MFJ-974/974H Balanced Line Antenna Tuner
Instruction Manual
5
When adjusting the tuner, use the lowest number on the
ANTENNA
and
TRANSMITTER
controls and the
highest letter on the
INDUCTOR
control that produces a good SWR. This will reduce tuner losses and increase
the power rating of the tuner.
In Case Of Difficulty
If the tuner fails to tune, please
double check
all connections and follow the tuning procedures again.
Be sure you are using
enough inductance
(low enough inductance letter) and have the
capacitors open far enough
(higher front panel numbers).
If the tuner arcs
at the rated power levels, please
double check
all connections and follow the tuning procedures
again. The power rating of this tuner is 300 watts PEP RF power. Be sure you are using the
least amount of
inductance
(highest number) and the
greatest capacitance
(lowest number) possible that still allows matching the
load on the operating frequency.
Note:
If this tuner arcs when operating on the 160 meter band, it may be necessary to reduce transmitter output
power.
If you are still unsuccessful, but the tuner does adjust and operate when used with a dummy load or another
antenna, please read the
Antenna System Hints
section.
Grounding Hints
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, 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).
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. 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.
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-974H/974. 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.