Tunable Cable Loop Indoor MW/SW Antenna

Loop antenna for MW/SW utilizing multi-conductor cable and a variable capacitor tuning system.

Based upon the Carpet Loop by David Moisan, N1KGH. The carpet loop is the original design. This is simply my own take on it.

For those who do not have a large space or ability to install an antenna for reception of distant shortwave stations and/or money to spend on often dicey commercial products, this loop antenna may present an adaptable solution.

How it Works

The concept is that a long cable with multiple conductors looped around a room (or other object/space) is used as a large inductor. A switch allows the number of turns in the inductor to be selected using the multiple wires in the cable. For example, if a 4-wire cable is used, switch position 1 uses only one of the wires, position 2 uses two of them, 3 uses 3, and 4 uses 4. More turns in use means more inductance and better reception of longer wavelengths.

This giant inductor is connected in parallel with a variable capacitor. Once a station is detected, the number of turns in the loop can be adjusted until the signal is strongest. Then the variable capacitor can be adjusted to bring the system close to or to its resonance point. Once this occurs, the signal oscillates between the capacitor and the inductor (cable loop), effectively strengthening the signal to the receiver by way of maximum power transfer.

The resonance point of the circuit is where:

f = -------------------
    2 * pi * sqrt(L * C)

where f is the frequency of desired signal in hertz, pi is approximately 3.14, L is the inductance of the inductor (henries), and C is the capacitance of the capacitor (farads). You do not need to know or understand this to build the antenna. Just there for the curious.

Parts List

(substitute whatever is in your junk box)

  • 2 x Male DB-9 Connectors. A single connector could be used for up to 4-conductor cable, but this makes the loop a bit unwieldy to install, as there will be no free end.
  • 2 x Female DB-9 connectors to match above.
  • 50 feet of 4-conductor flat telephone patch cable.
  • 1 x SO-239 (UHF) Female connector for radio.
  • 1 x Screw terminal for ground wire.
  • 1 x 2 Pole, 6 Position rotary switch from Radio Shack.
  • 1 x Single Pole, Single Throw mini toggle switch from Radio Shack.
  • 1 x 365 pF air variable capacitor from an old radio.
  • 2 x silicon diodes of any size. The cheapest ones are probably the best.
  • Wire to hook it all up. 22 AWG copper is fine.
  • Knobs, case, hardware to put it all together.

The circuit

Schematic of Cable Loop Tuner

The circuit is simple: a method to produce an effective inductor (loop) from a multi-conductor cable, a switch to select the number of individual conductors in use in this loop, and a variable capacitor to form the capacitive portion of the tank circuit.

Connections to the loop cable itself and to ground and your radio receiver can be made based upon personal taste and availability of parts. I used DB-9 connectors from old serial cables because I had them, and an SO-239 for the radio connection because it is common at HF frequencies and doubles as a banana plug socket. Ground is simply a bolt and a wing nut.

The back-to-back diodes act as a static discharge point to prevent static electricity from damaging your receiver. A high-value resistor (megohm) will work in their place just as well.

The cable

The cable is wired straight-through (Pin 1 on left end goes to pin 1 on right end, pin 2 goes to pin 2, etc.). The tuner unit's connectors are wired to produce a large coil using all of the wires in the cable.

Diagram of Cable Loop

Using the antenna

Set the variable capacitor (C1) to its midpoint, and the turn control at either end (longest or shortest). Find a fairly weak station, and adjust the turns control until the signal is strongest. Adjust the variable capacitor until signal is strongest and cleanest.

In some cases at lower frequencies (or any frequencies), a longwire configuration may work better. You can simply flip the switch S2 to lift the ground on the loop, converting the system into a longwire. The length of this wire can still be adjusted with the turns control. Open the capacitor all the way up or else it will serve as an attenuator. Or maybe not. Play with it and see what happens. Just play with the controls until you get the best signal.