eHam article by KØFF
The shielded loop is a receive only antenna for 160 or 80 that is popular and easy to build. Using a loop of 3/4”CATV hardline (approx. diameter = 6’for 160, 3’ for 80), it can be used either indoors or out, and can even be installed on a ground mounted rotor, to help null out specific noise sources. Many articles on the operation of this type antenna can be found, including a good one in ON4UN’s “Lowband DXing”. What is presented here is a simple mechanical way to build one, and an improved matching system. The 80 Meter loop can be stacked inside the 160 loop, for single feed, dual band operation. This type antenna is VERY narrow in frequency coverage. A high Q is maintained by the coupling transformer. Briefly: The SLOOP’s operating concept is a circle of coax with the shield open in the center (top) by a small gap. A coil formed by the inner conductor is tuned to the desired frequency with a variable capacitor and appropriate padders (in the bottom housing). Coax is fed via a 9:1 matching transformer.
All you need is 20’(160) or 10’ (80) of the 3/4” aluminum CATV cable, an aluminum conduit TEE* with waterproof gasketed coverplate*, a 9:1 Matching transformer*, tuning capacitor and padders*,coax fitting of your choice*, shrink tubing*, a short piece of 3/4” PVC waterpipe-slit, a plastic plug* and rust proof hardware*. The mounting “mast” is a 2 foot piece of 1/2” or larger waterpipe, with 1 foot driven into the ground, or a one foot piece clamped into a ground mounted rotor. Remember, the SLOOP is intended for receive only applications, never transmit into it.
First prepare the 3/4 inch aluminum shielded CATV hardline following these steps:
Test a sample of the cable you have on hand for fit in one of the two open holes on either side of the TEE. The cable (minus any plastic coating) should be a perfect fit, or a little tight. Try not to insert/remove your working cable too many times as the material itself will become compressed and a sloppy fit.
Assuming the sample is correct, proceed.
Remove the gasketed coverplate from the TEE, and set it aside.
Prepare the TEE housing by drilling 4 equally spaced 11/64” holes around the circumference of the opposite extensions, spaced 1/4” back from the edge (see photo). Plug up the bottom hole on the inside with a plastic plug to keep insects out. Now drill and punch the solid back wall of the TEE with appropriate holes for your choice of RF connector, and mount the connector. Go on to the cable prep steps.
Cut the length of cable to 20 feet. Prepare each of the two ends by removing 2-1/2 inches of the outer plastic covering if so equipped. Next remove 1-1/2" of the shield and insulator, leaving only the copper coated center conductor exposed. Clean this well, but don't scratch the copper off. Generally it is very thin. Before cutting the gap in the top center shield , form a circular loop from the cable. Doing so now will prevent strain on the gap, once it is cut.
Find the center of the length with a ruler. Here you must slit the outer shield only, causing a gap to exist. If the piece of cable you have is covered with a black plastic coating, remove just enough of it to gain access to the aluminum so you can cut the gap. Using a tubing cutter makes the task easier but it could also be accomplished with a hacksaw. Make two shallow cuts all the way through the shield, but not into the plastic insulation, 1/16" either side of the center line. Cut and peel away the 1/8" strip just created, and discard it. There should be a nice clean gap in the middle now. Apply a few wraps of SCOTCH 33+ or equivalent vinyl tape over that, and then install the two halves of the slit and trimmed 3/4” PVC waterpipe over the area with screws, as a stiffener. Slip some heat shrink tubing over the cable, center it over the stiffener, and shrink it down with heat from a heatgun or (carefully) from a torch.
Now insert each end of the prepared cable into the opposite holes in the TEE, and push with enough force to bottom it out. When both ends are fully and properly inserted, there will be a space of 3/4” between the ends of the center conductor. Across this space is where you will solder the fixed capacitor(s) and the variable capacitor in a later step. In order to install the 4 screws that secure the cable firmly into the TEE, first drill through the aluminum cable shield with a 1/16: drill bit. Don’t penetrate the plastic insulator. Try to make the pilot hole in the center of the screwhole so things line up later. An easy way to do this perfectly is put an 11/64” drill bit into the screw hole and give it a few light turns by hand, causing the point on the drill to make a small indentation, which is the exact center. Follow up by penetrating the aluminum with the 1/16” bit. Install all 8 self tapping screw to secure the cable to the TEE. Check with an Ohmmeter that the center wire did not become grounded
Install the coupling transformer next, with the primary wire ( long red wire) going to the center conductor pin on the RF connector, the ground wire (twisted red wires) to the lug on the connector, and the secondary wire(long orange wire) to either one of the main loop center conductors Solder all connections.
Solder the tuning capacitor across the two center conductor ends. It will take about 950 pF to tune 1840 kHz .Use a 910 pF silvered mica and a 110 pF trimmer. For the 80M version, it will take 650 (3.5 MHz) to 450 (4.0 MHz) pF depending on the band segment. Use the 110 pF trimmer,, and appropriate fixed caps to resonate.
**When stacking the 160 and 80 M SLOOPS, do not put a connector or transformer on the 80M loop, just the tuning caps. Run a single wire from the 80 M housing to the 160 M housing and attach it to the center conductor of the loop, Make sure the single coupling transformer also attaches to this junction. Upsize the mounting pipe to at least 1”, preferably 1-1/4”, by adding an appropriate reducer to the bottom spigot.
If for some reason you require a smaller loop, just increase the capacitance to compensate. SLOOPS from 3 to 8 feet diameter all seem to work well.
Tune the SLOOP with an MFJ 259 or equivalent analyzer. The dip in SWR will be dramatic when resonance is achieved.
73 and c’ya on TopBand!, Geo>K0FF *=Kit parts.