Revised 14 June 2003
E-5
However, if you wish to achieve perfection at some particular frequency, here is the
procedure to completely optimize the antenna for operation on a particular frequency:
1. If the minimum SWR is low enough to make you happy, proceed to step 6 below. If
not, then adjust the matching (front) stub for minimum SWR as indicated in the
subsequent steps.
2. First, find the frequency of minimum SWR (don't worry about the frequency where
this occurs yet).
3. Then, adjust the shorting strap location on the
matching
(front) stub to minimize the
SWR at that frequency.
4. Find the new frequency where the minimum SWR occurs (this should
NOT
be far
removed from the original minimum SWR frequency).
5. Repeat steps 2 and 3 above until you have the array minimum SWR set to 1.0 to 1 at
some frequency. Further adjustment of the
matching
(front) stub will not be
necessary.
6. If you can arrange to reach the element tips for adjustment, then adjust the element
lengths (all four tips by the same amount) to bring the minimum SWR point to the
desired frequency. The 2D-40A antenna will move about 14 kHz per inch of element
tip length change. As you might expect, the resonant frequency of the array will rise
with shortening element lengths.
7. If you absolutely
cannot
arrange to reach the element tips for adjustment (and the
required frequency excursion is less than one percent), then move the shorting bar on
the rear
tuning
stub to bring the antenna resonant frequency to the desired point.
However, if the frequency change is larger than one percent, please consider
resetting the element lengths if at all possible. The 2D-40A antenna will move its
resonant frequency by approximately 6 kHz per inch of
tuning
(rear) stub length
change. Shortening the stub raises the resonant frequency.
NOTE:
Pattern degradation due to moving the
tuning
(rear) stub length away
from the recommended setting is not symmetrical. Lowering the
antenna’s resonant frequency by changing the
tuning
(rear) stub length
will degrade the front to rear performance much more significantly than
raising the resonant frequency will. If the constraints of the installation
will prohibit adjustment of the element tip lengths for tuning, it is better to
err on the low side when selecting the target frequency during antenna
assembly.
