MFJ All Band Doublet Instruction Manual
3
be at least 30 feet above the ground in this configuration. The antenna radiates
mainly in the direction of the downward slope. The optimum angle of "slope"
will varying with the desired coverage distance and the frequency of operation,
but will almost always be somewhere between 45 degrees and almost vertical.
160 METER OPERATION AND GROUNDING
The MFJ-1777 will can be used on the 160 meter band as a
“
Marconi Antenna
.”
Operation on 160 requires a tuner and a good RF ground system.
Do not
try to
use this antenna in its supplied, center-feed configuration on 160 Meters, as you
risk damage to your antenna tuner and/or transceiver.
To use the MFJ-1777 as a Marconi antenna, connect together the bottom ends of
the feedline (ladder line) at your tuner. This allows the vertical feedline to
become a radiating element, working against the station ground. The feedline
should be kept as vertical and as clear of metallic objects as possible. Obviously,
an “indoor” configuration such as this has complications, namely, that the
feedline is “hot” inside. Take suitable precautions.
A good way to visualize an RF ground is to picture a large conductive "mass"
that the antenna pushes against. The "mass" should have a short connection to
the ground point and extend as far as possible in every direction. The ideal
ground system for 160 meters would have 120 straight evenly spaced radials at
least 140 feet long. A marginally acceptable ground system requires a
minimum
of four or more long (60 foot or more) straight radials. Other large
metallic masses (such as fences or water lines) should also be "tied in" to the
ground system.
The earth and fresh water are poor electrical "masses" because of their poor
conductivity (1 to 30 milliohms/meter). Although salt water is several hundred
times (5000 milliohms/meter) more conductive than dirt, copper is several
hundred times (millions of milliohms/meter) better than saltwater. Copper
radials obviously offer a much better ground than saltwater.
Ground rods almost never suffice for RF grounding in applications like this
because they connect to the lossy dirt. This is why radials are required when
feeding this antenna as a Marconi. Radials will improve your signal even if the
earth is wet or saturated with salt water.
Elevated counterpoise systems are also effective, but they also require as much
"electrical mass" as possible. A supplemental earth connection is always
required for lightning and low frequency ac grounding.
