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Tuning the Vertical
When the antenna elements are completely assembled to the base section, the overall length
measured from the top of the antenna to the feedpoint on the base section should be 67 feet, 1-3/4
inches prior to tuning. This allows you to make adjustments in the overall length to tune the antenna
to the center frequency you desire using the following information.
It’s best to use an antenna analyzer such as the MFJ-259B for determining antenna resonance. Use
the X=0 an +/- j0 readings to determine the resonant frequency. The SWR will be adjusted by the
impedance matching assembly mounted at the feed point once the vertical is resonant at the desired
frequency.
The
DXE-7580FS-VA-3
should resonate at approximately 3.65 MHz with the recommended
ground radial system installed and the vertical dimensions described in this manual. Resonance is
adjusted by the length of the vertical element sections. To raise the frequency, slide the top 1/2” OD
element into the 5/8” OD element.
If you are having trouble achieving resonance, make sure the element section lengths are correct.
Make sure you have at least 16 radials (32 are better), 65 feet long, symmetrically placed around the
vertical. Our test vertical employed 32 radials, 65 feet long. The difference in resonance from 16 to
32 radials is about 30 kHz.
The antenna resonant frequency may be centered at any point by merely adjusting the overall
length. To raise the base resonant frequency, shorten the element tubing stack.
As a rule of thumb, one foot of length should be approximately 50 kHz in frequency. A shorter
antenna length = higher frequency and a longer antenna length = lower frequency.
Typical Example:
Tuned for 3.65 MHz
Bandwidth at
2:1 is 450 kHz
