MFJ 1798 - Page 25

MFJ-1798 Vertical Antenna Instructions
24
in frequency if (or when) the antenna is relocated. Moving the antenna higher, or farther from
other objects, will usually cause an slight increase in the resonant frequency.
Caution: Always tune the lowest three frequency bands in order, starting with 80 meters.
Both 80 and 40 meters have narrow operating bandwidths. On 80 and 40 meters, the antenna will
probably not cover the entire CW or phone band. The antenna must be tuned to a favorite "area"
of 80 or 40 meters. An antenna tuner can be used to "stretch" the operating bandwidth on these
bands. Avoid running the maximum rated power when operating with a non-tuned SWR over
3:1.
The following tuning procedure should be followed when adjusting 80, 40, and 30 meters.
Warning
: Shortening the spokes from the looped end degrades safety and power
handling capabilities!
TUNE 80 METERS FIRST
For 80 meters: Removing
1"
from a single spoke will shift the resonant frequency up
approximately 10 KHz.
Select a frequency 50 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 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 the desired frequency is reached.
Example: The antenna measured 3.47 MHz, the desired frequency is 3.95 MHz (less .05 MHz)
for a difference of .43 MHz or 430 KHz. This would require the trimming of 43" from one spoke.
Instead, cut 4.3 inches 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 40 METERS NEXT (
after 80 meters
)
For 40 meters: Removing
1"
from a single spoke will shift the resonant frequency up
approximately 20 KHz.
Select a frequency 40 KHz below the desired operating frequency. Use this frequency as a target.