Telephoto View of the South-East Corner of the HF Delta-Loop Antenna at the Salt Lake City Receiving StationNylon pulleys at the corners automatically adjust loop leg tensions.
Nylon pulleys, ceramic insulators, and 350 lb breaking strength Dacron 1/8 inch diameter double-weave support ropes connect like this to the north-east and south-east corners of the HF delta loop installed at this site. Heavy-duty balanced 300-ohm 18 AWG 19-strand copper-clad-steel feed-line attaches to the west corner. The feed-line drops straight down toward earth from the loop feed-point to maintain balance. It enters the roof through the center of a 9-inch diameter steel stove pipe (used for no other purpose) that is grounded both at the top and bottom via multiple 8 AWG solid copper ground wires. The delta triangle is equilateral.
Loop corner pulleys provide several advantages:
- Loop leg tensions automatically equalize if flexible antenna wire is used. That makes tension adjustment much easier, because tightening any one support rope removes sag from all the legs of a loop simultaneously.
- Loop antenna wire is less apt to break where trees are used for support, because loop leg tensions automatically equalize as trees blow in the wind and as tree grow over time.
- It isn't necessary to measure and cut individual wire legs during construction. It is difficult to accurately measure long lengths of wire without a special tool designed for the purpose. Only the total length of a loop needs to be measured if pulleys are used. Most wire vendors are able and willing to accurately measure and precut one long length of wire for you.
- It isn't necessary to break antenna wire insulation at the corners of a loop if pulleys are used. The surface of copper wire quickly corrodes when exposed to air, especially moist salt-laden air near the sea or polluted air near industrial complexes. Furthermore, precipitation static and corona due to nearby thunderstorms are both much less apt to occur if antenna wire is continuously insulated.
- If isn't necessary to splice individual wire leg ends together at the corners of a loop, both simplifying installation and eliminating the potential for bad connections to develop in the future. Furthermore, precipitation static and corona due to nearby thunderstorms are both much less apt to occur if there are no sharp points on conductors at the corners of a loop.
Loop corner pulleys have only one significant disadvantage:
- Loop leg length ratios are relatively insignificant at wavelengths that are low compared to the total circumference wavelength of a loop. However, that isn't true at higher wavelengths. Both the high-frequency radiation pattern and gain of a loop are highly-dependent upon leg length ratios. It is important to maintain precise leg length ratios if the radiation pattern or gain at high frequencies are important.
The HF delta loop at the receiving site is uses military grade M76-HW-C-14-41-A PVC insulated 14 AWG wire composed of 41 strands of 30 AWG tinned copper. It is very flexible, has a breaking strength of 142 lbs, and very little stretch. The loop circumference is 170 feet, 9 inches. PVC insulation reduces the velocity factor of antenna wire. The loop length was reduced to compensate for that and make the antenna resonant in hams bands.
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