January 1963 Electronics World
Wax nostalgic about and learn from the history of early electronics. See articles
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
If you have been searching for a do-it-yourself VLF loop antenna
that can be resonated from approximately 14 to 25 kHz,
then look no more. This article from a 1963 edition of Electronics
World presents a relatively simple to build job that reportedly
provides excellent reception. At these frequencies a wavelength
is measured in miles, which makes even a simple dipole antenna
impractical, so the multi-turn loop is the only alternative.
It is the same principle that allows the little ferrite-core
antenna inside your AM radio to work so well when the shortest
wavelength in the commercial AM broadcast band
(535 - 1700 kHz in the U.S.)
is nearly 600 feet.
V.L.F. Loop Antenna
By Richard A. Genaille
Construction of a simple, low-cost sensitive loop ideally
suited for good low-frequency reception.
Fig. 1 - Schematic of loop with its tuning
and matching network.
The very-low frequencies from 3 to 30 kilocycles have experienced
a resurgence of use and interest about which many persons engaged
in electronics are not aware. The September, 1961 issue of this
magazine carried an article ("Below the Broadcast Band") in
which the author described some of the activities which are
taking place in the very-low and low-frequency bands and also
a simple low-frequency converter which could be constructed
for the reception of these frequencies. Briefly, the very-low
frequencies are providing a means of high-accuracy frequency
determination in much shorter periods of time than is normally
possible on the higher frequencies, and also the means by which
the U. S. Navy transmits messages to submerged submarines which
are scattered throughout the oceans of the world. Present and
proposed activities relative to the v.l.f. region will be discussed
later in this article.
So fascinating and useful have the very-low frequencies become
that the author decided to upgrade his receiving antenna system
for v.l.f. by constructing a loop antenna which would greatly
improve signal-to-noise ratio and would discriminate against
adjacent-channel interference. A considerable amount of experimentation
produced a v.l.f, loop antenna that, you will discover, is simple
to construct, efficient and inexpensive, and which has all of
the desirable electrical features that a loop antenna should
have. While the loop to be described was constructed for use
in conjunction with the low-frequency converter featured in
the aforementioned article, it has been designed with a feed-point
impedance of 52 ohms to accommodate standard 52-ohm coaxial
transmission line and the input impedance of some receivers.
It may be used on other very-low frequency converters or receivers
having other than a 52-ohm input impedance by the simple expedient
of impedance matching. Since receiver input impedances can vary
considerably, the author will describe the method used to impedance-match
to his converter as well as several simple methods of matching
to other input impedances.
Fig. 2 - Construction of the bifilar-wound
The decision to construct a loop antenna for v.l.f, in preference
to using a random length long wire or a simple half-wave dipole
was made after realizing that such antennas have several undesirable
characteristics. Providing that the long wire was made long
enough to have definite directional characteristics, it would
be quite difficult to make use of the directional features because
of the problem of positioning the antenna. The long wire is
inferior to a loop antenna for minimizing noise pickup. A simple
half-wave dipole at 20 kilocycles would be approximately 4 miles
long. To keep the v.l.f. antenna to a reasonable size and to
provide directional selectivity and noise reduction were factors
which led to the choice of the loop antenna as the most satisfactory
solution to the antenna problem.
Fig. 3 - Bridge method of measuring s.w.r.
when tuning antenna.
Since loop antennas are not as commonplace as the "garden
variety" of directional antennas, such as multi-element directive
arrays for amateur radio and TV use, a few words regarding loop
antenna operation may be in order for a better understanding
of the constructional details to follow.
Loop antennas have been widely used for many years in direction
finding systems particularly aboard aircraft and vessels. The
function of the loop is to sense the direction of the arrival
of radio signals emanating from a transmitter at a fixed location.
The basic loop antenna is simply a coil of wire whose diameter
is small in comparison to the wavelength to which the coil is
tuned. The ground-wave transmission from the very-low frequency
station causes vertically polarized waves to induce voltages
in the loop wire as these waves pass by the loop. The induced
voltages in the loop wire produce a loop current which depends
upon the positioning of the loop antenna with respect to the
Almost any convenient shape can be used for a loop antenna,
such as a square, triangle, octagon, or diamond. But regardless
of which shape the loop assumes, the maximum directivity is
along the plane of the loop with a distinct minimum or null
at right angles to the plane. The directive pattern of a loop
whose diameter is small with respect to the wavelength to which
the loop is tuned is similar to that of a doublet antenna, that
is, a figure-8 field pattern. The minimum or null, which is
broadside to the plane, is extremely sharp in a well-designed
loop antenna and is normally capable of giving bearing information
better than one degree in low-frequency direction finding work.
While the purpose of constructing the loop antenna, in this
case, is not that of direction finding, the presence of a sharp
null at zero and 180 degrees with respect to a fixed v.l.f.
signal source a reasonable distance away indicates that the
loop is functioning properly. This will make it possible for
us to eliminate undesirable adjacent-channel interference. The
absence of this sharp null broadside to the plane of the loop
antenna can be caused by locating the antenna too close to power
wires, other antennas, gutters and downspouts, or other metallic
objects and, in general, by poor symmetry of the entire loop
antenna circuit including the transmission line and receiver
input. The use of a balanced feedline arrangement and a push-pull
r.f. stage for the receiver input or a suitable-matching transformer
to match between a balanced feed point and an unbalanced line
can be accomplished to improve loop circuit symmetry.
Static electricity in the air is a source of much noise in
low-frequency reception and very often causes complete masking
of desired signals. Enclosing the receiving loop wires in a
non-magnetic metallic shield will greatly reduce noise pickup
thereby enhancing the over-all signal-to-noise ratio of the
receiving system. The loop wires are completely surrounded by
the shield except for a narrow transverse gap or break at the
apex of the loop electrostatic shield.
The v.l.f. loop antenna shown in the photograph can be resonated
from approximately 14 kc. to 25 kc. with the components specified
in the schematic diagram. In this range the feed point impedance
will be 52 ohms. The construction of this antenna is quite simple
and straightforward and the cost of the materials used represents
a very small investment of the performance obtained. All of
the component parts of the loop antenna circuit are readily
The schematic of the v.l.f. loop is shown in Fig 1. L1 is
continuous loop made up of 18 turns of #16 enameled or form-var-insulated
wire. L2, which is adjustable from 0.2 to 3 mhy, is used to
resonate the loop circuit to the desired frequency. T1 is a
matching transformer wound so that loop balance is maintained
while providing a match to a 52-ohm unbalanced line. Capacitors
C1 and C2 are good quality micas used to bring the over-all
loop tuning into the range of tuning coil L2. The tuning coil,
capacitors, and matching transformer are housed in an aluminum
box with dimensions of suitable size to permit freedom in making
the necessary connections.
The electrostatic shield for the loop wires is made from
a twenty-five foot length of soft-drawn copper tubing with a
1/2-inch inside diameter. This tubing is available from Sears,
Roebuck or any plumbing supply house. In many cases 25 feet
of tubing represents a standard length coil and was chosen so
as to avoid wasted tubing due to cutting. A length of cheap
plastic hose with a 3/8-inch i.d. and slightly less than a 1/2-inch
o.d. was used inside the copper tubing to protect the loop wires
during the pulling operation. The author felt that the small
extra cost of the plastic hose would be worth it to prevent
possible abrasion of the wire insulation and subsequent operational
troubles. The plastic hose also provides additional loop rigidity
lost by the necessity for a gap in the copper tubing at the
apex of the loop.
The first step in the construction of the loop proper is
uncoil and stretch out the 25 feet of copper tubing on a level
floor. The straighter the tubing the easier it will be to pull
through the plastic hose and wires in the following steps. After
the tubing has been straightened, solder a 1/2-inch copper tubing-to-outside
thread adapter to each end of the tubing. Conduit nuts of suitable
size may be used to secure the copper tubing to the metal box
which houses the smaller circuit components. Next, measure to
the exact center of the length of tubing and, using a tubing
cutter, cut the tubing in half. Keeping the tubing sections
together, insert a 27-foot length of plastic hose into one end
of the tubing and, by working it slowly, pass the hose through
both sections of the tubing so that approximately one foot of
hose remains outside each end of the copper tubing.
The author's loop must presently be turned
by hand although it is planned to install an antenna rotator
for it shortly.
At this point a single #16 wire should be worked through
the tubing-hose combination to facilitate pulling through the
bundle of 18 #16 loop wires. The loop wires should be cut to
a length of 27 feet and each wire tinned on one end. The bundle
of eighteen wires should then be soldered to the pulling wire
and the bundle carefully pulled through the entire length of
the loop tube. Incidentally, the tedious job of removing the
insulation from the wire ends can be simplified by using some
Sears, Roebuck No. 2779 Paint and Varnish Remover. The simplest
way to form the loop is to layout a circle 8 feet in diameter
on your basement or garage floor. Use a heavy marking pencil
so the outline may be easily seen. A word of caution, don't
construct the loop in the basement if you can't get it out of
the basement doorway.
In bending the tubing the author found that the 100 or so
pounds of his 13-year-old son standing on the tubing prevented
the tubing from moving as the circle was being formed. After
the circle has been formed the plastic hose and wire should
be cut back as shown in the photograph. Suitable holes may now
be drilled in the metal box and the tubing ends fastened to
the box with the conduit nuts.
A terminal board was installed in the metal box in order
to facilitate connecting the 36 wire ends into a continuous
loop of 18 turns. Connecting the wires properly can be accomplished
by using an ohmmeter or they can be "buzzed out" by use of a
dry cell and buzzer or pilot light bulb. If care is exercised
the center turn of the multi-turn loop can be identified and
marked at the time that the loop wires are being soldered together
to form the continuous coil thus avoiding the trouble of trying
to determine the loop center by electrical measurement. The
loop wire resistance is quite low, approximately 1 3/4 ohms,
and it is quite difficult to make accurate measurements in the
low resistance range of the ordinary ohmmeter. After the loop
wires are all connected, except for the two ends, tuning coil
L2 can be mounted and connected in series with the loop wires
at the midpoint. Capacitors C1 and C2 can be mounted and wired
except for the connection to be made to the matching transformer
T1. A suggested layout for the various components that are installed
within the metal box is shown in the photograph.
Line-matching transformer T1 is an important part of the
loop circuit in that it provides the impedance transformation
required while maintaining loop balance. Reasonable care should
be exercised in winding T1. The ferrite rod from a transistor-radio
loopstick was used as the core for this transformer. The original
windings were removed and 180 turns of #28 enameled wire close-wound
on the core. This winding will be the secondary and will connect
to the coaxial output connector J1. To determine how much wire
will be required for the primary and how much space will be
taken, wind 104 turns of #28 enameled wire over the secondary.
Remove the 104 turns of wire and after finding the center of
this length of wire, fold the length of wire in half so that
the wire is doubled. Now take the doubled wire and wind 52 turns
over the 180-turn secondary. Care should be taken to center
the primary winding over the secondary in order to maintain
balance. By connecting the wires of the primary as shown in
Fig. 2 we will have a bifilar primary which will have equal
capacities from its ends to ground. Some coil dope can be applied
to the windings of T1 keep the wires in place. When the line-matching
transformer has been completely fabricated it should be installed
in the metal box and wired into the loop circuit.
One end of the loop just prior to its being
tied into the box.
The loop antenna proper is now electrically complete and
should be mounted in a manner convenient to the builder. The
author used a combination of aluminum tubing, bamboo, and TV-type
antenna clamps to construct the supporting cross members. Be
careful not to use a solid metallic framework to support the
loop since the electrical operation of the loop may be seriously
affected by shorting across the electrostatic shield. A piece
of steel tubing was used for the lower part of the mast and
this piece of tubing was inserted into a slightly larger piece
of tubing which had been driven in the ground. As shown in the
photograph of the loop, the "Armstrong" method of rotating the
loop was used; however, the author plans to install a TV antenna-type
rotator in the near future.
Fig. 4 - An s.w.r. bridge modification for
improved v.l.f. use.
Tuning and Impedance Matching
Tuning the loop to resonate at a particular frequency in
the 14 kc. to 25 kc. range is not much of a problem. It is accomplished
in the same manner as tuning and impedance matching of higher
frequency antenna systems. A simple block diagram of the set-up
used by the author is shown in Fig. 3. The standing-wave-ratio
bridge used for adjustments is a home constructed bridge typical
of those described in any of the popular radio handbooks. The
audio signal generator used was a Heathkit AG-9 with a range
of up to approximately 100 kc.
Before starting the adjustment procedure it would be wise
to check the bridge to be used for satisfactory operation on
the very-low frequencies. If the bridge is to be used to check
a 52-ohm termination, connect a 52-ohm resistor to the output
or line side of the bridge and feed a signal of about 20 kc.
into the bridge. If, with the correct load, the bridge indicates
reflected signal, make the modification to your bridge as shown
in Fig. 4. The bridge should then operate satisfactorily on
the v.l.f. band.
Inside view of the aluminum box. A terminal
board or a piece of insulating material with holes drilled into
it can be used to support the connected ends of the No. 16 loop
With the bridge and signal generator connected to the antenna,
as shown in Fig. 3, set the generator to deliver output at the
frequency to which the loop antenna is to be resonated. Adjust
the loop tuning coil L2 until the reflected power reading on
the bridge reads a minimum. On the author's installation it
was possible to obtain a zero reading of reflected power on
the bridge at all frequencies between 14 and 25 kc.
Impedance matching to the converter or receiver with which
the loop antenna is to be used can be accomplished in a number
of ways. A satisfactory match to the author's converter was
made by winding 24 turns of #28 enameled wire on the converter
v.l.f. coil L1, as shown in Fig. 5. The coupling loop is wound
as close as possible to the original winding and on the slug
adjustment side of the coil. The converter v.l.f. coil was remounted
on the upper side of the chassis since it was found that a noticeable
amount of signal strength was lost by under-the-chassis mounting.
The same mounting hole was used and several smaller holes were
drilled in the chassis to accommodate the grid and coupling
coil connections which were run under the chassis. In addition,
a u.h.f.-type coaxial connector was installed on the chassis
to connect to the loop antenna transmission line.
Fig. 5 - Circuit modification to the author's
For the different input impedances which may be encountered
one may find it convenient to use one of the ultra-compact high-fidelity
audio transformers manufactured by UTC. The UTC type A-24 or
A-26 has a response of from 20 to 40,000 cycles with a primary
impedance of 15,000 ohms and 30,000 ohms respectively. The secondary
impedance for both of these transformers is 50, 125-150, 200-250,
333, and 500-600 ohms as required. By using either one of these
transformers in reverse it is possible to make a transformation
between the low impedance 52-ohm line and the high-input impedance
of a converter or receiver.
Another alternative in obtaining a higher feedpoint impedance
for the loop antenna is to replace capacitors C1 and C2 and
line-matching transformer T1 with a single .015-μf. capacitor
to close the loop coil. When the loop is resonated by the use
of tuning coil L2 one may tap at two points, one on either side
of the loop electrical center and each equidistant from the
center, and obtain a balanced feedpoint that is higher in impedance
than 52 ohms. On the author's loop, tapping of the first two
loop turns on either side of center gave a feed point impedance
of 150 ohms. This impedance could conveniently fed by the use
of two 75-ohm coaxial cables with the shields tied together
and the inner conductors connected -one each to each loop tap.
Since the loop turns are accessible in the miscellaneous components
box it is quite convenient to tap in this manner to find a satisfactory
Operation and Results
Operation of the v.l.f. loop antenna needs very little comment.
You will notice, after a trial "spin," that the loop exhibits
two very sharp nulls broadside to the plane of the loop and
that you have a much improved signal-to-noise ratio. Comparative
checks can be made which will vividly demonstrate the superiority
of the loop over that random long wire that you may now be using.
Stations now being heard by the author on the v.l.f. band
include the U. S. Navy's precise frequency-controlled transmitters
such as NAA at Cutler, Maine on 14.7 kc., NBA (Summit, Canal
Zone) 18 kc., NPG/NLK (Jim Creek, Washington) 18.6 kc. and others.
WWVL, the National Bureau of Standards' frequency-standard station,
which operates on 20 kc. from Sunset, Colorado can be heard
but not nearly as loud as the Navy "giants" which run in the
neighborhood of two million watts of power to their antennas.
Several foreign stations, such as GBR in Rugby, England on 16
kc. and FUB near Paris, France on approximately 17 kc., are
being heard consistently. Future plans for the very-low frequencies
include the construction of a new Navy transmitter to be located
in Australia which will undoubtedly be in the multi-megawatt
class and a proposed NATO station which will operate on 19 kc.
Recent tests using satellites have disproven the belief that
the ionosphere was a shield for the very-low frequencies with
the result that consideration is now being given to the use
of these frequencies for communicating between earth and outer
space. The ultra-low frequencies recently produced some astounding
results when a 400 cycle-per-second signal was transmitted a
distance of approximately 750 miles from Boron, California to
El Paso, Texas in tests conducted by the Air Force.
The very-low and low frequencies are, to be sure, more fascinating
than ever before. Constructing the novel v.l.f. antenna described
in this article will assure you of greater success and listening
pleasure while exploring the very-low frequencies.
Posted August 4, 2015