MFJ-805 Instruction Manual MFJ RFI Detector
INTRODUCTION
Your MFJ-805 is a sensitive handheld device that tracks down elusive spectrum
noises caused by microprocessors, high-speed switching, electro-mechanical
devices, automotive ignition systems, and many other RF noise generators. You
save time and money by isolating radiation sources quickly and installing
suppression on only those lines and cables that need it. In addition to
eliminating RFI affecting radio receivers, your MFJ-805 can help you to protect
sensitive consumer-electronics from disrupted operation caused by powerful
transmitter signals by tracking down the cables that pick up the signal. And, if
you're a RF experimenter, you may use your MFJ-805 to profile current
distribution on antennas, radials, and feedlines using only a low-level RF source.
Important Note:
electronic device that may be damaged when used improperly in a high-level
RF environment. Please read this manual carefully before attempting to use
your unit!
The MFJ-805 RFI Detector is a highly-sensitive amplified
RFI--THE REAL CAUSE
In Radio Receivers: The spurious signa ls and noise that make up "RFI" may
originate from many sources. However, to radiate and disrupt communications,
all sources need a wire or cable to act as an antenna. Not all cabling associated
with potential RFI generators radiate signals. Energy transferred over properlytreated cable-pairs or coaxial lines may radiate little or no energy at all.
Radiation can only take place when electrical signals appear as common-mode
currents on lines and cables. That's because common-mode current induces
electromagnetic radiation (also called EMR or radio waves). EMR carries may
carry unwanted signals over significant distances to radio antennas in the
surrounding area.
In Consumer Electronic Devices: The same process occurs in reverse when a
powerful transmitter interferes with a RFI-prone consumer product. Wires and
cables associated with that device may act like an antenna, picking up EMR and
transforming it into electrical current. This, in turn, can invade sensitive
electrical circuits and disrupts normal operation. However, without a common-
mode wire to serve as an antenna, powerful EMR from nearby transmitters can't
be transferred into to consumer devices at disruptive levels.
The Cure: If EMR causes RFI, the key to eliminating it is to pinpoint lines that
carry elevated levels of common-mode current and to install suppression on
them. Readily available suppression devices such as ferrite chokes and RFI line
filters diminish a line's ability to act as an antenna. This, in turn, destroys its
ability to transmit or receive unwanted EMR.
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MFJ-805 Instruction Manual MFJ RFI Detector
HOW YOUR MFJ-805 WORKS
The MFJ-805 is a highly-sensitive RF-current meter that picks up low-level
common-mode energy over a wide frequency range. This energy is detected,
boosted in strength by a high-gain dc amplifier IC, and applied to a sensitive
meter movement. The MFJ-805 responds to common-mode current at or below
levels likely to cause a RFI problem. Consequently, if you have an interference
problem, your MFJ-805 can detect it easily!
1. Eliminating Computer Hash and RFI in Communication Receivers:
With today's explosion of electronic consumer products, communication
radios are frequently plagued by hash, birdies, and transient noises. Your
MFJ-805 tells you which specific interconnecting cables in the area are "hot"
with RF. You treat only the offending lines, and ignore the rest. Your RFI
Detector also works in mobile installations for ferreting out alternator hash,
ignition pulses, and microprocessor noise on feedlines and power leads.
There's no faster or better way to isolate and block RF interference!
2. Eliminating RFI in Consumer Devices: The MFJ-805 also isolates cables
picking up high levels of RF energy from local transmitters, and indicates
when you've suppressed the problem. In most cases, you can substitute a
signal generator for the offending transmitter to run your tests. Your meter
can detect signals picked up by cables and wiring--even if they're attenuated
50 dB below the source! This lets you resolve many problems at very low
power, avoiding on-air interference and exposing yourself to high EMR
levels.
Important Note:
transmitter antenna with a 50-ohm signal generator set for +10 dBm output,
or connecting a MFJ-259 Antenna Analyzer to the antenna. The signal
source should be set for the transmitter's usual operating frequency.
For a low-level RF source, we recommend driving the
3. Using Your MFJ-805 Research and Testing: The MFJ-805 also work s i n
conjunction with low-power RF generators to plot current distribution on
antennas, networks, ground systems, and control lines. With a low-level RF
source applied, you may work legally and safely inside or outside of your
licensed frequency allocations (note that the detector coil may interact with
elements under test, altering antenna characteristics somewhat).
Important Precaution:
current detector in combination with a high-gain dc-servo amplifier. To avoid
potential damage, please use with caution around high-power transmitters.
This device uses a highly efficient clamp-on RF-
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