MFJ-935C
High-Efficiency Magnetic Loop Tuner
Instruction & Technical Manual
3
LOOP THEORY
A magnetic loop antenna is one that is characterized by low-noise reception, working well even
when mounted at ground level, and a circumference of less than
1
/
3
wavelength. The ideal small
transmitting antenna would have performance equal to a large antenna, and the MFJ-935C
approaches that performance. Bandwidth is quite narrow due to the extreme high-Q of the
tuned-circuit configuration when paired with a capacitor.
The components in a resonant transmitting loop are subjected to high currents and voltages
because of the large circulating currents in the high-Q tuned circuit formed by the antenna. It is
very important that the capacitors used in this antenna have a high RF current rating. Even a
100W transmitter develops currents in the tens of amperes, and voltages across the tuning
capacitor can be greater than 10kV. This consideration also applies to any conductors used to
connect the loop to the capacitor. Therefore care must be taken when choosing materials in the
loop. The best electrical connections possible are those using soldered or welded joints.
The heart of the MFJ-935C
High-Efficiency Magnetic Loop Tuner/Antenna Current Meter
™
is the
“Butterfly” loop-tuning capacitor, which has no rotating contacts. When coupled to a
low-resistance loop conductor, such as a copper strap or #10 stranded copper wire, it creates a
high-efficiency transmitting loop.
The efficiency of the MFJ-935C
High-Efficiency Magnetic Loop Tuner/Antenna Current Meter
™
increases with its height above ground. When traveling, a room above the ground floor makes for a
better portable operation experience with the MFJ-935C. At very low heights, close coupling to
the ground causes detuning and losses due to the twin factors of current induced into a mirror
image of the loop below the surface and the resistance of the image loop proportional to soil
resistance. Another loss component is due to current flowing in the soil via capacitance between
the loop and soil surface. These are all reduced by increasing the height of the MFJ-935C above
the ground as measured from the tuner box. An operational height equal to
1
/
2
the diameter of the
loop antenna is recommended to prevent detuning and excess ground losses when using the
MFJ-935C
High-Efficiency Magnetic Loop Tuner/Antenna Current Meter
™
loop antenna system.
In practice, ground losses are rather low above 14 MHz, so it is acceptable to operate close to the
ground on these bands. However below 14 MHz ground losses become significant and will
noticeably degrade the performance of the MFJ-935C when placed close to the ground. If elevating
the MFJ-935C is not an option, the loop should be mounted vertically.
It is possible to improve the efficiency of the loop antenna through the use of freeware antenna
modeling software. Use the software to create a model of the antenna (including wire length,
shape, height above ground, and orientation), and then vary the parameters until you optimize
your particular operational configuration. One source of free programs can be found at
-9-
Содержание MFJ-935C
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