[Dept. of Computer Science]

Tuned Loop Antenna


Our Department and University.
My radio collection; next item.

You can improve your Medium Wave reception with this little Do-It-Yourself project; you'll spend about 5 guilders on materials (or use your junkbox) and one evening to build it. Have fun and be surprised by the result!

Principle

The antanna consists of an LC (tuning) circuit formed by the air-core loop and a tuning condenser. High-frequent magnetic fields induce currents in the loop, and these are (passively) amplified by the resonance of the LC circuit for the frequency to which the circuit is tuned. The reaction is maximal if the field lines flow through the core, that is, the axis of the core is perpendicular to the direction of the transmitter, and minimal if the axis points in the transmitter direction.

The energy can be guided into your receiver in several ways.
(1) If you use a portable radio with built-in ferrite rod, just place it inside or near the loop.
(2) You can add a pickup coil, consisting of several windings isolated from the primary coil, and connect it to the antanna/ground input of your receiver.

The antenna improves reception of weak or interfered stations in several ways.

The antenna principle resembles that of the famous Kiwa Loop; I am not claiming that you'll have the same performance, but building your own loop is fun and a lot cheaper!

My Loop

The wooden frame, constructed from 17x28mm wood in my junkbox, defines a 50cm x 50cm square on which the primary coil is wound for 14 turns. The distance between turns is 1cm, with an exception of turns 7 and 8, which are 2cm apart to allow placement of the leg. The total wire length of the primary coil is about 28m (this was the most expensive part because the wire is 5ct/m!) and its DC resistance is about 1 Ohm. The tuning capacitor comes from the junk box.

The secondary loop consists of 4 turns of isolated wire.

I didn't design the antenna for low electric pickup so I do experience interference from electrical sources. Electrical pickup can be reduced by shielding the primary loop; perhaps my next DIY antenna will be shielded... The selectivity and directionality are nice for MW DX-ing.

Some ascii art

          wwwwwwwwwwwwwww
          wP           Pw
          w x         x w         w = wire wound around frame
          w  x       x  w         x = wooden x-shape made of two
          w   x     x   w             70 cm wooden rods
          w    x   x    w         L = leg
          w     x x     w         P = wooden pig, 18cm long but nearly
          w      x      w             invisible because it is in view
          w     xLx     w             direction
          w    x L x    w         S = stand
          w   x  L  x   w         C = Tuning cap with wire \
          w  x   L   x  w
          w x    L    x w
          wP     L     Pw
          wwwwwwwwwwwwwww
           \     L
            \C   L
            SSSSSSSSSSS

Are you satisfied?

That is great! Please let me know.
Brett D. Saylor from Pennsylvania reported excellent reception after building the project.

Gerard Tel, gerard@cs.uu.nl