MFJ 1798 - Page 26

MFJ-1798 Vertical Antenna Instructions
25
1- Measure the resonant frequency of the antenna. Determine the frequency shift required by
subtracting the initial frequency from the desired frequency. Use the result to determine the
proper amount to cut a spoke.
2-
If the calculated amount of element shortening exceeds 10"
, cut one inch from the inside
end of all six spokes for each 10" calculated.
3- Return to step 1. Keep repeating these steps until done.
Example: The antenna measured 6.95 MHz, the desired frequency is 7.25 MHz less .05 MHz (50
KHz) for a difference of .25 MHz or 250 KHz. This would require the trimming of 43" from one
spoke. Instead, cut one 4.3 inch from all six spokes and measure the new resonant frequency.
Assume the new resonant frequency is 3.82 MHz. A single spoke can now be trimmed 8" to adjust
the antenna to resonance.
TUNE 30 METERS LAST
For 30 meters: Removing
1"
from a single spoke will shift the resonant frequency up
approximately 40 KHz.
Select a frequency 20 KHz below the desired operating frequency. Use this frequency as a target.
1- Measure the resonant frequency of the antenna. Determine the frequency shift required by
subtracting the initial frequency from the desired frequency. Use the result to determine the
proper amount to cut a spoke.
2-
If the calculated amount of element shortening exceeds 8"
, cut one inch from the inside
end of all four spokes for each 8" calculated.
3- Return to step 1. Keep repeating these steps until done.
Example: The antenna measured 9.95 MHz, the desired frequency is 10.15 MHz less .02 MHz
(20 KHz) for a difference of .18 MHz or 180 KHz. This would require the trimming of 4.5" from
one spoke. Cut 4.5 inches from one spoke and measure the new resonant frequency. Assume the
new resonant frequency is 10.1 MHz. You would only have to shorten a spoke 3/4 inch to be
done.
TUNING 20 METERS
Tuning the 20m band is done almost the same way, except there is much less interaction in the
adjustments with the other band groups. Loosen the 6-32 screw up that applies tension to the 20
meter tuning rods. Sliding the 20 meter capacitor rods in (decreasing the rods total length) will
increase the frequency. Sliding the 20 meter tuning rods out (increasing the rods length) will
decrease the frequency (refer to Figure 8 on page 18).