> At 05:08 PM 6/30/2004 -0500, you wrote:
> >The parasitic element that reflects back toward the
driven element is called
> >the reflector. I think the name origin is obvious. If
it were shorter than
> >the driven element it would not exhibit this
characteristic. It is usually
> >placed 1/4 wavelength from the driven element so that the
magnetic field set
> >up in the parasitic element is 180 degrees out of phase.
This induces a
> >voltage in the driven element that is in phase with and
increases the
> >forward radiation.
Yes, that's basically correct. The Yagi is an end-fire
array, and endfire arrays fire in the direction of
**lagging** element phase.
The re-radiation from each parasitic element is inverted 180
degrees in phase just because it is re-radiation.
The spacing to element adds a time delay in space. If the
element is not exactly resonant it adds a phase shift
because of reactance in the element. So we have 180 degrees
inversion plus the delay in space plus or minus a smaller
delay or advance caused by element tuning.
Looking at a reflector spaced .2 wavelength you have a space
delay of 72 degrees, which is the same as a 288 degree lead.
This is inverted by 180 when reradiated for an effective
phase of 108 degrees lead. The reflector, when self resonant
at the operating frequency, has the equivalent phase in this
case of 108 degrees lead. Continuing past the reflector you
see this inversion cancels the primary radiation. The driven
element has the equivalent of lagging phase compared to the
reflector, and so that is the primary direction the array
radiates.
This is why a two-element array has a zero angle back-null
when the reflector is self-resonant. Self-resonant tuning is
nearly perfect for a reflector in a two-element array.
When acting like a director, the element is shortened. This
adds capacitive reactance, and causes a phase lead. The
phase lead is inverted by 180 and becomes a lag, which adds
to the spacing lag. (You have the same spacing delay effects
added.) This is why directors are tuned SHORTER than
resonance. Because the director is generally operated
further from resonance than the reflector, it often requires
closer spacing to have proper current ratios.
The resulting pattern is the vector sum of all the radiation
fields of the elements, as it is in any system.
Spacing and tuning BOTH set phase and amplitude, so
adjustments are a compromise. That isn't the case if you
drive all elements, you can make every element optimum phase
and current. Luckily with a three element we can obtain
nearly perfect 1:2:1 current ratios in the elements with
correct phase. That's why three element Yagi's can produce
such high F/B and gain, as good as a driven array the same
size.
73 Tom
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