MFJ-1784
Giant Box Fan Portable Loop Antenna
Instruction & Technical Manual
WARNING
•
Keep this antenna out of reach of adults, children, and animals. Any contact
with this antenna while transmitting, even at low power, will cause severe RF
burns and contact with lethal voltages.
•
Never place this antenna or its feedline close to electric power lines or utility
wires.
•
Do not stay near the antenna if you are transmitting more than 10 Watts of
power.
•
The MFJ-1784 is not weather proofed and will be damaged if exposed to water
and moisture.
•
Never operate this antenna near RF sensitive medical devices such as pacemak-
ers.
2.2
THEORY OF OPERATION
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-1784
approaches that performance. Bandwidth is quite narrow due to the extremely 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 above 10 kV. 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-1784 is the “Butterfly” loop-tuning capacitor, which has no rotating
electrical contacts. Coupling this capacitor to a low-resistance loop, such as the welded aluminum
used in the loop’s construction, creates a high-efficiency transmitting loop.
The efficiency of the MFJ-1784 increases with its height above ground. When traveling, a room
above the ground floor above ground level makes for a better portable operation experience with
the MFJ-1784. 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-1784 above the ground as measured from the bottom of the
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-1784.
In practice, ground losses are rather low above 14 MHz, so it is acceptable to operate close to the
ground on these frequencies. However below 14 MHz, ground losses become significant and will
noticeably degrade performance if the MFJ-1784 is placed close to the ground. If elevating the
-3-
Summary of Contents for MFJ-1784
Page 13: ...USER NOTES ...
