The IL-ZX Antenna for 40 Meters

L. B. Cebik, W4RNL (SK)

Every antenna design has a niche in the overall world of amateur radio antennas. The one described here has a quite small niche: it is for the individual who requires operation on 40 meters at low elevation angles, but who does not have the real estate to erect one of the SCV (self-contained vertically polarized 1 wavelength loop) antennas. The IL-ZX provides low-elevation angle radiation within a narrow operating bandwidth at low gain with a bi-directional pattern and reduced radiation at higher angles. It can be fed directly with 50-ohm coaxial cable, although a network antenna tuner will likely be useful for increasing the usable bandwidth.

IL-ZX is shorthand for Intermediate Loop-Impedance Transformation antenna. The design has some of the properties of a small loop, for example radiation off the edges of the loop rather than off the face. However, it does not require the level of mechanical care associated with small loops and replaces the capacitor with a simple capacitive gap, the spacing of which resonates the loop. The native feedpoint impedance of such a loop, about 1/2 wavelength in circumference, is around 10 ohms. By using a double-loop form of construction, the impedance is raised to about 40 ohms.

Small Loops

The small loop is defined by some experimenters, such as W5QJR, as a loop whose circumference is between 0.1 and 0.3 wavelengths. Figure 1 shows the elevation pattern of one such loop resonated at 7.2 MHz. The maximum gain for copper loops and lossless capacitors is relatively constant across the range of defined size at 0.4 to 0.45 dBi at low elevation angles. Feedpoint impedances range from 0.5 ohms for the smaller sizes to about 1.5 ohms for the larger sizes. Below 0.1 wavelength circumference, the loop gain drops rapidly, as does the feedpoint impedance.

Small loops require extreme care in construction, since every fraction of an ohm connection loss results in large increases in power lost to heat. Hence, 3/4" diameter copper water pipe, soldered at every joint, is a common material. The required resonating capacitor demands special care of construction and attachment. If one has the skills to build one, a small loop can be a very effective antenna. With a stepper motor operating the capacitor, a 2:1 frequency range of operation is easily possible with good results.

Large Loops

In contrast, a large loop is thought of as a full wavelength in circumference, such as the quad loop. This loop has a natural resonant feedpoint impedance of 125 to 130 ohms. Many users reduce this impedance with a 1/4 wavelength section of 75-ohm coax so that it presents a reasonable match to 50-ohm coax for the remainder of the run. The antenna offers a fairly wide operating bandwidth without further adjustment.

The full wavelength loop is capable of higher gain than a dipole placed at the center height of the loop. However, a large loop is about 1/4 wavelength on a side, about 35' horizontally and vertical on 40 meters. If fed at the bottom or top, the radiation pattern is largely horizontally polarized and subject to the same high-angle of maximum radiation as a dipole. Hence, low mounting heights reduce the effectiveness of this antenna.

Fed in the middle of one side, the antenna offers low angle radiation, largely vertically polarized. However, for maximum effectiveness, the antenna requires about 10' spacing above ground, raising its top height to about 45' or so. Figure 2 shows the pattern of a vertically polarized 40-meter large loop.

The full-size quad loop is but one of several SCV designs for achieving low angle vertically polarized radiation without need for a ground plane and without high angle radiation or reception of QRM and QRN from those upper angles. They have come into increased use by those who have directly or indirectly read into materials researched by ON4UN and others. Another entry in this series of notes attempts to put into perspective the entire spectrum of SCV antennas.

SCVs require significant real estate, either or both horizontally and vertically. The modern city lot or rental property does not always offer sufficient space even for a 40-meter SCV.

The Intermediate Loop

The Intermediate Loop (IL) is a small loop enlarged to approach 1/2 wavelength in circumference. Because the antenna approaches a natural resonant point, its operating bandwidth enlarges, reducing its gain at any single frequency. However, the antenna offers lower construction losses because the resonance can be established simply by adjusting the width of the gap at the top of the antenna. Capacitance from one wire end to the other is sufficient for the task, but the low-C high L nature of this circuit also contributes to broader response and lower gain. Figure 3A shows the outline of the basic IL, which has a natural resonant feedpoint impedance of about 10 ohms. Relative to a small loop with an adjustable capacitor, the IL-ZX is a one-band antenna.

The feedpoint impedance can be raised to about 40 ohms by doubling the loop and feeding only one of the wires, as shown in Figure 3B. (Hence, ZX = Impedance Transformation.) This method is essentially the same impedance transforming technique used in the folded dipole. With wires of the same diameter at any spacing, the transformation is 4:1. This transformation applies to both radiation and heat components of the impedance, so no magical reduction in losses occurs--and likewise, no magical increase in gain occurs. However, the feedpoint impedance is now more manageable for use with 50-ohm coax.

A second benefit of the double loop is that it offers the builder standard techniques of wire antenna construction. The loops may be spaced from 6" to 3' apart with corner CPVC spacers. Wire joints should be carefully constructed and soldered. The antenna benefits from the use of large wire sizes, with 1" wire showing an additional 0.5 dB gain over #12 wire. Therefore, one may wish to build the antenna from such materials as 450-ohm parallel line for each loop to simulate fatter wire. If such a method is selected, it is usually wise to solder a short across the parallel line periodically to ensure equal currents on each wire. (Do not short the two loops except at the top gap.)

Figure 4 shows two arrangements for the top gap. In one case, the loops are brought together as a point; in the other they approach each other as a bar across the loop ends. Since the gap is actually the dielectric space for a capacitor formed by the loop ends, the difference in construction can make a big difference in antenna size and adjustment. Models of the point- gap required about 18' per side for the antenna, with a gap between 0.2 and 1.0' wide, depending on spacing of the loops. The flat-gap antenna, for loops spaced at 2' and a gap of 0.8' required sides of only 17' each. The flat-gap construction will make side length a much more sensitive function of the loop spacing, since the capacitance between ends will change more radically with loop spacing and the consequential lengthening or shortening of the wires facing each other. In all cases, the builder should be prepared to do considerable experimentation to achieve resonance.

Performance

The IL-ZX offers the would-be 40 meter operator a relatively small antenna, no more than 18' per side. Its best low angle performance occurs with the center about 15' high and its bottom wire therefore about 6' off the ground. The high point becomes about 24' up.

The 2:1 VSWR operating bandwidth is about 100 kHz at 40 meters. However, a network ATU in the line should expand this without introducing significant losses on this lower HF band where a full wavelength of coax feedline is over 90' long (accounting for velocity factor).

The primary signal direction of the IL is like that of the small loop: off the edges of the loop, as shown in Figure 5. With a center height of 15' or so, the elevation angle of maximum gain is 21 to 22 degrees, similar to SCV angles. Front-to-side ratio is generally around 10 dB.

In the process of further experimenting with the IL-ZX design, I discovered that you can easily create a virtually circular low angle pattern--still of relatively low gain--by turning the IL-ZX "on its side." In this orientation, we need to raise the antenna to a base height of about 15' (for a top height of about 33') in order to eliminate excessive influence of the ground on one side of the antenna wire run more than on the other. At the 15' height, the impedance is about 64 + j15 Ohms, still an easy match for coax.

Figure 6 shows the circularized pattern at about the same take-off angle as the "upright" IL-ZX. Gain is not significantly different from one version to the other. Hence, which orientation you choose to use is largely a matter of the pattern that you desire and the ease of feeding the antenna at the side vs. at the bottom.

The principle disadvantage to the IL-ZX antenna is low gain. The antenna gain at maximum is about 3 dB less than that of a full size quad loop and about 4.5 dB less than that of a half square, when each of these is at optimum height. The reduction is less than a full S-unit in signal strength.

However, the antenna offers two advantages that offset the reduction in gain. First, although not as narrow in reception bandwidth as a small loop, the sensitivity of the antenna to reception noise is considerably less than that of a resonant dipole or large loop. Second, the attenuation of signals at higher angles (in the 45-degree elevation angle range) reduces the reception strength of QRN and QRM. Hence, the signal-to-noise ratio of the antenna should be quite good for signals in the desired main lobes of the antenna. Since most receivers have excess gain at 40 meters, reception of desired distant signals should be a matter of increasing either pre-filtration or post-filtration gain.

Even if we become very conservative and estimate performance at 6 dB down (1 S-unit) from an optimized half square, the transmitting success ratio should only go down in contest and pile-up conditions. For QRP operation, raising power from an initial 1 watt to a final 4 watts would restore signal strength at the reception end.

The IL-ZX is not by any means a perfect antenna, designed to outperform anything else on the market. However, neither is any other antenna. Every set of performance figures carries with it a set of operating specifications within which performance is measured. We too often ignore this fact when evaluating antennas.

If vertical and horizontal space are at a premium and skills needed to build an effective small loop are somewhere in the future, the IL-ZX may serve as an effective low radiation angle antenna in the interim until a perfect antenna site can be purchased. If you decide that you do not like the antenna, you can likely put the materials to use on other projects.

The ILZX Horizontally

Considerable interest has grown up in the last few years relative to the intermediate or interrupted loop used in a horizontal position. One or more such antennas--strung together for multi-band use--have appeared on the market within the British Commonwealth; one is called trhe "Cobbweb." However, a single-wire intermediate loop shows a very low impedance and requires a matching system for the ubiquitous coaxial cable feedlines preferred by many amateurs.

The ILZX form of the interrupted loop is quite usable in a horizontal position. In fact, with almost no adjustments, the vertical ILZX for 40 meters can be used horizontally. In the following notes, we shall build the model ILZX in the same way for horizontal use that we used for vertical applications. The #12 elements will be separated by 6" and form a square that is 18' on a side. The tips that approach each other will form a "spear tip" pair for ease of adjusting the gap to refine the source impedance. The tips will be 1' apart.

If we place the antenna at 50' above ground, we find a pattern resembling the one in Fig. 7. Note that the pattern is stronger along the axis formed by the feedpoint and the gap. The feedpoint impedance under these conditions is about 53 Ohms, with about the same bandwidth as the vertical version: 100-150 kHz or about 1/2 of the 40-meter band. Since the antenna is set for mid-band, a user would have to adjust the dimensions to favor either the CW or the SSB portion of the band.

One myth surrounding interrupted loops is that they have a circular pattern. They do not. Due to the current distribution along the wire, radiation from the region on each side of the feedpoint yields a stronger pattern on the feedpoint-gap axis. In order to develop a circular pattern, one must readjust the shape of the ILZX into a long rectangle with shorter feed-region and gap-region dimensions. An example of such an antenna appears in the "Experimental Omni-Directional Antennas for 6-Meters". The general proportions would be a partial guide to developing a truly omni-directional interrupted loop for any other band. However, expect to make considerable adjustments for differences in the wire spacing, the wire size as a function of a wavelength, and the shape of the wire ends at the gap.

The maximum gain of the horizontal ILZX is about 5.1 dBi at a 37-degree TO angle. The minimum or side gain is 3 dB less. Nevertheless, the pattern shows considerable side-pattern development, as displayed in Fig. 8. The graphic shows both the vertical and horizontal components of the total pattern. The vertical components are largely a function of ground reflections, but they still contribute to the overall useful radiation. Since 3 dB difference between the main and cross axes amounts to about half an S-unit, the radiation might be considered to be adequate for omni-directional operation.

Compared to a dipole, the horizontal ILZX holds its own quite well, as demonstrated in Fig. 9. I modeled a resonant dipole at 50' above average ground for comparison. The dipole's maximum gain is about 1.1-dB higher than the maximum for the ILXZ. However, the dipole shows about 7-dB difference between its maximum and minimum gain, where the minimum is off the ends of the antenna. Note that for dipoles well under 1-wavelength above ground, we do not obtain a true figure-8, but only a peanut. Brought closer to ground, the pattern becomes a broad oval.

Since the ILZX has a naturally oval pattern, it better approaches the omni-directional pattern favored by many hams who have only a single, fixed-position antenna. Erecting an ILZX requires only an 18' by 18' space, but does require 4 corner support posts for the 40-meter version. A version for 20 meters would require only a 9' by 9' space and might be supported on a single mast with fiberglass spreaders. The higher the frequency, the easier the ILZX will be to support. Because the antenna has a pattern that approaches the omnidirectional, it requires no rotator. However, it does call for orienting the strongest axis in the direction(s) of the most favored communications targets.

Although the antenna looks something like a beam, it is not. Hence, it will not provide the QRM attenuation to the sides and/or rear of a beam. Indeed, the gain is less than that of a dipole (and hence considerably less than the gain of any well-designed beam). That is the price one pays for omnidirectional coverage. About the only way to obtain more gain from the ILZX is to extract it from the high-angle radiation. One (impractical) scheme for doing so is to stack and feed in-phase two ILZXs spaced 1/2-wavelength vertically. The result is about 3-dB more gain in every direction.

The horizontal ILZX is suited to an exceptionally wide variety of construction techniques, depending on the frequency of operation and the exact layout of the loop and gap structures. Nested multi-band version may use a fairly low impedance line to connect feedpoints. The system of closed sleeve coupling sometimes works best when the main feedpoint is the highest frequency loop. Wire interactions will require loop adjustments, especially for the inner loops. As well, expect significant current on the inactive band loops and consequential modifications of the overall pattern on some bands. Finally, if one or more bands seem hard to bring into line, try moving the composite feedpoint to a different element relative to the one initially used. Be certain to check the SWR bandwidth for each trial arrangement before finalizing the selection.

For a multi-band antenna, you may have better luck separating the bands. 20-15-10 provides less element-to-element interaction than a 5-band version of the antenna, although the harmonic relationship of 20 and 10 meters may show some pattern deviations. Of course, a second smaller array for 17 and 12 meters makes a good antenna to stack on top of the tri-band model.

The ILZX principle of raising the feedpoint impedance simplifies the matching problem that faces single-wire interrupted loops. However, it requires greater care in supporting the double-wire loops. The wire problem might be resolved by using TV twinlead or 450-Ohm window line. Such insulated transmission lines will likely require adjustment of the dimensions downward by 2 to 5 percent to account for the antenna velocity factor of the vinyl coatings.

Every variation of the horizontal ILZX will demand ingenuity and considerable experimentation. As well, remember that the horizontal ILZX resembles every horizontal antenna in the relationship of its elevation angle of maximum radiation to the height above ground. The original vertically polarized ILZX provided low-angle radiation, but suffered gain losses due to its proximity to ground. The horizontal ILZX provides more gain, but at higher elevation angles until the antenna is at least 1/2-wavelength above ground. The higher the operating frequency, the easier it is to meet the height requirement for long-distance communications.

Happy experimenting!

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