Construction of the 40 / 60 / 80m TX loop
pa0nhc 20181106 / 20200612
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An optimal circular shaped loop is probably more expensive to construct. And it has only 0.2dB to 1.4 dB less losses on 40m and 80m. Unnoticeable.

You could make a maximal efficient circular shaped loop by using VERY THICK coax, for instance :

39mm Cellflex LCF114-50 (170 Euro / 10m).

50mm Cellflex LCF158-50 (200 Euro / 10m).

A circular shaped , full size MONO BAND loop for 80m, made with 20m 39mm Cellflex LCF114-50, costs 340 Euro. 
Its diameter will be 6.4m. But it is in principle weather proof, and needs no painting. 

The loop is insulated from mass.
Two 10k bleeding resistors inside the matching box (at the top of the loop) discharge static charges.
The shown "ground" point is at 
the output of common mode choke L1,
preventing noise input.

1cm air gap at the feed point.

Cheaper square loop.
All tubing copper water pipe.
Knees brass filled with thin copper sheet and electronic tin solder.

1cm air gap at the feedpoint.

Center insulator : PVC pipe. Inters and waterproof with Vaseline petroleum jelly.
Clamps are made of stainless steel M6 threaded rod, washers and (lock) nuts.
All insulators are pieces of 6mm Trespa plate.

Version still without the matchbox at the top.

        My square loop. 
Square shaped with water pipe copper radiator : Top pipe 35mm diameter, sides 28mm diameter, bottom 22mm diameter. copper water pipe. Due to the diameter differences, the 90 degr. brass knees do not fit exactly. No problem. The space between pipe and knee is therefore filled with thin copper sheet. Then firmly soldered using standard electronic tin-lead solder. You could also use 35mm pipe for all parts at more costs.

As the RF currents in this antenna are relative small compared with much smaller loops, and only 4 connections exist, tin solder will not cause any noticeable signal loss.
For mechanical strength two M4 stainless steel bolts are drilled through all knees (and pipes).
But if you want silver soldered : go ahead. Silver solder could better withstand VERY low temperatures (< -20C).

The bare copper tube is weather-proofed by first cleaning it by sanding, then applying two layers metal-primer, and three layers of blank metal paint (or boat lacquer).
Let harden in-between.

Trespa plates serve as insulators. You also could use 6mm Plexiglas.

Clamps are made from stainless steel M4 threaded rod, washers and lock nuts.
The loop radiator is connected to the tuning box by the water proof screening of pieces RG214 silver plated coax and water tight N-connectors.

The loop is fully insulated from mass. Two 10k bleeding resistors inside the matching box (at the top of the loop) discharge static charges to the coax feeder screening.

The wooden mast (5.4cm x 7.2cm x 3.3m) is supported by three pre-stretched polyester flags line guy wires connected at the top. Better use garden composite materials for the mast. No painting, no rot. If possible, use a longer mast and place the loop higher. It could lower received noise levels.

The mast is clamped bolted to a big satellite dish stand, which is secured by 8 concrete tiles (each 2 stacked). I used thinly galvanized muffler clamps, but stainless steel is preferred. Cover the thread in vaseline (petroleum gel).
Two horizontal wooden bars support the lower part and the middle of the loop.

Two 5mm pre-stretched polyester guy wires are connected to the ends of the middle support bar. They prevent "no shaking" of the loop, and metal fatigue after long time in stormy weather. (Surprisingly strong vortex winds can occur between housing blocks !).

            Installing the tune box under a roofing cabinet.
A "Roodfing cabinet" over the tune box protects it against weather. It is made from thick Trespa (Plexyglass) plate. It is screwed together with stainless steel wood screws, and glued with waterproof (silicone) glue. Before screwing, pre-drill the screw holes wide enough to prevent splitting of the plate.

The ABS tune box containing the tuning-C, DC-motor + gears and heating, is screwed into the back wall of the roofing cabinet. 
At the outside of the lid of the tune box, a Trespa (Plexy glas) plate can be screwed and glued. This stiffens the thin ABS box lid, ensuring a tighter closing lid. All holes and stainless steel hardware must be water proofed using waterproof glue or Vaseline.