[Antennas] End-fed half-wave religion

[Cletus W Whitaker]

de WB2CPN South Central Pennsylvania   2002.07.15

Perhaps the arguments will converge if we examine what the 1/4 wave
stub is, and replace it with something we all know.  A 1/4 wave stub,
if examined from the top where the two sides are open, is a paralel
resonant circuit which has an extremely high impedance.  If enough
of these stubs are close enough and in a row we end up with half of
a waveguide.  We can make a "symbolic" J-Pole, (it's not going to
look much like a J-Pole), by building a center-tapped coil and
capacitor connected in paralel, and standing a 1/2 wave dipole on
one end of the coil.  The shield of the coax connects to the center
tap of the coil, the center conductor connects to the coil a few
turns up (or down) from the center tap.  How far up and down will
depend on the characteristics of the coax, the "Q" of the L and C
combination, and the "Q" of the 1/2 wave section.  The 1/2 wave
section is fed by the extremely high voltage which is produced by
the resonant coil and capacitor in paralel.  Current in the 1/2
section flows up and down, zero current at the bottom and at the
top, maximum current at the middle. Current in the resonant
coil and capacitor flows round and back.  The current in the coax
flows through the part of the coil that it's tied to.  Now, some
commercial people stick a second 1/2 wave on the other side of
the 1/4 section, and use the 1/4 wave section as a support for
the whole antenna, but then it's not a J-Pole anymore, is it?
                            73  Clete