6
The SWR bandwidth and directional performance bandwidth of this system is highly dependent
upon the characteristics of the vertical elements used in the array. The Four Square System Phasing
Unit is a switchable modified hybrid coupler. It provides proper currents and phase at each antenna
element when the correct electrical length 75 Ω feedlines to each vertical antenna are used.
Each of the four vertical antennas used in the Transmit Four Square system must be very similar to
each other. If the elements have different impedances or radiation characteristics, system
performance will suffer.
Each side of the square must be 1/4-wavelength in free space at the desired operating frequency. For
example, each side must be 64.7 ft for a center frequency of 3.800 MHz.
Note that the free space formula 246/f (MHz) is used for array spacing - not the more familiar 234/f
(MHz) used for antenna element lengths, which takes element end effect into account.
Array spacing is important in providing the deepest rear null – commonly observed when checking
the front-to-back ratio (F/B).
The
TFS4
Four Square System Phasing Unit is placed in the physical center of the square, where
exactly equal 1/4-wavelength 75Ω coaxial cables connect each vertical element to the Four Square
System Phasing Unit. Note that the cable lengths are different from the free space length of the
square sides. This is because the cables have a different velocity factor than air. Radio frequency
signals move through the cables slower than they move through air.
For best performance, you should install a good ground radial system. 32 straight radials, most
about 1/4 wavelength long, are recommended at each of the four vertical antennas. Some of the
radials will cross in the middle of the array. These radials may be allowed to cross if well-insulated
from each other at crossing points, but it is better if they are bonded as described in the installation
section. Either method will provide suitable performance. What cannot be tolerated are poor or
intermittent connections between radials.
As with any properly installed quarter-wave vertical antenna, signals arriving at lower angles of
radiation will be enhanced more than very high angle signals. Phasing is optimized for very low
wave angles. This means array directional performance will generally be better on more distant low
angle and on local ground-wave signals.
While greatly dependent on many variables, (such as the radial system, angle of arrival of signals,
etc.), the gain of a properly designed and installed four-square array will be on the order of 5 dB
over a single vertical element.
Typical F/B ratios will approach or exceed 20 dB, minimizing interference from the side and rear
directions. See various reference books such as "
ON4UN’s Low Band DXing
" for more in-depth
discussion of four-square theory and performance.