MFJ MFJ-805 User Manual

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: The MFJ-805 RFI Detector is a highly-sensitive amplified 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!

RFI--THE REAL CAUSE

In Radio Receivers: The spurious signals 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 commonmode 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: For a low-level RF source, we recommend driving the 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.

3.Using Your MFJ-805 Research and Testing: The MFJ-805 also works in 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: This device uses a highly efficient clamp-on RFcurrent detector in combination with a high-gain dc-servo amplifier. To avoid potential damage, please use with caution around high-power transmitters.

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