of the antenna is due to the the wire geometry and spacing that is chosen to produce a feed point impedance near 50 Ω.
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Due to the significant inductive feed point reactance, Cebik suggested using twisted pair series capacitive loads (one on either side of the feed point) to neutralize the reactance. The capacitance needed (each load) for this version of the antenna (50.4 MHz) is ~9.2 pF. Since the loads are capacitive and in series they act like resistors in parallel. The net capacitance at the feed point is ~4.58 pF. The twisted pair loads can easily be made using #20 AWG twisted doorbell wire available from a number of sources. The lengths of the twisted pair loads needed to provide the capacitance can be measured using an antenna analyzer. Be sure that each load measures as close to 9.2 pF as possible.
Figure-2 is a diagram of the twisted pair capacitive loads at the feed point.
Figure-3 is a table of wire dimensions (inches) for an implementation of the antenna using #14 AWG THHN stranded copper wire at a height of ~20′ and optimized for operation @ 50.4 MHz. The width of the gap is 1″ and the vertical distance between the two horizontal loops is 4″.
Figure-4 is a bar graph of the predicted SWR for the frequency range (50.1 – 50.6 MHz).
Figure-5 shows the predicted 3-D radiation pattern (cursor elevation 14°) @ 50.4 MHz.
Figure-6 shows the predicted radiation pattern (elevation slice) @ 50.4 MHz.
Figure-7 shows the predicted azimuth radiation pattern @ 50.4 MHz.
The IL-ZX is certainly an interesting antenna and well worth building especially if you don’t want to deal with an antenna rotator. The antenna also provides good gain at a height of 10′ but presents a somewhat higher takeoff angle than at a height of 20′.
Special thanks to AC6LA for the help in modeling the IL-ZX antenna discussed in this article. Modeling the antenna was greatly facilitated by using AutoEZ. The optimizer and resonate features of AutoEZ were especially helpful in determining the optimum wire lengths and the feed point capacitive load values. Thanks also to W5BIG for the tips on how to measure the capacitance/ft. of #20 AWG twisted pair doorbell wire with an AIM-4170C antenna analyzer. If you’re looking for an antenna analyzer be sure to review the highly acclaimed AIM-4300.
The EZNEC+ V6 and AutoEZ models of the antenna are available from the link below in zipped format.