MFJ 1798 - Page 24

MFJ-1798 Vertical Antenna Instructions
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re-installed after the higher bands are adjusted, if this is more
convenient. The lower bands can then be adjusted starting with 80
meters, since they can be reached from a step ladder. The antenna
must be mounted vertically with the base of the antenna around
five or six feet above ground to make tests and adjustments
accurate.
Caution:
The ground mounted adjustments remain fairly accurate only when
the antenna is more than 30 feet from surrounding metallic
objects or antennas. There may be a small upward shift in the
resonant frequency of the antenna on 80, 40, and 30 meters if the
antenna is relocated in location further from surrounding
objects. This shift is greatest on 80 meters.
The SWR can be measured by using a transmitter and SWR bridge, or
an SWR Analyzer. The best location to make SWR measurements is at
the base of the antenna. If the measurements are not taken at the
base of the antenna, the antenna must be fed with a reasonably
short length of good quality 50 ohm coaxial cable. When using a
transceiver and SWR meter, set the transceiver to the lowest
power possible while making measurements.
TUNING THE ANTENNA ON LOW FREQUENCIES
Measure and record the frequency of lowest SWR occurs on each band. The MFJ-1798 is tuned
for lowest SWR either at or below the lowest frequency of each HF band.
The following is a typical low frequency chart of initial measurements on a new antenna before
tuning:
80 meter 1.2 : 1 @3.49 MHz 30 meter 1.2 : 1 @10.06 MHz
40 meter 1.2 : 1 @6.95 MHz
NOTE:
If the resonant frequency is lower than your equipment can detect, completely remove
one spoke. See the hints at the beginning of this section.
Trimming one spoke will result in the following frequency change:
80 m:
1"
trimmed off a spoke moves the frequency up approximately 10 KHz
40 m:
1"
trimmed off a spoke moves the frequency up approximately 20 KHz
30 m:
1"
trimmed off a spoke moves the frequency up approximately 40 KHz
The lower frequencies (80 through 30 meters) can be adjusted while the antenna is mounted on a
temporary mast five to six feet high. The antenna's resonant frequency may shift slightly higher