MFJ-993BRT 300 Watt IntelliTuner
TM
Automatic Remote Antenna Tuner Instruction Manual
© 2011 MFJ Enterprises, Inc
MFJ-993BRT Version 1A
7
Grounding Hints
To minimize RFI, single wire feedlines (such as used with true Windom or longwire antennas) should be
kept away from other wiring. Radiation will be minimized if the single wire feeder runs parallel and
reasonably close to the wire that connects the tuner to the outdoor ground. The antenna feed wire should
be adequately insulated to prevent arcing or accidental contact.
A good earth ground and RF ground should always be connected to the case of the MFJ-993BRT. A
wing-nut post marked GROUND is provided for ground connections. For safety, please use good DC and
RF grounds. It is particularly important to have a good RF ground when using a single wire feeder as the
tuner needs something to "push" against in order to force current into the single wire feedline. If a good
RF ground is not available, RF will usually find its way back into the power line (RFI), transmitter audio
circuits (RF feedback), or the operator (RF burns). Metal underground water pipes and ground rods may
provide good DC and AC safety grounds, but they are often inadequate for RF grounding. RF grounds
work much better when "spread out" over a large area. Metal water pipes, heating ducts, and fences may
work (especially if they are connected together with multiple wires), but the best RF grounds are radial
systems or multi-wire counterpoises. Radials and counterpoises provide large, low resistance surfaces for
RF energy. RF and lightning travel on the surface of conductors. Braided or woven conductors have high
surface resistance to lightning and RF. Ground leads for RF and lightning should have wide smooth
surfaces. Avoid the use of woven or braided conductors in RF and lightning grounds unless the lead
needs to be flexible.
There are a number of articles in the technical literature that can advise on how to properly ground the
antenna system for both lightning and RF signals.
Antenna System Hints
Location
For the best performance, an end-fed wire antenna should be at least one quarter-wavelength long at the
operating frequency. Horizontal dipole antennas should be at least a half-wavelength long and located as
high and clear as possible. While good RF grounds help the signal in almost any transmitting installation,
it is extremely important to have good RF grounds with long wire or other Marconi-style antennas.
Matching Problems
Most matching problems occur when the antenna system presents an extremely high impedance to the
tuner. When the antenna impedance is much lower than the feedline impedance, an odd wavelength
feedline converts the low antenna impedance to a very high impedance at the tuner. A similar problem
occurs if the antenna has an extremely high impedance and the transmission line is a multiple of a half-
wavelength. The half-wavelength line repeats the very high antenna impedance at the tuner. Incorrect
feedline and antenna lengths can make an otherwise perfect antenna system very difficult or impossible
to tune. One example where this problem occurs is on 80 meters when an odd quarter-wave (60 to 70
feet) open wire line is used to feed a half-wave (100 to 140 feet) dipole. The odd quarter-wave line
transforms the dipole's low impedance to over three thousand ohms at the tuner. This is because the
mismatched feedline is an odd multiple of 1/4 wavelength long. The line inverts (or teeter-totters) the
antenna impedance. A problem also occurs on 40 meters with this same antenna example. The feedline
is now a multiple of a half-wave (60 to 70 feet) and connects to a full-wave high impedance antenna (100
to 140 feet). The halfwave line repeats the high antenna impedance at the tuner. The antenna system
looks like several thousand ohms at the tuner on 40 meters. This places enormous strain on the tuner,
since voltages can reach several thousand volts. This can cause component arcing and heating.
