Phased Array of Dipole-on-Ground Antennas for HF Reception
Matt Roberts - matt-at-kk5jy-dot-net
Close-spaced phased arrays exhibit useful directivity characteristics for reception
of skywave signals. The dipole antenna is a natural and simple element type
for a phased array antenna, and the placement of untuned (nonresonant) horizontal
antenna elements very close to the ground provides a pattern that is a useful
building block for constructing such arrays. Combining the concepts developed
in the loop-on-ground with those from the
small loop array, and substituting a nonresonant
dipole for each array element, another type of space-efficient phased array can be
realized. This array has a far-field pattern that is very similar to that of
the array of small vertical loop antennas, combined with the space-efficiency and
stealth of the Loop-on-Ground.
The loop-on-ground is not the only form factor for a ground-mounted horizontal
antenna. A simple dipole has a nearly identical pattern when mounted on the
surface (is a dipole-on-ground a DoG?), and realizes a different kind of space
efficiency. The dipole-on-ground yields a similar electrical efficiency
(peak gain) as a square loop when each leg of the dipole has the same length as
each side of the loop. This means that the dipole's space distribution
trades length for width. That is, a dipole is a thin line across the ground,
but it is twice as long as any side of a square loop with the same electrical
performance. The main lobes of the dipole are off the ends of the wire,
and like the loop-on-ground, the pattern is vertically polarized. In order
to use the dipole-on-ground in an endfire array, the elements are positioned
end-to-end, in a straight line.
Note that the dipole-on-ground is not a "low dipole", such as is used
for an NVIS transmit antenna, or otherwise mounted a few feet above ground.
This antenna element is deliberately mounted on the surface in order to
completely cancel the horizontal response in the antenna pattern.
The remainder of this article describes a phased array of ground-mounted dipole
antennas, very similar to the phased array of vertical loop antennas. In
fact, most of the material in that article with
respect to feedlines and phasing is applicable here. Please see the loop
array article for details on feeding endfire arrays.
Figure 1 is an EZ-NEC model of a
two-element endfire dipole-on-ground. The elements are each 180 inches
long (15ft), with 60 inches (5ft) of separation between the elements at their
closest point. The overall length of the antenna is 35'.
Figure 1: EZ-NEC Antenna Model
As with the vertical loop array, the dipole array
requires preamplifiers to isolate the antenna from the delay lines. Otherwise,
the asymmetric reflections within the delay lines will spoil the front/back ratio
of the pattern. Alternatively, the individual elements can be loaded with
resistance to match them to the feedlines, but doing so decreases the element gain
significantly, which causes even more preamplifier gain to be needed. So the
simplest solution is to use equal feedline lengths between the untuned, unloaded
elements and the preamplifiers, and then place the delay lines between the
preamplifiers and the combiner.
When properly phased, the array can produce a pattern similar to the array of
small vertical loops:
Figure 2: 40m Elevation Plane
Figure 3: 80m Elevation Plane
Figure 4: 160m Elevation Plane
The azimuth profile of this antnena is essentially the same as with the vertical
loop array. Azimuth plots are shown here for 80m response, at the elevation
of peak response (~27°) and at 10° elevation.
Figure 5: 80m Azimuth Plane, 27° Elevation
Figure 6: 80m Azimuth Plane, 10° Elevation
The shape of the azimuth pattern is similar down practically to the horizon.
This is the predicted azimuth at 5°:
Figure 7: 80m Azimuth Plane, 5° Elevation
The delay lines used in the model for 40m and 80m are 133° and 156°,
respectively. As with the vertical loop array, the required delay is
dependent upon the spacing between the elements.
The main feature distinguishing this design from the
vertical loop array is its physical profile.
The long, thin profile of this antenna makes it ideal for installation almost
anywhere. The centerline of the antenna "points" in the direction
of the main lobe, and the antenna direction can be electrically reversed by
swapping the delay lines on the receiver side of the preamplifiers. This
makes aiming the antenna much easier than an array built from the loop-on-ground
for 40m and 80m is also similar to the vertical loop array, with values of
8.9dB and 9.2dB, respectively. This places it on par with
a well-designed and constructed Beverage
antenna that is several hundred feet long, yet this model fits in a space
that is only 35' long.
Copyright (C) 2016,2017 by Matt Roberts, All Rights Reserved.