Gary W Thorburn


A traditional amplified tuned box loop antenna for MW DXing

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A Here's a view of the loop antenna. The square is about 2.5 feet on a side. The "primary" (tuned side) consists of 12 turns of stranded 16-gauge insulated wire. The "secondary" (pickup loop) is a single turn of the same kind of wire. Because both are the same color, the pickup turn cannot be distinguished in this photo. The windings are spaced evenly across about 3.5 inches, but the winding is not especially taut or precise. The loop is the right impedance to tune the upper two thirds of the MW band with a 10-365pf tuning capacitor, but this design uses a varicap diode to provide the capacitance for remote tuning. The loop has so much internal capacitance, that its not possible to tune the entire MW band with a standard tuning capacitor. There are formulas for calculating the parameters of these windings, but I encourage an experimental approach. Try 12 turns, but be prepared to add or remove depending on how it tunes. Even if you do the math, be sure you will have to do some empirical tweaking in the end anyway. I can't recall where I found the mount, which enables me to use this on any standard tripod, it was salvaged from some old piece of camera gear.
Here you can see the MVAM109 tuning diode, plus two capacitors, C1 and C2 in the circuit diagram below. The schematic diagram shows C2 being a variable capacitor, but I removed that when I found that switching in and out a fixed capacitance was sufficient for me to tune the entire MW band, plus some LW beacons. C2 can be determined experimentally, I used 680pf. It is switched in for the bottom part of the MW band, down to about 380 kHz.
This is the amplifier circuit board. The visible yellow pair leads in from the single turn pickup loop. The switch cuts out the 8 AA cells (the power switch) and the "Motorola type" jack (car radio antenna type) is the 50-ohm output to the receiver. I made a cable from 50-ohm coax which has a Motorola plug on one end, and a PL-259 on the other, for the 50-ohm input to my Drake R8A.
This is the circuit diagram. There are three parts: 1) the remote tuning device, which supplies 0-9 volts to 2) the MVAM109 varicap to tune the loop. The varicap diode and the additional capacitance and switch sits on the frame of the loop antenna. The remote tuning device enables the loop to be tuned from a more comfortable position, and is fed thru a piece of thin flexible coax. 3) The amplifier circuit is based on a Motorola TV IF amplifier chip, the MC1409. It is powered by 8 AA cells, which last a long while. I find that 12 volts works better than 9 volts. The amplifier circuit is based on a Motorola application note. The input to the amplifier is taken from the one-turn pickup which forms the "secondary" of the loop. Thanks to K6QGH, who redrew this nicely for me.