ANTENNAS FROM THE GROUND UP
The bi-directional phased array is a venerable antenna type that we should not overlook in our quest for a good wire antenna. In this episode, we shall review a pair of old-timers: the 8JK "flat-top" array and the expanded Lazy- H. But first, a little nomenclature. Fig. 1 shows three common terms used in phased-array work. An antenna element is collinear whenever it is two or more half wavelengths long, since it can always be analyzed as a series of half wavelength elements. In multi-element arrays using long elements relative to a given frequency, we often drop the term to emphasize other features of the array. However, the 8JK and the Lazy-H that we shall examine will consist of collinear elements on many frequencies on which we operate them.
An array is an end-fire antenna whenever the main direction of radiation is in a plane with the elements. Yagis are end-fire arrays, and so is the 8JK. An array is called broadside when the main direction of radiation is at right angles to the plane of the elements. The 1 wl quad loop and the Lazy-H are broadside arrays. In phased array work, the difference tells us something about the array. For example, end-fire phased arrays generally reverse the phase-line with a half twist between elements, not only in the 8JK, but in the LPDA as well. Broadside arrays are generally fed in phase.
Let's begin with an 8JK, named after its developer, W8JK. It consists of two elements, normally about a wavelength long each spaced from 1/8 to 1/2 wl apart. The elements are fed 180 degrees out of phase by a simple means: run two parallel feedlines to the center point between elements and give one side a half twist. The lines can actually be any length so liong as they are equal. See Fig. 2 for a sketch of the basic 8JK. In the last episode, we looked at a 44' wire antenna for 40-10 meters. On 10, it was an EDZ. The 8JK that we shall look at here consists of two 44' elements spaced 22' apart. Like the single wire, we shall place them at a height of 66' so that you can make comparisons between the single and double wire arrays.
The following table provides modeled performance data on the 8JK for 40-10 meters. The azimuth patterns for the array, each at its own TO (take-off or maximum radiation elevation) angle, appear in Fig. 3.
General Performance of a 44' 40-10 Meter 8JK
Freq. Max. Gain TO VBW HBW Feedpoint Z
MHz dBi Deg Deg Deg R +/- jX Ohms
7.15 9.0 26 30 66 1 - j 92
10.1 10.5 19 22 64 15 + j 138
14.15 10.8 14 15 60 132 - j 463
18.1 11.3 11 12 54 26 - j 98
21.2 11.4 10 10 47 23 + j 3
24.95 11.9 8 9 40 30 + j 131
28.5 11.6 7 8 32 142 + j 441
The 8JK, in the form shown here, gives significant gain improvement over the single wire 44' antenna. The spacing used here, 22', provides peak gain on 12 meters, with over 11 dBi gain on 17-10 meters, and over 10.5 dBi on 30 and 20 meters. The key limitation of the array appears on the lowest 2 bands: a very low resistive component to the composite feedpoint impedance at the junction of the two phasing wires. Some change in the values can be obtained by changing the line lengths (together to keep them the same), but the element length on 40 and 30 meters is falling below 1/2 wl. Hence, the composite impedance will be low, with a consequential tendency for higher losses. The losses result from the fact that any connection and wire losses will claim a higher percentage of the power supplied to the array assembly. Hence, the array might best be used from 20 meters on upward.
Note that there is an upper limit to the use of the array. On 10 meters, the pattern shows the typical EDZ ears, indicating the formation of a new lobe set. Extending the wire length to 1.5 wl on 10 meters would yield a 6-lobe pattern. As a result, the 8JK is best considered to have a 2:1 frequency range for best operation. Nevertheless, over this range, the array provides very even gain from band to band, with the low TO angles that result from both wires being at the top height of the array.
Unlike single-wire 44' element in 40-10-meter use, the 8JK is not as easily placed into a triangle for full horizon coverage. Hence, its best use is where there are main target communications regions both fore and aft of the array. In my location (Tennessee), setting the array for Europe on one side would yield good results with VKs and ZLs in the other direction.
The end-fire 8JK we have just examined has a broadside counterpart: the Lazy-H. Conventionally, a Lazy-H consists of two 1-wl wires placed a half wavelength apart vertically. The wires are fed in phase by running equal-length parallel feedlines to the center point between them, with a main feeder to the station. However, the version that we shall explore will be set up as an expanded Lazy-H. For 10 meters, the wires will be 1.25 wavelengths long or about 44'. The spacing will be 5/8 wl or 22'. Fig. 4 gives an outline sketch of the array. Again, the phase-line feeders can be any equal lengths. In both this case and the 8JK, the phase line feeders are 450-Ohm, 0.95 VF line, such as might be found in vinyl "window" line. If we use a different line having either a different Zo or a different VF, then the charted composite feed impedances will differ from those given. However, the principles of operation will not change.
The two wires of the array are stacked vertically,. Since we have arbitrarily set the top wire height at 66', the bottom wire of the Lazy-H will be at 44'. Relative to the 8JK and to the single 44' wire explored in the last episode, the Lazy-H will have slightly lower TO angles. The lower angles result from the fact that the TO angle is a composite of the angles that result from the two wires, with the lower wire contributing a higher angle. Hence, the net of the two is slightly higher than for an array with all wires at the 66' level.
The Lazy-H will also show considerably more range in the gain levels, unlike the even gain of the 8JK across the bands we wish to use. The following table gives performance numbers, with Fig. 5 showing azimuth patterns.
General Performance of a 44' 40-10 Meter Expanded Lazy-H
Freq. Max. Gain TO VBW HBW Feedpoint Z
MHz dBi Deg Deg Deg R +/- jX Ohms
7.15 6.4 33 44 99 10 - j 97
10.1 8.1 24 27 85 48 + j 103
14.15 9.0 17 18 73 385 - j 395
18.1 10.9 13 14 61 43 - j 126
21.2 12.5 11 12 52 22 - j 17
24.95 14.6 10 10 41 18 + j 115
28.5 15.1 8 9 31 64 + j 425
Like the 8JK, the Lazy-H shows no very large extremes in reactance. Hence, most ATUs can accommodate these values. However, the impedance values that appear at the tuner terminals may vary widely from those shown in the chart, depending on the length of feedline from the antenna assembly to the shack. Only 40 meters shows a quite low impedance, although it may be high enough to be usable without undue loss. However, careful construction to minimize wire-junction losses is advisable.
Because the lower wire presents such a high TO angle on 40 meters, the gain on that band is actually lower than the gain using a single 44'. However, the reduction is only a little over a half dB, which some may find acceptable to acquire the improved gain on the higher bands. (A full-size dipole for 40 meters would have a gain of about 7.5 dBi if placed at a height of 66'.) Perhaps the most interesting question is how the Lazy-H achieves such fine gain values, especially from 15 meters upward.
Fig. 6 provides part of the answer by comparing the elevation patterns of the single-wire 44' element and the Lazy-H on 10 meters. Both the single wire and the Lazy-H have horizontal beamwidth that are virtually identical. Therefore, looking only at the vertical pattern will give a good idea of where the Lazy-H gets its added (5 dB) gain. The single wire--and the 8JK, for that matter--show a series of lobes all the way to a vertical angle, and the lobes are of nearly equal strength. In contrast, the Lazy-H, with its vertical stack of wires, tends to suppress upward radiation, leaving more power within the lowest lobe.
This same phenomena occurs at least down through 20 meters. Fig. 7 compares the elevation patterns for the single-wire antenna and for the Lazy-H. Again, since the horizontal beamwidths of the two antennas are so nearly equal, the elevation pattern can tell an accurate story. In this case, we can estimate the area under the two upper lobes of the single wire and compare it to an estimate of the area under the corresponding part of the Lazy-H pattern. The differential shows up as stronger lobes in the Lazy-H. As well, the vertical beamwidth of the Lazy-H array is larger, giving a bit more coverage to the possible angles of arriving signals.
As we did for the single 44' wire antenna, we can design a triangle to hold 3 of the 44'-element Lazy-Hs. The antennas will show minimal interaction, and we can offset one or more of them to obtain the best headings for our desired target areas. The same structure and feed-switching system that we showed in the last column can be used with the Lazy-H with no changes.
If we had the room, both vertically and horizontally, an array of 88' Lazy-Hs would provide similar coverage for the bands from 80 to 20 meters. Unless we could also double the height, performance will be down just a bit. However, even with a top height of 75', the array will give outstanding performance, with 13-14 dBi gain on 20 meters. If we have to reduce the spacing from the optimal 44' between wires down to say 35' (« wl on 20 meters), we would still have an exceptional bi-directional array.
The 8JK and the Lazy-H are extremely simple examples of phased arrays, but they are also very effective antennas. And they are relatively inexpensive!
Updated 11-06-2003. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.
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