Topband: : antenna question

[Tom W8JI]

> once upon a time a long time ago i saw somewhere in a magazine or book,
> or maybe someone told me, or i heard at a ham club meeting or flea
> market, or i had a dream while in heart surgery, or my wife told me that
> a 300 ohm TV twin lead dipole is not all twin lead, but the twin lead is
> shorter than a half wavelength  and at each end a wire continues to make
> half wavelength length of the dipole.
>
> anyone do a dipole this way? or heard or read etc etc about this
> technique?

With balanced line, that idea makes very little (if any) difference in how 
the antenna works around resonance.
This is because the antenna is not in differential mode between conductors. 
The antenna is in common mode.

Think about antenna currents and voltages. In order for the velocity factor 
to play a role, there has to be a considerable proportion of the electric 
field between the conductors. Currents are the same direction and phase.

There isn't much transmission line action between the conductors.

As a matter of fact if you build a normal folded dipole from twinlead and 
cut it exactly to resonance, and then cut the wire going past the center and 
simply parallel them to make it a normal fat dipole of the same size, the 
resonant frequency and bandwidth does not shift.  The only thing that 
changes is the feed impedance, which goes up about four times.



73 Tom