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A Dual-Band Flexi-Dipole
KA4LBE presents, in simple terms, the operation and
construction of a portable, dual-band, flexible dipole antenna made by
connecting center-fed dipoles in parallel at their feedpoints.
By Benson Smith,* KA4LBE |
A close-up of the completed antenna center
area. |
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The connecting of two or more center-fed
dipoles in parallel at their feed points is a method used by many hams.
This parallel feeding saves feedline by requiring only one. For more
information than is presented here, refer to several articles in the ARRL
Antenna Book.1, 2
The purpose of this article is to discuss in simple terms the operation,
pros and cons, of such an antenna system and to present a unique
construction of the system. The author and XYL KA4LBD use a multi-band,
parallel-fed dipole group spanning 160 through 30 meters that allows
operation on harmonics through 12 meters. The system works very well.
Construction
We will begin with the construction of a roll-up, two-band, portable,
center-fed half-wave dipole. It is based on the use of TV-type 300-ohm
twin-lead transmission line as the radiating elements. The twin lead1 must
be unshielded. The foam type is harder to use than the plain, flat ribbon
type and offers no advantage in this application. Check to see that the
type you plan to use is flexible.3
Our primary design goals were:
1. Build an antenna that does not require a tuner for effective operation.
2. Two-band operation.
3. Very portable, roll-up capability.
4. Easy to build.
Figure 1 shows a sketch of the antenna. Notice that the upper side of the
twin lead has been trimmed to a shorter length (B) than the overall length
(A). Because of the element spacing, it makes no difference if the shorter
or longer dipole is on the top. The overall length is the dipole element
for the lower frequency band, while the shorter length creates the dipole
for the higher frequency. The two dipoles are fed at their centers as in
all center-fed half-wave dipoles. We will limit our design to 6 meters
through 70 cm.
An objection to this type of antenna is the loading on one of the dipoles
from the other. With antennas built of separate conductors, tuning
variations can occur if the wires move in relation to one another. Also,
when building them using separate conductors, length predictability is
poor. Using twin lead to create two-band operation overcomes these two
objections, as the separation between element is fixed.
The overall length of a dipole is found by the formula:
OA feet = (492/Fo) ¥ K
where Fo is the design frequency in MHz and K is a combination of
modifying factors (see text).
In so-called “free-space” there is no ground, no other conductors nearby
the antenna, and the dipole is made of infinitely small diameter
conductor. In that case, the value of the K factor in the formula above is
one. Here on Earth we find totally different cases. Designing a dipole for
HF and using conductors with diameters near that of AWG #12 copper wire
and mounting this dipole in our atmosphere and at about 1/2 wavelength in
height above ground and away from other conductors results in a K value of
about 0.95.4
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© Copyright 2004, CQ
Communications, Inc. All rights reserved. This material may not be
reproduced or republished, including posting to a website, in part or in
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