Instruction manual

MFJ-945D Instruction Manual
7
Water pipes provide good dc and ac safety grounds, but they are often inadequate for RF
grounding because they are long single conductors. RF grounds require large "spread out"
surfaces with direct multiple connections to the equipment ground point. Water pipes, heating
ducts, or multiple ground rods may work (especially if they are all connected together with
jumper wires), but the best RF grounds are radial systems or multi-wire counterpoises.
RF and lightning travels on the surface of conductors. Braided or woven conductors have high
surface resistance to lightning and RF. Ground leads for RF 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
For operator safety a good outside earth ground or water pipe ground should always be installed
and connected to the case of the MFJ-945D. Make certain the safety ground also connects to the
transmitter and other station accessories. A wing nut post marked GROUND is provided for
ground connection(s).
Location
For the best performance, an end-fed longwire wire antenna should be at least one quarter-
wavelength long at the operating frequency. Horizontal antennas should be at least a half wave
long and high and clear of surrounding objects. 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
foot) open wire line is used to feed a half-wave (100 to 140 foot) 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 foot) and connects to a full-wave high impedance antenna
(100 to 140 foot). The half-wave line repeats the high antenna impedance at the tuner. The
antenna system looks like several thousand ohms at the tuner on 40 meters.