Projects, info & thoughts from Dick's lab
Most recent update 10-6-201060kHz Shielded Loop Antenna
This antenna was constructed to use with a Dymec DY-5842 VLF Receiver, primarily to receive WWVB on 60kHz. The Dymec optional shielded loop antenna was built as a 4 foot diameter circle with 50 feet of cable attached. The nominal inductance of the Dymec loop antenna is specified as 1.43mH and it has no tuning capacitor of its own -- tuning is done with switched and variable capacitors inside the DY-5842. For 60kHz reception, the nominal total tuning capacitance is 5nF, which includes the antenna and cable C.
This antenna is a square loop approx. 31 inches (outside) on a side, and it has 4 loops of 4-conductor telephone cable with 22-gauge wires, for 16 turns total. It has approximately 25 feet of two-conductor shielded connecting cable, fitted with a UHF Twin-Ax connector that mates with the DY-5842's input jack.
Copper-pipe shielded loop antenna
For those wanting to build their own, I assembled the pieces of 3/4" copper pipe and fittings, and a short piece of plastic tubing for the shield gap, around the four lengths of 4-conductor cable, to avoid messy lubrication and difficult pulling of wire through the corner ells. The mechanical fit of the copper is good and I epoxied the ground-down plastic barb-type nipple into the first two short lengths of copper pipe. Once completely assembled, I used a Bernz-O-Matic torch to quick-heat and sweat a little solder into each joint to close the shield electrically and add mechanical strength.
Plastic-pipe shield gap
I used a plastic conduit box lined with copper mesh shielding instead of a metal conduit box because I liked the big mounting ears of the plastic box. I had intended to mount it from an attic rafter. The nice copper mesh came from a Michael's hobby and craft store, a chain-store operation. I don't know what crafts people use it for, but it's very good stuff for electronics and can be cut with scissors yet is sturdy and holds shape well.
If I were doing it again, I'd use a metal conduit box.
Conduit box interior shield
Conduit box interior wiring
The conduit box had the correct holes for the copper-sleeve-to-threaded connectors. I cut the copper mesh and folded it into a box shape to fit into the plastic box, then scissored out the holes for the threaded ends to go through. The terminal strip in the box makes the connections between the ends of the loop and the connecting cable's pair of conductors, and the bolt through the box wall that holds the terminal strip also provides the ground connection between the mesh and the cable's shield.
Twin-ax cable end connector
Low-precision measurements made at the twin-ax connector at the end of the cable:
Inductance = 1.47mH
Capacitance = 2.5nF