MFJ -1796 Instruction Manual page 8

MFJ-1796W WARC Band Vertical Antenna
[ ] 22. Mount the antenna in it's permanent location. Slight re-tuning may be necessary.
FREQUENCY AND SWR ADJUSTMENT
This antenna covers wider frequency ranges on the higher bands, and narrower segments on the
lower frequency bands. The 60 meter band has the narrowest range of operation (approximately
20 KHz) and is the most sensitive to adjustments.
The entire antenna must be accessible during initial coarse tuning and testing. Any repair or
adjustment to the antenna after it is installed on a tall support will make adjustments difficult and
time consuming. It is best to install the antenna on a short temporary mast or pipe that is located
in a reasonably clear location for tuning. The antenna should be mounted vertically with the
base of the antenna around six feet above ground to make tests and adjustments easy.
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 to insure proper results. If using a transceiver and SWR
meter, set the transceiver to the lowest power possible to make measurements.
The normal resonant frequency of this antenna is at or just below the bottom of each
amateur band. This allows the user to "trim" a small amount off the inside end of the
capacitance spokes to raise the resonant frequency. Conversely, adding a longer capacitance
spoke will lower the resonant frequency of a loading assembly. Spare spokes are included in
case you need to lower the resonant frequency of the antenna.
CAUTION: Always start tuning on 60 meters and adjust each band progressively higher in
frequency. Adjustment of a lower frequency band will always have the most effect on the next
higher frequency band. The tendency of the interaction is that if you move one band higher ALL
the other bands move higher, but only very slightly. It is always best to "shoot for" the lowest
end of the range you intend to use and "trim in" by adjusting the bottom loading assembly after
the antenna is in it's final location. ALWAYS work from the bottom band up.
Tuning the Antenna
1. Measure and record the frequency that the lowest SWR occurs on for each band. The lowest
SWR should be at or below the bottom end of each HF band. The SWR should be below 2:1 at
resonance on each band.
The following is a typical chart for initial measurements of a new antenna before tuning:
60 M 2.5:1 at 5.0 MHz 17 M 1.7:1 at 17.8 MHz
30 M 1.6:1 at 9.7 MHz 12 M 1.7:1 at 24.8 MHz
If the resonant frequency is lower than your equipment can detect take one spoke off to raise the
resonant frequency. Measure the resonant frequency and calculate the approximate resonant
frequency as if the spoke was in place using the chart on the next page.