Vectronics VEC-896 - Manual (Page 12)

VEC-896 Vertical Antenna Instruction Manual
11
Please read the following hints:
••••
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.
••••
If you suspect the resonant frequency is lower than your equipment can detect, for
example below 6.0 MHz, take one 40 meter spoke entirely off the antenna (top and bottom
hats). Make another attempt to measure the resonant frequency. If you still are unable to find
the resonant frequency, check another band. If none of these tests produce a good SWR on
any frequency, substitute a 50 ohm load for the antenna to test the feed line. If the SWR
checks good, you will have to double check the antenna assembly for shorts or opens at the
feed point and the connector box.
••••
If the antenna operates normally higher than the band after one spoke is removed,
install all the spokes and trim one spoke simultaneously from top and bottom hats in one inch
steps until you find the resonant frequency within the range of your equipment.
••••
Once you find the resonant frequency, use the chart located on the following page to
estimate the amount of spoke length to cut so the antenna will resonate at your favorite
section of the band.
Important: Always start tuning on 40 meters and adjust each band progressively higher
in frequency.
TUNING THE ANTENNA
Remember: This antenna is a balanced dipole. If changes were made to one side only, the
dipole will be balanced. For that reason, any changes you make on the bottom
capacitor hat should be done in the same manner on the top capacitor hat to keep
the dipole balanced.
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:
40 M 1.6:1 at 6.99 MHz 10 M 1.6:1 at 27.97 MHz
20 M 1.2:1 at 13.9 MHz 6 M 1.2:1 at 51 MHz
15 M 1.2:1 at 20.85 MHz 2 M 1.2:1 at 146 MHz