1): The length to avoid is nothing more than a half wavelength, which
translates the same impedance from end to end. i.e., the high Z open end
translates to a high Z antenna base end. This results in minimum radial
current. At 10' up, they're close to free space (0.47 vs 0.5 WL). At 2"
above the ground, the length to avoid is still a half wave, but
shortened by the near ground velocity number. Your 2" high length is
almost identical to my on ground one. Very good correlation....
2): Rudy's original premise exactly. If the radials are the same
dimension as the short vertical, you achieve 90% of the performance
possible. I merely resonated them exactly, to achieve the lowest radial
impedance. The dimension similarity was purely serendipitous...Sometimes
you win!
Brian K8BHZ
Two things become obvious:
1) As the radial height is lowered the "length to avoid" gets shorter. When
the radials are at 10 ft the large drop in efficiency happens at a radial length of ~0.47
WL (~250 ft at 1.85 MHz). When the radials are at 2 inches the efficiency dip happens at
a radial length of ~0.36 WL (~191 ft).
2) For both the straight vertical and the inverted L, the highest efficiency
happens at a radial length of ~0.2 WL (~106 ft). But if you make them shorter,
say the length of the vertical portion of the inverted L (60 ft, ~0.11 WL),
you're really not giving up very much.
_________________
Topband Reflector Archives - http://www.contesting.com/_topband
|