MFJ-945D Instruction Manual
8
This places enormous strain on the balun and the insulation in the tuner, since voltages can
reach several thousand volts. This can cause component arcing and heating.
The following suggestions will reduce the difficulty in matching an antenna with a tuner:
1. Never center feed a half-wave multi-band antenna with a high impedance feedline that is
close to an odd multiple of a quarter-wave long.
2. Never center feed a full-wave antenna with any feedline close to a multiple of a half-wave
long.
3. If this tuner will not "tune" a multi-band antenna, add or subtract 1/8 wave of feedline (for
the band that won't tune) and try again.
4. Never try to load a G5RV or center fed dipole on a band below the half-wave design
frequency. If you want to operate an 80 meter antenna on 160 meters, feed either or both
conductors as a longwire against the station ground.
To avoid problems matching or feeding any dipole antenna with high impedance open wire
lines, keep the lines around these lengths. The
worst possible
line lengths are shown in
brackets:
160 meter dipole; 35-60, 170-195 or 210-235 feet. (Avoid 130, 260 ft)
80 meter dipole; 34-40, 90-102 or 160-172 feet. (Avoid 66, 135, 190 ft)
40 meter dipole; 42-52, 73-83, 112-123 or 145-155 feet. (Avoid 32, 64, 96, 128 ft)
Some slight trimming or adding of feedline may be necessary to accommodate the higher
bands.
WARNING:
To avoid problems, a dipole antenna should be a full half-wave on the lowest
band. On 160 meters, an 80 or 40 meter antenna fed the normal way will be extremely reactive
with only a few ohms of feedpoint resistance. Trying to load an 80 meter (or shorter) antenna
on 160 meters can be a disaster for both your signal and the tuner. The best way to operate 160
meters with an 80 or 40 meter antenna is to load either or both feedline wires (in parallel) as a
longwire. The antenna will act like a "T" antenna worked against the station ground.
Technical Assistance
If you have any problem with this unit first check the appropriate section of this manual. If the
manual does not reference your problem or your problem is not solved by reading the manual
you may call MFJ toll-free at 1-800-647-TECH (8324). Outside of the continental USA. call
601-323-5869. You will be best helped if you have your unit, manual and all information on
your station handy so you can answer any questions the technicians may ask.
