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A VIRTUAL GROUND BALUN FOR THE SUPPRESSION OF ANYONE OF THE TWO BRANCHES OF A ½ WAVELENGTH OPEN DIPOLE ANTENNA
by Francesco Errante
Scientific purposes
a. The balun described hereby enables to demonstrate that
the ½ wavelength open dipole antenna, also known as "Hertzian Dipole"
is not an "elementary antenna" (by definition an elementary antenna is
an aerial where the condition of resonance and radiation cannot take place without
the presence of all its parts) but it is, instead, an "elementary array"
of 2 radiating/captating elements of a physical length equal to ¼ of a wavelength each, which are electrically
arranged in a counterphase, while being fed in the middle of them.
b. The balun described hereby enables to demonstrate that once a virtual ground node
is available, it is possible to receive and transmit radio-electric signals through the
space by employing a single radiating/captating element of a physical length equal to ¼ of a wavelength,
without the need for a natural nor an artificial ground plane.
c. The balun described hereby enables to demonstrate that each of the two branches of an ½
wavelength open dipole antenna, at its frequency of resonance, exhibits an absolute impedance value of 35 Ohm, unbalanced and
referred to the virtual ground node.
d. The balun described hereby enables to demonstrate that each of the two branches of an ½
wavelength open dipole antenna, exhibits a phase-angle difference of +/- 90 degree with respect to the virtual ground node.
Circuit description
In the field of radio-engineering, a multitude of systems are well-known
for matching an ½ wavelength open dipole to an unbalanced transmission line,
they are usually referred to as balun (a compound term meaning "balanced-unbalanced").
State of the art balun(s) do not allow feeding of the two branches of
an ½ wavelength open dipole independently from each other and as a result of that,
they do not allow to verify whether it possible to suppress anyone of them without
interfering with the condition of resonance and irradiation of the remaining one.
The invention described hereby allows to feed each of the two branches of
an ½ wavelength open dipole independently, so that they can become electrically independent
from each other allowing, therefore, to suppress anyone of them without repercussions
on the condition of resonance and radiation of the remaining one.
(similarly, a device for doubling the ½ wavelength folded dipole antenna
is described here)
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This result has been achieved by means of a lumped-constants radio-electric circuit, based
around a broad-band radio-frequency electric transformer(1) winded on a binocular type
ferrite core having a primary winding(2) exhibiting an impedance value equal the one of the transmission line(3) being used and a center-tapped secondary winding(4)
exhibiting an impedance value of 35 + 35 Ohm.
The said impedance values are referred to the virtual ground.
("virtual ground" is defined as a point in an electrical circuit that appears
to be at ground, but is not actually attached to ground, it is therefore,
a node having a 0 degree phase angle difference with respect to ground and has the
same electrical potential as the Earth")
The virtual ground node has been obtained by providing the transformer's
secondary winding(4) with a center-tap(5), effectively splitting the secondary
winding in two sections. The virtual ground is then made available to the whole of
the network by a short electrical connection(6) to the device chassis(7).
The transformer's working point is optimized by compensation with the employment of high-voltage
RF duty capacitors(8).
The suppression of one of the two branches of the open dipole is made by manually
operating the high insulation resistance switch(9) diverting the radio-electric signal from the transformer(1)
off the branch of the dipole(11 or 12) and into one lead of the non-inductive 35 Ohm dummy-load(10)
while the other lead of the dummy-load is attached to the virtual ground circuit.
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