Richard Clark wrote:

EZNEC+ ver. 4.0

Dipole in Ring of Sources 11/2/2005 10:00:48 PM

--------------- LOAD DATA ---------------

Frequency = 70 MHz

Load 1 Voltage = 4.783 V. at 23.52 deg.
Current = 0.06643 A. at 23.52 deg.
Impedance = 72 + J 0 ohms
Power = 0.3177 watts

Total applied power = 2000 watts

Total load power = 0.3177 watts


Taking the determination above as the "standard" I then have
progressed to place an NBS yagi in three space about the center to
obtain its best result.

All such expressions (x,y,z) of the placement of the NBS yagi are with
respect to its "driven" element.

0,0,0 Power = 0.2091 watts
.5,0,0 Power = 0.2198 watts
1,0,0 Power = 0.1429 watts
1.5,0,0 Power = 0.1026 watts
2,0,0 Power = 0.1601 watts
2.5,0,0 Power = 0.2113 watts
3,0,0 Power = 0.1571 watts
3.5,0,0 Power = 0.06028 watts
4,0,0 Power = 0.04128 watts

So, within one quadrant, and over the space of roughly a wavelength,
and at intervals of roughly one eighth wavelength, nothing emerges as
being equal to the "standard" above. Except perhaps a hidden peak
between 0,0,0 and .5,0,0. To investigate this:
.25,0,0 Power = 0.2286 watts
examining further:
.125,0,0 Power = 0.2219 watts
nope, examining further:
.375,0,0 Power = 0.2278 watts
nope, examining further:
.30,0,0 Power = 0.2291 watts
nope, examining further:
.35,0,0 Power = 0.2285 watts
nope, looks like the one before at .30,0,0 is the new sweet spot.

Now, to proceed to investigate the other quadrants to see if there is
symmetry:
-3.5,0,0 Power = 0.03997 watts
0,3.5,0 Power = 0.005925 watts
0,-3.5,0 Power = 0.005859 watts

This last offers that on the Y axis there is a strong symmetry, and
along the X axis there is a moderate symmetry. Now, in regard to both
the X and the Y axis, there is a moderate symmetry. If we were to
look at the fine data attempting to find the peak, we should notice
that the "center" of the antenna lies between the "driven" element and
its reflector. My having chosen the "driven" element as the nominal
center was in error and my guess is that if I re-visited the same
quadrant test above, with that new center at the sweet spot, then we
would find very strong symmetry in all four quadrants. I will add
that the Y axis data supports this due to its strong symmetry that is
relatively immune from the choice of antenna center - at least at this
scale.

Putting that aside, it is enough to suggest that barring an
exquisitely positioned peak of rather a sharp rise, then the yagi
exhibits a poorer response compared to a dipole of approx. 1.4dB.

Others are encouraged to investigate further to reclaim that missing
dB or to put the horns to my error.

73's
Richard Clark, KB7QHC

Hi Richard,

I think what you're seeing is the 3-D interference pattern generated by
your sources. I'm not sure that really tells us very much about the
antennas themselves. You'd need to surround each of the antennas with a
uniform field in order to compare them. By uniform, I mean the field
intensity toward the antenna is the same in any direction.

Thanks, AC6XG