W1TAG LF Receiving Loop
Basic description:
The loop is square, 6 feet on a side. There are 14 turns of #12 THHN wire in total,
with 7 turns on each side of center.
Construction:
The loop frame is constructed of 1.5" PVC plastic pipe. From the center hub,
(4) 54" pieces end in tees with 9" pieces on each end to form the spreaders.
Each side of the spreaders, (7) 1" holes are drilled, 1" on centers for the #12 wire.
The wire must be threaded through the holes, so winding was done in sections that were
soldered together.
Click for winding picture.
The two 7-turn sections are connected together in the center, with the option for a
center tap (not presently used). From the tee at the bottom of the loop, a 4 foot
section of PVC is slipped over a 1" steel pipe which is anchored to the ground.
The loop therefore turns on the steel pipe inside the PVC.
Tuning and coupling:
Click for schematic of tuning unit.
The two wires from the ends of the loop are brought to a tuning box clamped to the PVC
below the loop. The loop is series tuned into a coupling transformer, with no ground
connection on the loop side of the transformer. That transformer is presently a 1:2
stepup wound with 10:20 turns on an FT-82-77 core. Higher turns ratios could be used
to improve pickup at the expense of bandwidth. Tuning is presently done with 8 small
relays controlled by a switchbox in the house, allowing binary combinations of
50, 100, 200, 400, 800, 1600, 3200 and 6400 pF. The loop can be roughly tuned from
over 400 kHz to 45 kHz. Plans are underway to add a motorized trimmer capacitor, and
then tighten the coupling.
Connection back to the shack:
Presently, the 20 turn secondary of the coupling transformer is connected to 100 feet
of RG-58 back to the shack. There is no preamp at the loop! In the shack, the RG-58
connects to a 50 ohm in/out preamp (designed by W1VD)
Click for schematic. I plan to
replace the RG-58 with a balanced, shielded, twisted pair cable this year.
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