Vectronics VEC-896 - Manual (Page 14)

VEC-896 Vertical Antenna Instruction Manual
13
20, 15, and 10 Meter Bands
Now tune progressively higher frequency HF bands by using the same technique used for the
40 meter band. Set the target frequency 30 KHz lower than the desired operating frequency for
these bands. 20 meters must be the second HF band adjusted, 15 the third, and 10 meters last.
After adjusting 10 meters go back and check the other bands. Tighten all spoke screws. (Use the
chart above for cutting at each band.)
TUNING THE HIGH FREQUENCY BANDS (VHF)
The six and two meter bands are tuned by lengthening or shortening the threaded portion of the
stubs. See Figure 9. This adjustment is very coarse. If the frequency is still too low with the
stub element extension at a minimum, the unthreaded ends of the stubs can be trimmed. Once
again the lower frequency stub (6 meters) should be adjusted first.
The six meter stub covers from 50 to 54 MHz as it is adjusted. Be careful because the third
harmonic resonance of the six meter stub will show up as a low SWR on the third harmonic of
it's setting, usually between 150 and 160 MHz. The range of the two meter stub allows
frequencies as low as 130 MHz to be covered with the stub fully extended and 148 MHz with the
stub fully contracted.
Final frequency adjustments can be made by trimming spokes on the bottom hat only when the
antenna is permanently mounted. The minor adjustment of bottom spokes without the
symmetrical trimming of the top spokes is perfectly acceptable so long as the frequency is
changed less than 30 KHz on 40 meters and 60 KHz on the higher bands with the final
adjustment. The result of attempting to move the resonant frequency too far with only the
bottom spokes will be an increase in the SWR of the antenna at the resonant frequency.