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A VIRTUAL GROUND BALUN FOR DOUBLING THE ½
WAVELENGTH FOLDED DIPOLE ANTENNA
by Francesco Errante
Scientific purposes
a. The virtual ground RF BALUN TRANSFORMER described hereby allows
the doubling of the ½ wavelength folded dipole's radiator.
b. A virtual ground RF BALUN TRANSFORMER, as described hereby, for
the doubling of the folded dipole's radiator, allows to demonstrate that in a ½
wavelength folded dipole antenna its radiator acts as it was made of two distinct
radiators identical to each other and coinciding with each other, while being fed in
a counterphase arrangement.
Circuit description
The Author, by means of a particular radio-electric
circuitry for the suppression of anyone of the two branches of a ½ of wavelength
open dipole, has previously demonstrated that it is, indeed, possible 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. On that occasion, the Author has introduced the reader to the
concept and the method for generating a virtual ground node for the separation of the
dipole's branch currents.
The ½ wavelength folded dipole antenna has a single wiring radiator and, therefore,
does not allow the independent usage of the currents flowing on it, both in a
clockwise and anti clockwise way.
The present invention allows to split up the folded dipole radiator into two
nearly-coinciding wirings in order to demonstrate that the single folded dipole
radiator acts as if it was made of two identical but distinct wirings coinciding with
each other and each working cyclically for the duration of a semi-period.
This result has been achieved by means of a lumped-constants radio-electric circuit,
based around a broad-band flux-coupled radio electric balun 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(10) being used and a center-tapped
secondary winding(3) exhibiting an impedance value of 150 + 150 Ohm.
The said impedance values are referred to the virtual ground node(4 & 6). (a
virtual ground node 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 is made available to the whole circuit by means of a very
short electrical connection(6) between the transformer center-tapping(4) and the
balun chassis(5).
The transformer's working point is optimized by compensation with the employment of
high-voltage RF duty capacitors(9).
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It is important to highlight the fact that the two
independently fed branches(7 & 8) can also be independently rotated around their
common axis to give a variable radiation pattern which ranges from a typical dipole's
radiation pattern, if the arms are parallel to each other or a quasi-omnidirectional
radiation pattern in the case of the arms being placed perpendicularly to each-other
(turnstile arrangement).
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