is mounted in the the earth or it may be grounded through a wire
between the mast and a copper ground rod driven into the ground.
The shield side of the coax will be connected to this side of the
antenna system. This side of the antenna system is supposed to be
at ground potential.
Shortly after my installation of the Butternut HF9V I did
some reading from various sources about the performance
of vertical antennas. Particularly elevated verticals which
are used with what I essentially had, a ground plane. My
radials were elevated and tuned which changed the definition
of the antenna from a traditional quarter wave vertical
fed against ground, to something more like a half wave dipole
with one leg mounted vertically (that is the main element
assembly) and the other leg being more horizontal (that
being the resonant radial system). I believe the article
I read may have been in issues of QST Magazine but dating
a couple of decades ago. If I can come up with those articles
again, or specifically the one which influenced my decision
to modify my system I will post the info here.
The difference between a ground plane antenna and a center
fed half wave antenna (typically a dipole) is that with
a ground plane antenna, the radial system is at DC ground
potential. Or ideally at DC ground potential. Earth ground
may be a more accurate term. We are familiar with the old
CB ground plane antennas which are mounted at the top of
a mast which is earth grounded and the radials are electrically
as well as physically connected to the mast and thus the
earth ground. The center radiating element may actually
be DC grounded as well by including a matching stub between
the center element and the base to which both the radials
and ground are connected. The mast will be grounded in some
way. The mast itself will be grounded because it
Click to see larger image
In truth, that doesn't mean we have a good ground, but
the point is, we do have a DC connection to earth where a half
wave dipole antenna will have both legs isolated from earth ground.
Even though the shield of the coax will generally be connected
to one leg of the antenna and at the other end of the coax the
shield will be connected to a ground inside the station, the antenna
will be above ground on both sides.
The Butternet in its original configuration had the radials at
ground potential. In addition to the radials, the shield side
of the coax feed line and the bottom of the 80 and 40 meters matching
coil were at ground potential as well. The lug intended for attaching
the radial system was located on the lowest tube section which
is isolated from the upper system by an insulator. The lower tube
section is meant to mount either in the ground (if the antenna
is ground mounted) or to a supporting mast which is assumed to
be grounded if the antenna is to be elevated. That is the same
point the shield side of the coax attaches to as well as the bottom
of the 80/40 meter matching coil.
I wanted to isolate the lower part of the antenna with the radials
from earth ground. The only way to do that would be to isolate
the area from the lower tube which is going to be grounded when
it is mounted. I fabricated the assembly seen in the picture below.
Basically just a plate with a PVC pipe sleeve fitted in the center
through which the antenna lower mount passes through and insulates
the two assemblies. The radials are attached to the plate at the
four corners. The shield side of the coax is also attached to
this plate as is the bottom of the 80/40 meter matching coil.
This makes it the equivalent of the coax shield being attached
to one side of a dipole antenna.
Before I continue, I must explain the purpose of the blue nylon
line tied to one of the radial tie points. The HF9V has two wire
elements. One for 15 meters and one for 6 meters. These wires
run parallel to the main center element and are supported at each
end by standoffs from the main element. One end of each wire is
"fed" by having it in direct electrical contact with
the main element. The other end of the wire is dead ended but
supported by an insulated standoff so that it remains straight
and ideally rigid.
Initially the wire which was supplied with the antenna assembly
for these two elements was insulated, stranded wire. That did
not seem rigid enough and with movement in the breeze, the wire
seemed to flex too much and thus affect the tuning of those two
bands. I replaced the stranded wire with solid aluminum wire which
was much more rigid and did not move as much. That worked pretty
well but over the course of several years the standoffs themselves
are going to weaken a little and tend to be pulled by the tension
placed on the wires. The wires should be somewhat tense so that
they do not "flop around" in the breeze. We always have
a breeze here in North Texas where the idea of a dead, calm day
is almost unheard of. So the antenna is always going to be swaying
I found that I could not keep proper tension on my 6 meter element
and thus the wire tended to flex a bit. The flexing did not affect
the tuning of the 15 meter element but it sure did the 6 meter
one. Now perhaps my perceived "good idea" to go with
the more rigid wire for these elements was not as good as it appeared
to be, but on the other hand, it may not have mattered. The fact
is, as the antenna flexed in the direction of the element which
is mounted on the side of it, that caused the element wire to
flex and bend a little and resulted in it bowing just a little.
When it did, it was a little closer to the main element and the
band tuning was affected. As I work the lower portion of the 6
meter band I liked to have the tuning bandwidth run from 50 MHz
to close to 51 MHz. Something like that. When the element became
detuned, it lowered the resonant frequency so that it was somewhat
below 50 and much closer to 49 MHz.
There were probably a number of schemes I could have applied
to correct this problem but I elected to take the simple approach
for now. I detached the lower end of the 6 meter element wire
and tied the nylon line to it and ran it down through the center
of the 30 meter coil form and down to where I could tie it to
the radial base plate. The idea was to allow me to access it if
need be to adjust the tension. So far I have not had to do that
and there is just enough tension on that wire to keep it straight
and thus the tuning on the 6 meter band is constant.
So how well does the antenna work? Performance
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