MFJ-935C High-Efficiency Magnetic Loop Tuner Instruction & Technical Manual
2 THE MFJ-935C
2.1 INTRODUCTION
The MFJ-935C High-Efficiency Magnetic Loop Tuner™ is a small, versatile, high-efficiency device
that turns any wire or coax loop into a high-efficiency multi-band transmitting loop antenna
system designed for 50-ohm use at 150 Watts maximum input and all modes. It consists of two
primary components:
• MFJ-935C High-Efficiency Magnetic Loop Tuner/Antenna Current Meter ™
• Wire or Coax Loop (not included)
The MFJ-935C High-Efficiency Magnetic Loop Tuner/Antenna Current Meter™ (Figure 3) has two
functions. First, it uses various lengths of wire to create a very-high-Q tuned circuit that is used as
a transmitting loop antenna. An MFJ-19 low-loss Butterfly capacitor with no rotating contacts
(available separately for your own homebrew projects) is the heart of this circuit. The second
function is as a matching network that matches the high-Q transmitting loop circuit to your 50Ω
coaxial cable.
Figure 3: The MFJ-935C
RF grounds, radials, or a counterpoise system are not required, though a safety ground is
recommended. While the MFJ-935C High-Efficiency Magnetic Loop/Antenna Current Meter
Tuner™ can tune any shape loop (circle, square, rectangle or any odd shape), a wire or length of
coaxial cable approaching a quarter wavelength shaped as a circle is the most efficient
configuration.
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