Magnetic Loop Antenna
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Magnetic Loop Antenna
Although a magnetic loop antenna(aka small loop antenna) is very
compact, its efficiency is close to a half-wavelength dipole if
carefully built. The key point is to make the loop element with
material of low ohmic resistance. Otherwise, ohmic loss in the
element causes disappointingly low efficiency of the antenna. For
the same reason, joints between elements or joints between an
element and a tuning capacitor need be soldered or
silver-soldered. Aluminum is rarely used for the elements because
it cannot be soldered. Therefore, homebrewers often use thick
rigid copper water pipe with, for example, 1/2" to 2"
diameter.
However, I built my own small loop antenna with strips of printed
circuit board stock. They are fairly lighter, much easier to
solder and cheaper than the copper pipe. Copper foil of the PCB
is very thin. Therefore, one might think that the resistance is
much higher than the copper water pipe. However, you should
remember so called "skin effect" of HF current. HF
current tends to flow only in the thin surface region of the
conducting material. So the thin foil does not necessarily cause
more ohmic loss than thick conductor like copper tube. The deep
region of the conductor has almost nothing to do with the
conduction of HF current. Thin conductor with large surface area
may work even better.
The skin effect is characterized by a quantity called the
"skin depth". The HF current at the skin depth below
the surface of the conductor is 1/e (= 1/2.718) times the current
flowing on the surface. The skin depth of copper is about 21
microns at 10 MHz. Very thin. The skin depth decreases as the
inverse of the square root of the frequency. If the copper foil
is a few times thicker than the skin depth, it is practically
thick enough for the HF current of the specified frequency.
I got ten strips of surplus PCB stock with copper foil on both
sides at a price of 200 yen(approximately US $2) at a surplus
shop in Tokyo. Each strip is about 40 cm long and 7 cm wide. The
copper foil is about 50 microns thick.
The copper foil thickness devided by the skin depth is 50/21,
about 2.4. I would, therefore, say the thickness of 50 microns is
practically OK for 10 MHz or higher frequencies.
Seven strips of PCB are connected together to form about 270
cm-long and 7 cm-wide strip, and curved into a circular loop of
about 90cm diameter. Each joint between adjacent PCB strips is
overlapped by about 2 cm. Each overlapping part is tightly
connected with two screws. To make a good electric contact at the
joint, self-adhesive copper tape is sticked and solderd on the
boards on both sides of the joint.
Connections between the element and the tuning capactor need be
done using thick and wide copper leads, e.g. copper braid of
coaxial cable. The contacts should be also soldered. Simple
screw-fixing is not good because of the contact resistance, which
would result in again a low efficiency.
The rest of the construction is not much different from the
ordinary small loop antennas. For the details of constructing
small loop antennas, please refer to the reference articles
listed below.
Results. The Loop was installed horizontally near a window of my
apartment at the ground floor of a five-story building. The loop
is only 3 m high above the ground and 1.3 m apart from the wall.
SWR is 1.5 or less for 10 MHz and 14 MHz. In several months after
the installation, I worked: many JA stations ranging from north
end of JA, Hokkaido to south end, Okinawa, and also one Korean
station with 10 MHz HB 3-watt CW transceiver. On 14 MHz band. I
worked with stations of far east countries like Korea, China,
Asiatic Russia, Taiwan and Hongkong with 3-watt HB CW
transceiver.(My QTH is Chiba, 30 km to the east of Tokyo)
Homebrew is fun.
REFERENCES Transmitting Short Loop Antennas for the HF
Bands, I1ARZ Roberto Craighero, Communications Quarterly 1993
Summer/Autumn. Small High Efficiency Loop Antennas for
Transmitting, The ARRL ANTENNA BOOK 17th EDITION
pp.(5-11)-(5-16).