I should like to revisit the subject of 3/8 wavelength antennas which brought
several interesting postings on 17 May.
I had long thought that it was best to get the point of maximum current in a
vertical radiator as high as possible, for it would then be farthest away
from lossy ground. Therefore, I was especially interested by Tom's statement,
supported with facts I had not previously thought about, that centering the
point of maximum current in the vertical antenna, or in the vertical portion
of an inverted L antenna, yields maximum radiation in the vertical plane.
If, one had, say, a 120-foot vertical antenna, should one just feel great
about having a full-size quarter-wavelength antenna on Top Band and leave
well enough alone, or should one then add top-hat capacitance of such an
amount that the current maximum moves up the vertical from ground? And given
that the current distribution is not symmetrical when referenced to the
center point of a vertical antenna such as this (the way it is around the
center of a half-wave center-fed dipole), should one, by adding top-hat
capacitance, be aiming to position at the 60-foot center point, not the
current maximum, but rather a point on the current distribution curve such
that the median of the current distribution curve occurs at the center point;
in other words, the sum of all the current distributed over the top 60 feet
would be equal to the sum of all the current distributed over the bottom 60
feet? If this is indeed a desirable objective, it seems that it would then
be quite a trick to measure the currents in each half to ascertain if the
goal had been achieved.
I do not know if W1BB's objective with the 3/8 wavelength antenna was to
elevate the point of maximum current or increase the feedpoint impedance, or
both.
For those to whom a convenient feedpoint impedance is important (it is not a
primary consideration for me), a 3/8-wavelength vertical (ALL vertical) with
two conductors in a folded fashion is said to present a resistive 200-Ohm
impedance. Such an antenna derives from John Kraus, W8JK's efforts, as
described in "Multiwire Doublet Antennas, Radio, May 1939. Here, he
illustrates a 3/4-wavelength, center-fed folded dipole, with the unfed
conductor open opposite the feedpoint. Half of the 3/4-wavelength antenna
gives one a 3/8-wavelength radiator, which, when positioned vertically, has
the two feedline attachment points connected to one of the antenna's
conductors and to ground; the end of other antenna conductor (opposite the
feedpoint) is unconnected. Obviously, a 200-Ohm feedpoint impedance invites
use of a 4:1 balun when a 50-Ohm transmission line is used.
The feedpoint impedance of any antenna, regardless of its length, will vary
greatly when it is bent into an inverted L configuration, according to how
much of the antenna is vertical and how much is horizontal. A brief article
in CQ of December 1953, drawing upon an earlier work published in the Bell
Telephone Laboratories Record of May 1949, shows a 1/4-wavelength, base-fed
radiator to have a feedpoint impedance of 36 Ohms when it is 100-percent
vertical. This figure drops to 34, 30, 26, 20, 15, 9 and 4.5 Ohms when the
vertical portion of the 1/4-wavelength radiator is 80, 70, 60, 50, 40, 30 and
20 percent, respectively. By using two or three conductors, the feedpoint
impedances noted can be multiplied by four or nine, respectively. CAUTION:
Do not be fooled into thinking that folded elements beneficially affect an
antenna's radiation resistance; they do not.
73,
Charles, W2SH
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