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Although the 1λ wire loop antenna (square or triangular (delta)) has long been established both as a single-element antenna or, with a parasitic reflector loop, as the respected two-element quad beam. it is usual practice to have the loop in the vertical plane, providing either horizontally or vertically polarized signals according to how it is fed. A loop antenna mounted In the horizontal plane is still something of a rarity despite having been discovered by serendipity (the happy accident) over 20 years ago by the late Peter Pennell, G2PL, who for many years was a prominent DXer.

A TT Item In July 1968 (later Included In many editions of ART) described how, during some severe gales, he took the precaution of lowering his three-band quad, mounted on a tilt-over mast, so that the quad elements were in effect firing straight upwards, with the reflector loop touching the ground in places. TTcontinued: "Under these conditions, he found the performance of the aerial to be superior to that of a resonated vertical on all three bands (typically receiving S9 from VK on 14, S7 from W6 on 21 and 28MHz). The feeder SWR was little different from that in the usual vertical position... the 28MHz driven element was about 7ft above ground and that of the 14MHz element about 12ft ... Tests at G2PL suggest that the angle of radiation compares with a dipole a half-wave above ground, and he feels it would be a simple matter to erect such a system using four (short) vertical posts." Personally, I would not expect a low, horizontal loop to have much low-angle radiation, although entirely suitable for NVIS (near vertical incident skywave) short-range working.

A few years later (June 1972 TT and digested in later editions of ART) S M de Wet, ZS6AKA, reported on a series of "experiments with multiband loop aerials" including loops 1λ and 2/3λ (with stub matching), rectangular and triangular loops in both vertical and horizontal planes and with the horizontal loops at heights of about 35-40ft: Fig 1. He pointed out (as G6XN had done earlier) that a 1λ loop (unlike a ½λ dipole or ½λ loop) provides a low-impedance (resistive) feed-point not only at f but at all harmonics of f. His 1λ (3.5MHz) loop had an input impedance of about 100Ω at 3.5MHz, rising to roughly 200Ω at 28MHz, with intermediate impedances at 7, 14 and 21 MHz.

Fig 1
Fig 1. Some of the loop antennas tested by ZSSAKA In the early 1970s. No unbalanced line currents were detected with either twin or coaxial feeder despite the absence of a balun transformer.

General observations made at the time by ZS6AKA seem worth recalling:

  1. the loop tuned much more broadly than a dipole;
  2. the voltages along the loop were much lower than a dipole;
  3. although the loop requires a minimum of three supports, the extra support was usually easy to find in practice - the best results were obtained with the loop horizontal, but it did not greatly effect results with the loop horizontally slanted or even vertical;
  4. when the input is balanced, the furthest mid-point may be earthed;
  5. when the loop is fashioned in rhombic shape it does in fact become a rhombic directional aerial at the higher frequencies with a low input impedance;
  6. if the shape of the loop is changed from circular to square, triangle, rectangle and the like, the radiation resistance decreases as the enclosed area decreases - this means that a shape may be found which has an input impedance of approximately 50Ω and
  7. there are many shapes and mounting configurations which remain to be explored.

ZS6AKA noted that on lower frequencies the loop (as with a low dipole) provides omnidirectional radiation, but breaks up into a series of lobes at higher frequencies.

That was back in 1972. Coming up to date, Doug DeMaw, W1FB, provides 'A closer look at horizontal loop antennas' (QST May 1990, pp28, 29, 35) providing E- and H-plane radiation pattern diagrams for a large multiband loop antenna, computer-generated by Harold Johnson, W4ZCB, using an IBM computer with MININEC software (see July TT). W1FB's toop (Fig 2) is in the form of a square with 132ft 2¾4in sides at a height of 50ft, and the radiation patterns are at 1.9, 3.84, 7.16, 14.2, 21.2 and 28.5MHz: Fig 3. If moved up a band, the patterns should hold approximately correct for a scaled-down loop with 66ft sides at a height of 25ft The patterns resemble those noted by ZS6AKA.

Fig 2
Fig 2. The 1.9MHz/muitiband horizontal loop at 50ft as used by W1FB.

Fig 3
Fig 3. E- and H-plane radiation patterns for the loop of Fig 2 derived by W4ZCB using MININEC computer-simulation lor 1.9, 3.84, 7.16, 14.2, 21.2 and 28 5MHz.

With this large multiband loop, W1FR found that "the system produces 14, 21 and 28MHz performance that is on par with, and sometimes better than, that of my commercial triband Yagi at 55ft." He concludes his article as follows: "The point is that, if you have space for one, you can use a horizontal loop as a multiband antenna. You need not tailor it for 160m. A 75 or 40m full-wave loop will usually fit into a city lot. The higher you erect it above ground, the better its performance will he. But, don't give up the notion of a loop If you can't get it high above ground. Height extends the useful working distance of a loop, but many loops at low heights still permit good DX results at the higher end of the HF spectrum. The improvement in noise rejection during receive may be sufficiently rewarding to justify putting up a large piece of wire, especially if you live in a noisy neighbourhood.

"One word of caution: Wire that has thick polyvinyl insulation (such as No 14 (American) electrical wire) causes the antenna resonance to be somewhat lower than the formula dictates. Apparently the propagation factor of the wire, when used in a closed loop, causes this phenomenon."