Grounding Hints; Antenna System Hints - MFJ Legal Limit IntelliTuner -998RT Instruction Manual

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MFJ-998RT Legal Limit IntelliTuner Automatic Remote Antenna Tuner Instruction Manual
arcing which can damage the relays in the tuner. Check your transceiver owner's manual to see if yours
has a foldback circuit. The use of the AM mode normally will drop the radio to around 20 to 40 watts or
so. This is a convenient way of reducing power and supplying a continuous carrier for tuneup.

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.
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-998RT. 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. 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. When using a single wire feeder, 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 provide good DC and AC safety grounds, but
they are often inadequate for RF grounding because they are single conductors. Ground rods by
themselves 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.
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 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 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
© 2011 MFJ Enterprises, Inc 17
MFJ-998RT Version 1A