Vectronics VEC-896 - Manual (Page 13)

VEC-896 Vertical Antenna Instruction Manual
12
If the resonant frequency is lower than your equipment can detect take one spoke off (top and
bottom hats) 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.
Warning: Shortening the spokes from the looped end degrades safety and power
handling capabilities!
Note: Always tune the lowest frequency bands first.
TUNING THE LOW FREQUENCY BANDS
Note: Always trim the same amount of spoke length from both capacitor hats of the antenna
(top and bottom) to keep the dipole balanced.
40 Meters Band:
1. Select a frequency 50 KHz below the desired operating frequency. Use this frequency as a
target.
2. Measure the resonant frequency of the antenna at this band. Determine the frequency shift
required by subtracting the initial frequency from the desired frequency. Use the results and
the chart below to determine the proper amount of spoke length to be cut.
3. If the calculated amount of spoke shortening exceeds 8", cut one inch from the inside end of
all six spokes for each 8" calculated.
4. Return to the first step. Keep repeating these steps until done.
Example: The antenna measure 6.95 MHz, the desired frequency is 7.25 MHz, less 0.05 MHz
(50 KHz below desired frequency) for a difference of 0.2 MHz or 200 KHz. This
would require the trimming of 8" from one spoke in each top and bottom hat. Instead,
cut one inch from all 6 spokes (top and bottom hats) and measure the resonant
frequency. Assume the new resonant frequency is close to the wanted one. A single
spoke (from top and bottom hats) can not be trimmed to adjust the antenna to
resonance.
40 M: 1" trimmed off a pair of spokes equals approximately 25 KHz
20 M: 1" trimmed off a pair of spokes equals approximately 100 KHz
15 M: 1" trimmed off a pair of spokes equals approximately 175 KHz
10 M: 1" trimmed off a pair of spokes equals approximately 250 KHz