MFJ-9234 Mini Loop Tuner
Instruction & Technical Manual
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LOOP THEORY
LOOP-ANTENNA BACKROUND INFORMATION
A
small loop
antenna is characterized by low-noise reception. It typically works well even when it is
mounted at ground level and has a conductor length (or circumference) of less than 1/3 wavelength.
An "ideal" small transmitting loop can theoretically performance as efficiently as a large antenna, and
small loops often do approach that level of performance. However, bandwidth is always more
narrow due to the very hi-Q of the tuned-circuit configuration.
Components in a resonated transmitting loop are subjected to high current and voltage because of
the large circulating currents developed in the high-Q circuit formed by the antenna. It is important
that capacitors used in this antenna have a high RF current rating because even a small 10-25 W
transmitter may develop a peak current of several amperes. This consideration also applies to any
conductors used to form the loop or connect to the capacitor (a short piece of #14 wire may have
more resistance than the entire loop conductor).
Soldered or welded electrical connections always
yield the lowest resistance.
The heart of the MFJ-9234
Mini Loop Tuner
TM
is the loop-tuning capacitor. When coupled to a low-
resistance loop conductor, it provides high efficiency.
As the loop antenna is elevated, its efficiency improves accordingly. When traveling, a room at some
elevation above ground level makes for a better portable operation experience. At very low heights,
close coupling to the ground causes detuning and losses due to current induced into a mirror image
of the loop below the surface -- with resistance of the image loop proportional to soil resistance.
Another loss component is caused by current flowing in the soil via capacitance between the loop and
soil surface.
An operational height equal to 1/2 diameter of the loop antenna is recommended to prevent detuning
and excess ground losses when using the MFJ-9234
Mini Loop Tuner
TM
loop antenna system. This
means the tuner should be at that recommended height, since it is connected to the bottom (ends) of
the loop, whatever the loop antenna configuration: circle, square, hexagonal, etc.
For operation on the 14 MHz band and higher, ground losses are a minimum near ground, so it is fine
to operate on the ground floor. For the 7 MHz band and lower, ground losses become significant on
the ground floor. To reduce ground losses, operate on a second or third floor.
For minimum ground loss when operating near ground, the loop should be mounted vertically. For
higher elevations (relative to the wavelength), horizontal mounting will also give low ground losses.
Using
freeware
-modeling programs, it is possible to improve the efficiency of the loop antenna system
by varying the parameters until you optimize your particular operational configuration, even while
portable. One source example for free programs is G4FPQ’s Web site:
http://www.btinternet.com/~g4fgq.regp/
.
