The antenna consists of an LC (tuning) circuit formed by the air-core loop and a tuning capacitor. 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 antenna/ground input of your receiver.
The antenna improves reception of weak or interfered stations in several ways.
- Radio signals consist of electric and magnetic waves, and while a longwire or dipole reacts to electric fields, the loop reacts to magnetic fields. Because interference of atmosphere or applications is electric, it is ignored by the loop.
- The loop is quite directional, and you can pull in a remote station A that is overshouted by a closer or stronger station B (on the same or an adjacent channel) by nulling out B.
- The tuning circuit is sufficiently selective to ensure that the resonance is appreciably stronger for the chosen frequency than it is for adjacent channels; this is a great help in reducing Adjacent Channel Interference (ACI).
The wooden frame
First of all cut two pieces of 2 inch X 1 inch timber, each piece measuring 25 inches long. Half along each of these legs cut a piece out of the legs exactly the size of the other leg, so that the two legs can be screwed together in the form of an X. Then cut 4 pieces of timber 6 inches long, these are screwed on the end of each leg to form feet, the primary wire coil is also wound around these four feet.
Firmly fix in place one end of the 120 foot length of wire to the centre of the frame, where the two legs cross, feed the wire to the end of one of the legs, and wind it around the four feet for 18 turns, then feed the remaining wire back to the centre of the frame using the same leg as where the wire began, this is most important, both ends of the wire must go from the winding coil to the frame centre along the same leg. Affix a tuning capacitor salvaged from an old radio to the centre of the frame, where the two legs cross, using glue or whatever, I use two small nails hammered into the legs, so positioned to hold the tuning capacitor in place. Then cut the wire ends to make a neat connection to the tags on the tuning capacitor and solder the wires in place.
By now you should have a fully working loop antenna, ready and waiting to serve up the world on your radio.
CopyRight by Chris Ridley document originally available at http://www.geocities.com/SunsetStrip/Underground/8585/loop.html