ANTENNAS FROM THE GROUND UP

17. At Cross Purposes
or Vertically-Oriented, Horizontally Polarized 1 wl Loops

L. B. Cebik, W4RNL



There are many ways to make a 1 wl loop: a triangle, a square, a diamond, a hexagon, a circle. . .. What these loop antennas have in common is that they are resonant on a chosen fundamental frequency.

Physically, we can set the antennas in three positions: horizontal (parallel to the ground), sloping (at some chosen angle), or vertical (at right angles to the earth. Horizontal and vertical orientation are the most fundamental. We have already presented some notes on horizontally oriented loops, so let's focus on vertically oriented loops.

Before we dig into these loops, let's set up another distinction: horizontal vs. Vertical polarization. First, all low antennas (under 1/2 wl in height) have both horizontally and vertically polarized fields. So the question is not about exclusive polarization, as it might be if we were talking about a 2-meter Yagi at a height of 10 wl. Instead, the question is one of dominance.

Second, the polarization of a loop's transmissions (and receptions) only indirectly affects the far end of the communications circuit. For the most part, the ionosphere randomizes the polarization of signals. Polarization of the antenna's pattern plays a major role in the shape of the radiation and reception pattern, and it is this shape that affects signal strength, signal-to-noise ratio, and whether the antenna favors high or low angle radiation.

We shall look at vertically-oriented, vertically-polarized loops (VOVPLs) in a future installment, but for now, let's concentrate on vertically-oriented, horizontally-polarized loops (VOHPLs).

Some folks like to think of VOHPLs as folded dipoles opened up into sundry geometric loops. That is fine, if you remember to stress the dipole aspect of the matter. We can expect virtually any VOHPL to act like a pair of dipoles, one above the other. The resulting patterns will usually be a compromise between the pattern of the higher dipole and the pattern of the lower dipole.

The graph below (Fig. 1) charts the angles of maximum radiation for a 40-meter dipole, a 2-element wire Yagi, and three typical VOHPLs, the right-angle delta, a square loop, and a horizontal rectangle. The chart is based on practical considerations of lower HF-band wire antennas. Foremost is the fact that amateurs will install a wire antenna as high as possible, given the many constraints of backyard construction. Thus, the heights listed are for the top wire of each antenna. The premise is that if one can get a wire up to a given height, then whatever the wire antenna choice, its topmost wire will be at that height.

In all cases, the dipole and the wire Yagi have lower angles of maximum radiation than any loop. The explanation is simple: half of the loop's power is being distributed to a wire well below the peak of the antenna.

One can thus expect a dipole to outperform a vertically-oriented loop with respect to low-angle gain. The natural question is this: why install a loop? There are three very significant positive reasons why one might choose a loop:

1. The space between supports is too short for a dipole, but the supports are high enough to support the loop.

2. There is only a single high support, thus favoring one of the Delta loops.

3. The loop is inherently more immune to noise pick-up than an antenna with free ends, and the improvement in signal-to-noise ratio is worth the loss in other performance factors.

The pattern of the VOHPL on its fundamental frequency is essentially the same, whatever the precise shape of the loop. Figure 2 overlays azimuth patterns for a dipole, a rectangle, and a delta loop, all at top-wire height of 40' and with a 45 elevation angle. Patterns for increased height begin to show the peanut shape we associate with dipoles. Operationally, the rectangle most closely approaches the performance of the dipole, since its lower wire is highest among the loops.

Let's tabulate some of the properties of the various VOHPLs we might choose as an antenna. All dimensions are for 40 meters (7.15 MHz), and are approximate, since actual resonant length will vary with height above ground. Likewise, feedpoint impedance figures are ballpark to guide initial feedline thoughts.

All antennas were modeled on EZNEC-Pro over medium ground with #14 copper wire. All models have been stabilized by increasing the number of segments per half wavelength until further increases showed no significant changes. For new modelers, this technique is standard practice in the generation of reliable NEC models. Although dipoles may be stable with the minimum number of segments per half wavelength, antennas with more complex geometry, such as loops, may require more segments per half wavelength.

Right-Angle Delta: This antenna, as shown in Figure 3, uses a right triangle to hold down its height. Fed in the center, the impedance is 170-220 Ohms, depending upon height. The circumference is about 141', with a 58.4' base and a 29.2' height. Resonant dimensions will vary with antenna height.

Height          T-O Angle   Gain
Min.    Max.    (Degrees)  (dBi)
10.8    40      90         4.3
20.8    50      64         5.3
30.8    60      46         5.6
40.8    70      37         6.1
50.8    80      31         6.9

Equilateral Delta: This antenna, as shown in Figure 4, is taller than the right-angle triangle. Fed in the center, the impedance is 105-135 Ohms, depending upon height. The circumference is about 144', with a 48' base and a 41.5' height. As with the right-angle delta, resonant dimensions will vary with antenna height.

Height          T-O Angle  Gain
Min.    Max.    (Degrees)  (dBi)
1       42.5    79         -1.6
8.5     50      67         3.4
18.5    60      53         5.0
28.5    70      42         5.6
38.5    80      35         6.2

Square Quad Loop: The square loop, as shown in Figure 5, is often used as the standard for loops, although there is nothing electronically significant about its shape over and against other loop shapes. Fed in the center, the impedance is 115-145 Ohms, depending upon height. The circumference is about 145', with about 36.2' per side, with the exact resonant dimensions varying with antenna height.

Height          T-O Angle  Gain
Min.    Max.    (Degrees)  (dBi)
3.8     40      55         3.5
13.8    50      46         5.3
23.8    60      38         5.9
33.8    70      33         6.6
43.8    80      28         7.3

Diamond Quad Loop: The diamond loop, as shown in Figure 6, is usually too large both vertically and horizontally for lower-HF use. However, the builder can shrink the diamond vertically and stretch it horizontally. Fed in the center, the impedance is 120-140 Ohms, depending upon height. The circumference is about 144', with about 36.1' per side, with variations in resonant length owing to antenna height.

Height          T-O Angle  Gain
Min.    Max.    (Degrees)  (dBi)
(A maximum height of 40' is not feasible
with this configuration of the diamond
loop.  Note that the lowest maximum height
is about 52'.)
1       52      53         4.8
9       60      45         5.5
19      70      37         6.1
29      80      32         6.8

Rectangular Loop: The rectangular loop, as shown in Figure 7, is among the most practical of VOHPLs, since one can tailor its precise dimensions to the space available. The model used here, fed in the center, has an impedance of 200-270 Ohms, depending upon height. The circumference is about 138', with 49' horizontal wires and 20' vertical wires. However, exact resonant dimensions will vary with antenna height.

Height          T-O Angle  Gain
Min.    Max.    (Degrees)  (dBi)
20      40      63         5.5
30      50      46         5.7
40      60      37         6.2
50      70      31         6.9
60      80      27         7.7

Dipole: As a standard for reference, here are the numbers on the half-wavelength center-fed dipole, as shown in Figure 8. The model used here, fed in the center, has an impedance of 50-80 Ohms, depending upon height. The approximate resonant length is 67', but will also vary with height. A folded dipole will yield just about the same figures, with a higher feedpoint impedance.

Height          T-O Angle  Gain
Min.    Max.    (Degrees)  (dBi)
40      40      50         5.9
50      50      39         6.2
60      60      32         6.8
70      70      27         7.6
80      80      24         8.0

2-Element Wire Yagi: For reference, here are the numbers on a 2-element wire Yagi, shown in Fig. 9. I note it here because it uses only about the same amount of wire as a full wavelength loop The driven element is 66' long, while the reflector is 70' long. The element spacing is 20'. The feedpoint impedance is between 50 and 60 Ohms, depending upon height.

Height          T-O Angle  Gain
Min.    Max.    (Degrees)  (dBi)
40      40      41          8.8
50      50      35          9.5
60      60      30         10.0
70      70      26         10.5
80      80      23         10.8

Figure 10 provides the azimuth and elevation patterns for the wire beam at a height of 50'. Although the antenna will not outperform a rotatable Quad or Yagi at 200', it will show significant gain and front-to-back ratio in its fixed position--hopefully aimed at DX.

Throwing the 2-element wire Yagi into the mix has a purpose: to force you to think in three dimensions about the space available for antennas and about all the options that may be available. There are other directional wire designs, such as the Moxon rectangle, which we shall review in another installment.

Loop performance does not significantly change if fed at the center of the top rather than the center of the bottom. Inverting the delta designs so that the long horizontal wire is on top will improve performance, since a longer horizontal wire is now at maximum height. However, figures will still not outperform the basic dipole at the same maximum height. With the inverted delta, half the power still radiates from a lower conductor.

Most of the loop designs will require matching at the antenna feedpoint, along the line, or at the equipment position (an ATU). Therefore, parallel feedline has been the transmission line of choice, especially if multiband operation is contemplated. For single-band operation, networks, X:1 baluns, and line sections have all been successfully used to match any of the loops to a 50-Ohm coaxial feedline.

However, many antenna builders choose the VOHPL because it permits multiband operation with an ATU (or dual band operation with simpler matching means). So let's look next time at multiband loops.



Updated 9-8-99. 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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