MFJ-939 Plug&Play IntelliTuner Automatic Antenna Tuner
Instruction Manual
2006-2015 MFJ Enterprises, Inc.
25
GROUNDING HINTS
CAUTION
For operator safety, a good outside earth ground or water pipe ground should always
be installed and connected to the case of the MFJ-939. Make certain the safety ground
also connects to the transmitter and other station accessories. A wing-nut post marked
GROUND is provided for ground connections.
To minimize RFI, single wire feedlines (such as used with 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.
For safety, always use good DC and RF grounds. It is particularly important to have a good RF ground
when using a single wire feeder since 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 water pipes and ground rods provide good DC and AC safety grounds, but they are almost useless
for dependable RF grounding.
RF grounds work much better when "spread out" over a large area, especially when they employ multiple
connections directly to the equipment ground point. 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.
ANTENNA SYSTEM HINTS
Location
For the best performance, an end-fed longwire 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 quarter-
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.
