Thread: Antenna future
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Old January 23rd 04, 05:36 PM
Richard Harrison
 
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Art, KB9MZ wrote:
"Antenna engineers have become so focussed on the half wave patterns
that they have completely ignored the low efficiency portions at the
ends of the half wave antenna. Future antennas most surely will remove
these low efficient (sic) radiator parts together with the addition of
coupling techniques that will help to move away from the yagi syndrome."

Don`t hold your breath!

Antenna engineers are focussed on 1/2-wave antenna patterns because
those are the patterns produced by 1/2-wave antennas. A half-wave
antenna is resonant without a reflection from the earth or anything
else. Antenna system resonance is essential to remove reactive impedance
to antenna current flow. No current flow, no antenna operation.

The ends of a dipole have nearly zero radiation because current at the
ends is nearly a zero sum of incident and reflected currents.The H-field
is thus cancelled. Radiation ends where the current ends. A 1/4-wave
back from the reflection point, incident and reflected currents are
in-phase and strong radiation is possible.

What Art calls the "yagi syndrome" is a preference for an antenna which
has only one feedline attachment point and gets about as much gain per
length of wire as any. Size is important for wind loading in addition to
antenna cost and performance. The yagi is a big performer in spite of
its small size.

Yagi elements must be nearly 1/2-wave in length because that`s the
minimum length required to accept significant induced current in a
parasitic element=A0far from ground.

If you were to chop off the ends of a 1/2-wave antenna, you would have
to replace them with another mechanism to bring resonance back to the
shortened dipole. These artifices are almost always lossier than the
lost conductor removed from the antenna. A capacitance hat is an
exception, but this is hardly smaller.

At night it is often more rewarding to look for something lost, not
because the site seems probable, but because the search site is the only
illuminated spot.

Best regards, Richard Harrison, KB5WZI