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Topband: 1/4 and 3/8-wavelength vertical radiators

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Subject: Topband: 1/4 and 3/8-wavelength vertical radiators
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Date: Wed, 19 Jun 2002 15:21:28 EDT
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 

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.


Charles, W2SH

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