Appendix B - Rfi Considerations - microHAM DXP Manual

A few guidelines to eliminate problems caused by RFI:
1. Proper grounding of all electronic equipment is critical. A modern station contains many, diverse,
types of interconnected and interrelated equipment: transceiver, power amplifier, computer,
control boxes, switch boxes, and power supplies. Each of these must be individually grounded
with a separate connection to a single common ground point, thus forming a star ground
connection.
Proper grounding of computers, both "desktop" and laptop is often overlooked. A separate
ground connection should be run from the computer to the station common ground point. The
best place to ground a computer is a screw with a good connection to the case. On a laptop, this
is often the retaining screw on a D-sub connector (e.g, VGA output); on a "desktop" it is often the
screws holding the power supply.
It is absolutely important to prevent ground currents from flowing to the common ground point by
way of the signal cable. If you use a microHAM "keyer," a good test is to remove the DB15/DB37
connector and USB cable from the keyer and measure the resistance from the shell of the
DB15/DB37 to the shell of the USB cable. There should be NO MORE than FIVE (5) Ohms (and
preferably less than TWO Ohms) between them.
Note: many PC manufacturers fail to provide an adequate connection between the shell of the
USB connector and the PC case. If this is the case, a connection can be established by bridging
a folded piece of aluminum foil between the shell of the USB connector and the PC case.
2. Power all your equipment from a single wall outlet. The "safety ground" often exhibits excessive
noise between power outlets - often due to other equipment powered from the same branch
circuit. It is always better to avoid this source of noise/interference. It is also a good idea to check
the power distribution for loose connections, reversed neutral/ground, open ground and other
wiring problems.
3. Sometimes, the USB cable can be a source of RF interference - the cable might have
inadequate shielding or the data transceivers in PC might be improperly designed causing data
flowing inside the cable to be reflected as a common mode current on the shield of the cable.
This common mode current can radiate a significant "digital noise." If this is the source of your
problems, it can be significantly reduced or eliminated using ferrite chokes on both ends of the
cable. Two or three turns through a #31 mix toroid are better than the common snap-on ferrites
of unknown mix.
4. Often, another cause of RFI problems is a common mode current flowing along the antenna
feedline into the shack. It is a common misconception that the only thing required of a feedline is
that it have low SWR. Unfortunately, a low SWR does not guarantee low common mode current.
These common mode currents are conducted into the shack where they can radiate from the
feedline, induce currents in any nearby metal object, and/or be conducted into the interconnected
equipment. Common mode currents on a feedline are indicated by problems that differ in
intensity from one band to another or from one end of the band to another, by problems that
change when a feedline is moved or its length changed, where the problem moves from one
piece of equipment to another based on band, and/or where the severity changes with transmit
power level. The solution is to use common mode chokes to prevent the current from entering
the shack.
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APPENDIX B – RFI Considerations
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