The "Loopy" Remotely Tuned AM Loop Antenna


          UPDATE: I have added the Shield of the Pickup Loop to the Drawings. The break in the Shield can be made in the Center of the Loop or by connecting the Shield to the center conductor of the side which is the ground side of the feed in wire. If you look at the picture above there is a break in the center of the pickup loop. Loopy is a project for people who have already built an AM Loop Antenna that is tuned using a variable capacitor. This project uses a Varactor Diode. This allows you to tune your antenna remotely. THIS IS NOT A TRANSMITTING ANTENNA!!!

          The shielded wire that runs from the Control Box to the Varactor Board is used for two things. The control voltage goes up to the Varactor Board to control the diode and the signal comes back down the same wire to the Control Box and out to the radio. This is also how remote amplified antennas work.

          The above photo is my Winter Loop. I put it together and use it during the Winter when the AM band is quiet. I turn the Loop in the direction of stations I want to pick up. It is built out of PVC pipe. Speaker wire is used for the main loop. The pickup loop is sheilded wire. This project is not about the loop it is about controlling a loop remotely. The loop in the picture is a test bed. It is a unit I used to test all kinds of ideas. You can see the main speaker wire loop and the one turn of shielded wire as a pickup. You can also see the Varactor Board and the sheilded wire running off the antenna to the Control Box.

Inside the Control Box

          The Control Box puts out Zero to Nine volts depending on where the variable resistor is tuned! The box contains an on off switch, Variable Resistor, 9 volt battery and two outputs jacks. One output sends 0-9 volts up to the antenna and the second one sends the signal to the radio.

Diagram of the Control Box

          The Varactor Board is very simple! It contain just a handfull of parts. I built it using some terminals that are mounted on a piece of grey PVC. Wood will also work!

Varactor Board

          You will notice on the Varactor Board some Ferrite Beads. These are and option. The shielded wire that runs from the Control Box to the Varactor Board can pick up noise on its outer shield. The noise goes up the wire and modulates the antenna and then comes back down the wire to the radio. The Ferrites do help! Using a piece of shielded wire for the pickup loop is the next step. Get everything working before you experiment!

Varactor Board Diagram

          The Loopy System is very simple! Any 400 uuf Varactor Diode can be used. It is a diode! It can only work is it is put in the circuit the right way!

System Diagram Loopy

          It is at this point I will again say you need to have built a AM Loop Antenna and got it working before attempting this project! Check out is easy! With a voltmeter on the output of the Control Box make sure you see zero to nine volts as you turn your Variable Resistor. Then put the meter across the Varactor Diode and make sure that voltage is getting to the Varactor Diode. That conferms you have the control voltage going out of the Control Box through your shielded cable to the Varactor Board. The voltage might swing lower than 9 volts because of the resistors on the Varactor. Good Luck!

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