|
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. The virtual ground RF balun transformer described hereby
allows to demonstrate that in a ½ wavelength folded dipole antenna, its
radiator ( having a caracteristic impedance of 300 ohm) 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, having each of them a
caracteristic impedance of 150 Ohm, referred to the virtual ground node.
c. The virtual ground RF balun transformer described hereby
allows to demonstrate that the ½ wavelength folded dipole radiator and, indeed,
any other resonant loop, be it made of any shape, with both a single turn or a
multiple turn winding, when it is correctly fed with a sinusoidal
radio-electric signal of a proper wavelength, it is always interested by
electric currents that flow along its entire length from one end to the other
in a clockwise direction for a semi-period period and anti-clockwise during the
next semi-period or viceversa.
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).
|
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).
|
