Transcript The Off Centered-Loaded Dipole - Navy

THE OFF CENTER-LOADED DIPOLE
A PHYSICALLY SHORT 160 METER ANTENNA
by NNN0IAD
Introduction
Many operators have never given 160 meters a try simply because of the formidable antenna
length required for this low band. If you live in the city or have a home on a small suburban lot you
may have ruled out operation on the top band. Well, there are obviously a number of ways to
circumvent this problem, but as one would expect, the results can be as varied as the approaches.
I have found objections to most approaches. For instance, short vertical antennae can be base
loaded, but the radiation impedance of these antennae can be as low as one or two ohms. Think
about Ohm’s Law and what happens to current when you put 50 or 100 volts across one ohm. The
current ( I = E/R) flow could quickly damage your equipment without special precautions. This solution
normally requires the use of an elaborate radial system with good electrical integrity to increase the
radiation impedance and keep transmission line currents reasonable. A physically short dipole fed
with open line can be efficient, but you need a good antennae tuner to match the complex impedance
of the line to your equipment. Open feedline is popular, but my objection to this method is that you have
to be very careful about making sure that the line is really balanced. An unbalanced line can cause
you to have a shack full of RF. There is currently a lot of debate as to whether exposure to this RF can
cause leukemia. Why take the chance?
The Off Centered-Loaded Dipole
Of the various schemes in existence, the off centered loaded dipole is the one that I like the best.
Since it is a true dipole, it has a counterpoise and does not require radials. It is loaded by placing
inductors in each leg of the dipole. Thus, both the physical length and impedance can be adjusted by
“positioning” the inductors along the line length of the antenna. No tuner is required and the antenna is
directly fed with 50 ohm coax. When operated on or close to the resonant frequency stray radiation in
the shack will not be a problem.
The antenna described here was designed for NAVY-MARINE CORPS MARS and is resonate on
2025 KHZ (USB window 2023.5 KHZ). The antenna is not very broad banded but operates well on the
top of the 160 meter amateur band with an SWR of about 1.5::1.0. I have experimentally determined
that the resonant frequency roughly changes about 20 KHZ for every foot length of wire. Increasing the
length of each side of the dipole by 24-inches will bring the resonance down to 1950.0 KHZ, making it
suitable for amateur service on 160 meters.
The antenna is normally installed as an inverted-vee, with the center up at about 35 feet and the ends
up at about 12 feet above ground level. The antenna can also be installed horizontally, but keep in mind
that the resonant frequency will go up slightly unless the antenna is lengthened a few inches.
The position of the coils (L) will determine the overall physical length of the antenna and also what
inductive reactance will be required to match the feed point impedance to a 50 ohm line. The chosen
parameters are given below:
physical length:
position of coil:
resonant freq. (f):
inductance (L):
inductive reactance:
48% of a full length half-wave dipole
23% of the distance from center to end insulator
2023.5 KHZ
58.7 mh
746.5 ohms
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THE OFF CENTER-LOADED DIPOLE
by NNN0IAD
Two loading impedance matching coils are required, one for each side of the dipole. The coils have
to be constructed so as to provide 58.7uh of inductance with a reactance of 746.5 ohms at the desired
resonant frequency. The following construction technique has been successfully employed to build
several of these antennae for the 160 meter band. The coils were wound using 80 turns of wire wound
on a coil form made from 2-inch (1-1/2-inch ID) schedule 40 PVC pipe. Radio Shack speaker 16 gauge
wire (P/N 278-1267) is supplied on a 50-foot roll. The speaker wire pair can be easily separated,
providing two strands, one for each of the two required coils.
The coil (L) details are given below:
Page 3 shows a diagram of the completed antenna. The dimensions provided an SWR of 1::1 at the
location where the antenna was tested. It was taken down and erected at another site and with only a
minor adjustment of the height of the ends, the antenna performed flawlessly. It is helpful to have an
SWR meter, or antenna analyzer, to make minor adjustments to the antenna to tune it for your site.
Note: This is a reprint of an article that appeared in the Apil 1990 copy of NAVMARCORMARS Scuttlebutt.
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THE OFF CENTER-LOADED DIPOLE
by NNN0IAD