The Field That Stays at Home
1942 QST Article
is a really good introductory article on electromagnetic (EM) fields
as they pertain to inductors, transformers, and antennas. It appeared
in the April 1942 edition of QST. The
had only been in existence for eight years at the time and was pretty
much just figuring out how to regulate the heck out of everything. The
author discusses compliance issues for these newfangled RF devices that
were becoming more and more numerous. Interestingly, the first sentence
says, "Every time you threw the transmitting switch in pre-war days...,"
bringing to mind how the Feds banned Amateur Radio during most of World
War II for security reasons as well as to assure that scarce resources
went toward building and servicing military gear as needed. Many Hams
offered their gear either as a donation or for the pittance that was
offered by the government.
April 1942 QST
of Contents]These articles are scanned and OCRed from old editions of the
ARRL's QST magazine. Here is a list of the
QST articles I have already posted. All copyrights (if any) are hereby acknowledged.
On December 8, 1941, the day after
the Japanese attack on Pearl Harbor, the FCC issued a "Notice
to All Amateur Licensees" that began thusly: "All amateur licensees
are hereby notified that the Commission has ordered the immediate suspension
of all amateur radio operation in the continental United States, its
territories, and possessions."
See all available
vintage QST articles.
The Field That Stays at Home
Fundamental Principles and Practice of
By Clinton B. De Soto, W1CBD
"The induction field is the
field that stays at home." - Prof. R. R. Ramsey
you threw the transmitting switch in pre-war days and sent power surging
up into your antenna, two quite distinct electromagnetic fields were
set up around it. One of these was the radiation field, the energy in
which left the conductor and travelled off into space. The other was
the induction field; its energy did not go off into space, but instead
returned each time to the parent conductor.
Simply because its
energy does travel off into space (and perchance into the intent eavesdropping
ears of erstwhile J's, D's, etc.), the radiation field is no longer
available for your use. But the once-forgotten induction field, which
sends its energy only a little way and then yanks it back again like
a yo-yo ball, now offers an interesting field (resemblance to pun purely
coincidental!) for experimentation.
Older Than Radio
Even before Maxwell's theory of radiation had been
reduced to practice, experimental communication was carried on by means
of electrostatic and electromagnetic induction. As early as 1865, Dr.
Mahlon Loomis signaled between two mountain tops eighteen miles apart
by its use. In the latter 19th century many of the world's greatest
scientific minds experimented with inductive effects over distances
of a mile or more - Thomas A. Edison, Alexander Graham Bell, Oliver
Lodge, A. W. Heaviside and many others.
But with Hertz' demonstration
of radiation as a practical phenomenon, the remote induction field came
to have little more practical importance in communications than as a
source of annoyance to telephone and telegraph men running into cross-talk
from parallel wire lines.
Until 1938, that is. In that year
Philco engineers, seeking a remote-control tuning system without the
inconvenient multi-wire cables and accompanying complexities, evolved
the idea of using an inductively coupled r.f. transformer with its primary
and secondary spaced as much as 75 feet. The primary of this transformer
was supplied from a small battery-powered oscillator, and the voltage
induced in the secondary fed a supplementary amplifier in the b.c. set.
Naturally, because of its resemblance to a conventional transmitter
and receiver, questions arose concerning the legality of the device.
After considerable debate pro and con and a hearing or two, the FCC
finally handed down rules covering the subject. For the record, those
rules are reproduced here in their entirety:
GOVERNING THE OPERATION OF LOW POWER RADIO FREQUENCY DEVICES
2.101. General. Pending the acquiring of more complete information regarding
the character and effects of the radiation involved, the following provisions
shall govern the operation of the low power radio frequency electrical
devices hereinafter described.
2.102. Apparatus excepted from
requirements of other rules. With respect to any apparatus which generates
a radio frequency electromagnetic field functionally utilizing a small
part of such field in the operation of associated apparatus not physically
connected thereto and a distance not greater than
the existing rules and regulations of the Commission shall not be applicable
(a) That such apparatus shall be operated with the
minimum power possible to accomplish the desired purpose.
(b) That the best engineering principles shall be utilized in the generation
of radio frequency currents so as to guard against interference to established
radio services, particularly on the fundamental and harmonic frequencies.
(c) That in any event the total electromagnetic field produced
at any point at a distance of
from the apparatus shall not exceed 15 microvolts per meter.
(d) That the apparatus shall conform to such engineering standards
as may from time to time be promulgated by the Commission.
Exceptions; interference to radio reception. The provisions of sections
2.101 and 2.102 shall not be construed to apply to any apparatus which
causes interference to radio reception.
2.104. Inspection and
test; certificates. Upon: request, the Commission will inspect and test
any apparatus described in sections 2.101 and 2.102, and on the basis
of such inspection and test, formulate and publish findings as to whether
such apparatus does or does not comply with the above conditions, and
issue a certificate specifying conditions of operation to the party
making such request.
And so we have the regulatory justification
for all such miscellaneous gadgets as the "Mystery Control," wireless
phonograph record players, phantom volume controls in p.a, systems and
the like. We have also an interesting field for interim experimentation
in connection with limited-range communication and remote control devices.
The purpose of this article is not so much to suggest the possibilities,
however, as to underline the fundamental principles and to provide a
compilation of circuit and design data.
and Induction Fields Are Created
It may help in understanding the distinction between the two kinds of
fields to review the phenomenon of radiation, as far as that can be
reduced to simple, non-mathematical concepts. First of all, consider
a simple coil with d.c. flowing through it. A magnetic field is set
up around the coil, extending into space for a certain distance with
a strength depending on the magnitude of the current. This field has
a certain polarity, depending on the direction of current flow.
Fig. 1 - The relationship between induction and radiation fields
about a coil or loop antenna at various distances. Below each diagram
the distance in terms of wavelength is indicated. At a point very
near the coil (λ/20) the induction field strongly predominates.
At the distance ;r (center) the two are equal in the plane of the
loop. When the distance is a full half-wave (bottom) the radiation
field is the stronger.
Now suppose the current is instantaneously cut off. The field collapses
and the energy in it returns to the coil. But if, the very instant the
current stops flowing in one direction, it starts flowing again in the
opposite direction with equal magnitude, an equal and opposite electric
field will be set up before the original field can return to the coil.
Unable to return home because the new field has forced it out, the original
field sets forth on a journey through space.
radiation - the successive detachment of one electrical field after
another in a series of waves as each is succeeded by another with each
reversal of current. In an ideal system with instantaneous reversals,
all of the energy in each field would be radiated, and none would return
to the coil. In actuality, each succeeding cycle from the a.c. generator
feeding the coil represents a slow rise and fall of potential. This
gradual building up to peak amplitude and corresponding gradual decay
allows time for some of the energy to return to the coil before the
new field becomes strong enough to send it away. The slower the rise
and fall - i.e., the lower the frequency - the more of the initial energy
returns to the coil. Thus at audio or very low r.f, most of the energy
succeeds in returning and very little is radiated. At the higher radio
frequencies, on the other hand, the cycles come along so fast that the
electric field - even though it travels 186,000 miles per second - has
little time in which to go out and return, and as a result most of it
gets detached and is radiated into space.
The part of the field
that returns to the coil is the induction field, while the part that
is detached is called the radiation field.
The most obvious
difference between the radiation and the induction fields is that the
radiation field is the weaker near the antenna and the stronger at a
distance. This is illustrated in Fig. 1, based on studies made by Professor
Ramsey, which shows the relative strength of the two fields at various
relative distances identified in terms of the operating wavelength.
Specifically, the radiation field varies inversely as the distance,
while the induction field from a coil varies inversely as the cube of
the distance. The two fields are always equal at a distance equal to
the velocity of light divided by the angular velocity.
This expression will be recognized as that used in the FCC rule to state
the maximum distance at which the measured field may not exceed 15 p.v.
per meter, to ensure that no interference is caused radio services.
Actually, since the two fields are in time quadrature this measured
value represents 1.414 times the true value of either field alone, but
as far as the receiving antenna or coil is concerned it has no social
prejudices and responds to both fields as one.
Thus we have
our first design rule - the measured field strength at the distance
must not exceed 15 μV per meter. This distance, expressed
in feet, is shown in Fig. 2 for frequencies throughout the region normally
used for the purpose.
Calculating Field Strength
Fig. 2 - Maximum distances in feet at which 15 volts per meter field
strength is permitted under FCC Rule 2.102 for various frequencies.
The question then
arises - how to determine that the field strength at the maximum distance
does not exceed the legal limit of 15 μ V per meter? Few amateurs
have access to standard signal generators and the rest of the equipment
that would be necessary to do the job properly.
is possible to compute the induction field using nothing more than an
antenna ammeter of the thermocouple or hot-wire variety and a little
care. Knowing the current in and the physical characteristics of the
transmitting coil, the field strength at any distance can be worked
out by the following formula:
E = field strength at the receiving coil in
microvolts per meter;
N = number of turns in transmitting coil;
T = radius of transmitting coil in centimeters;
I = current
through the transmitting loop in milliamperes; and
d = distance between
centers of the transmitting and receiving loops in meters.
can be stated to give the current required in a given loop to produce
the maximum permissible field at any wavelength:
where λ equals the operating wavelength and the other
values are the same as in (2).
The physical dimensions of the
transmitting coil or primary are not of particular importance if rated
current is obtained. The important thing is that both dimensional and
electrical measurements be accurately made, and that enough power be
available from the oscillator to cause rated current (or slightly less,
for a margin of safety) to flow in the coil.
If the current
is held at the calculated value, the induction field remains constant
regardless of frequency. Unlike the radiation from a coil or antenna,
which varies with frequency, the induction field is independent of frequency.
In fact, the strength of the induction field is a function solely of
the current, the area of the coil, the number of turns and the distance.
Almost any simple oscillator circuit can be used in the transmitter.
Ordinarily not more than 2 or 3 watts input will be required, unless
a loop of very few turns and small area is used. One suitable transmitter
design was described on page 42 of March, 1942, QST.
Fig. 3 - Simple induction-field transmitter circuit.
C1 - 850-1500
mica padding condenser with 0.005 μfd. fixed mica in parallel.
C2 - 500 μμfd. midget mica.
C3 - 0.002 μfd. midget mica.
R1 - 50,000 ohms, 1 watt.
RFC - 25-mh. r.f. choke.
- 20 turns No. 12 antenna wire, 18-in. dia., spaced diameter of
L2 - Coil to tune to frequency in use. For 50-60 kc.:
150 turns No. 22 e., close-wound on 4-in. dia.
4 - Circuit of the Philco control unit.
C1 - 200 μμfd.
- Mica trimmer. C2 - 0.05 μfd.
R1 - 500 ohms.
L1 - 53 turns,
S - Filament switch (on pulsing device;
used to key oscillator)
shows an even simpler circuit, suitable for use on c.w. or for sending
remote-control pulses. It can also be modulated to perhaps 50% (another
6J5 as modulator would do the job) for voice work. The coupling tap
is adjusted to give the calculated current as determined from (2). If
more power is required, a 6V6 with No.2 grid tied to plate may be substituted
for the 6J5.
The circuit of the Philco "Mystery Control" battery-operated
portable control unit is shown in Fig. 4. Used between 350-400 kc.,
the type 30 tube with 45-volts on the plate gives 130 milliamperes
of r.f. current in the 675-mh. coil (122 mw. power, Q of 216).
The oscillator power transformer should preferably have an electrostatic
shield between primary and secondary. Transformerless supplies are
not advisable for this application because of the excellent possibility
of increased radiation. R.f. line filters in the 115-volt leads will
help to limit radiation.
In this connection, it is interesting
to note that the power wiring, etc., is much more likely to create interference
to other radio services by radiation than the emanations from the transmitting
loop alone. The efficiency of the coil as a radiator is very poor compared
with almost any straight-wire antenna, such as a power line. This is
particularly true when the receiver being interfered with is equipped
with an antenna rather than a loop.
The Receiving Coil
So much for the transmitter. Let's talk about the receiving
coil or the secondary of the inductive transformer now. Assuming the
loop is located in a known field, the following rules apply:
The voltage induced in the coil is directly proportional to the
area of the loop (frequency and number of turns remaining constant).
It is directly proportional to frequency (turns and area constant).
It is directly proportional to the number of turns (frequency
and area constant).
From the above, it can readily be shown
that the induced voltage is proportional to the inductance of the coil.
Disregarding strays, the inductance increases as the square
of the turns ratio, the area remaining constant. Conversely, the inductance
increases as the square of the radius, the number of turns being constant.
All this leads to the conclusion that with a given inductance
the induced voltage will be greater the larger the area of the coil.
In practice other considerations enter, but the rule holds in general.
For maximum sensitivity, therefore, every effort should be made to make
the area of the coil as great as possible, and to keep the inductance
as high as possible and the capacity - both tuning and distributed -
Since the induction-field system is, after all, nothing
more than an air-core r.f. transformer - even though the primary and
secondary may be spaced thousands of feet - it is quite possible to
compute the field induced in the secondary coil at any distance provided
only the dimensions of the coils and the current in the primary are
is the voltage induced in the receiving loop, I the current in the transmitting
coil in milliamperes, f the operating frequency in kc., r1 and r2 the
radii in centimeters of the transmitting and receiving loops respectively,
N1 and N2 the number of turns in the respective loops, and d the distance
in meters between them.
The effective signal at the grid of
the first amplifier tube when the receiving loop is at the distance
(coils co-planar) in a 15 μV field can be computed roughly by
the following simple relationship:
is the grid input voltage, r the radius of the receiving loop and N
the number of turns. Q refers to the tuned circuit, and can be considered
as that of the receiving loop.
The Q of the coil must be made
as high as possible, since the voltage applied to the grid of the first
amplifier tube in the receiver will be equal to the induced voltage
times the Q of the tuned circuit. This is determined by a number of
factors, including wire size, shape and insulation. At the frequencies
normally used Litz wire would be desirable, but unfortunately it is
not now readily available. Any available solid wire between No. 24 and
30 will result in a satisfactory coil. Smaller wire than No. 32 or larger
than No. 20 is not recommended for these frequencies.
diameter required in coils of this type makes it impossible to use the
form factor normally considered best at the frequencies in use. However,
by using single-layer windings, spacing the wire slightly or using double-cotton-covered
wire, and keeping the insulating or supporting material in the coil
to a minimum, a coil of good Q can be made.
Inductance of Loops
The inductance of large-diameter coils is difficult to calculate exactly,
but it can be worked out with fair accuracy by using Nagaoka's formula:
Fig. 5 - Constant K used in calculating inductance of loops using
formula in text, based on the ratio of diameter to length of winding.
Fig. 6 - Typical methods of constructing small loops. The construction
at (A) results in a particularly strong and efficient coil if rigid,
non-warping material is used as a base. Heavy electrical fibre board
or light Masonite are satisfactory. A form using crossed dowels,
as at (B), is easier to make but somewhat more difficult to wind
smoothly. Equal tension must be maintained on all sides. The dowels
should be slotted just the width of the wire, to cause the turns
to pile up properly. In either type, d.c.c. wire is advised in winding.
The form and winding should be thoroughly dried and impregnated
with coil dope (shellac may be used, if nothing better is available).
where r is the radius in cm., N the number of turns,
b the winding length of the coil in cm. and K a constant obtained from
This formula is accurate only for single-layer solenoid
coils, but approximations sufficiently close for ordinary work (there'll
be some trimming and paring anyway) can be made by taking the periphery
of coils of other shapes and dividing by
to obtain T. Thus a square loop 30 inches on a side, an equilateral
triangle with 40-in. legs or a circle with a circumference of 120 inches
all have the same value of r = 19.1 inches.
In the case
of spiral coils (as in Fig. 6), for r use the mean radius - i.e., one-half
of the inside diameter plus b.
Fig. 6 shows two satisfactory
methods of loop construction. For the higher frequencies large cardboard
containers, such as oatmeal boxes, make suitable forms. Self-supporting
basket-weave coils can be made with heavy wire by arranging wooden pegs
in a circle and interwinding a solenoid coil around them in similar
fashion to the "spider-web" spiral winding of Fig. 6 - (A).
Undoubtedly the toughest
problem in setting up an induction-field system is the receiver, at
least if you aren't going to be satisfied with anything less than a
completely independent unit. It takes a good bit of gain to build 15
microvolts (more or less) into a usable signal, and gain means stages
and tubes - and sometimes trouble. It's a good idea, too, to keep the
selectivity of the amplifier high, because reduced bandwidth means reduced
noise, and the ultimate limit in the useful range is determined by the
equivalent sideband noise input ratio.
The best system
without doubt is the use of a low-frequency converter and your present
communications receiver. The unit described by Goodman in the March
issue, beginning on page 15, can be applied to the present problem.
The tuned circuits must, of course, be modified to fit the chosen operating
frequency, but that isn't much of a job, particularly since most induction-field
work will be more or less fixed-tune, anyway, and there won't be any
tracking problem. And stage might be desirable to improve the signal-to-noise
ratio if work is to be done at the extreme limit of the useful range,
but all in all the complication hardly seems worthwhile.
If a receiver is built especially for the job it logically becomes a
straightforward r.f. amplifier, with two or three stages depending on
the frequency and required sensitivity, each stage individually tuned.
In designing such a receiver three quantities should be known - input
signal voltage, E1, gain per stage, and required voltage out of the
detector, Eo. Divide Eo by E1 to get the overall gain required, and
then in turn divide that figure by the stage gain to arrive at the number
of stages needed.
I.f. transformers constitute logical interstage
coupling devices. Standard replacement transformers are available for
frequencies such as 125, 175, 262, 375, 455 kc., etc.: consult a serviceman's
manual for types and sources.
Those who have ancient b.c.l.
superheterodynes or parts therefrom kicking around, or who have access
to such sets in local radio graveyards, may find themselves all set
up with a gold mine.
The old low-frequency i.f. transformers
- standard i.f.'s in the '20's ranged from 30 to 115 kc. should work
well in this job.
Fig. 7 shows the circuit of a receiver built around a set of small
175-kc. u.f. transformers and used in the 150-200-kc. band. It was made
originally for remote-control purposes, and therefore ends in a carrier-operated
relay tube. When a signal is received the diode biases the grid of the
6R7 triode, reducing the plate current and releasing the relay contacts.
For communication work the triode could be connected as an ordinary
audio amplifier, by the addition of a 0.01-μfd. coupling condenser
and 1-megohm grid resistor. A transformerless power supply is used.
Even at 150 kc, the useful range is only a fraction of a mile, and
therefore it is the lower frequencies that look most attractive for
induction-field work. If no transformers suitable for those frequencies
are available, tuned impedance coupling can be used as shown in Fig.
8. Ordinary lattice-wound r.f. chokes may be used as coils.
With this type of circuit the gain per stage is approximately:
where Gm is the amplifier tube transconductance in mhos,
I is the operating frequency in kc., L the inductance in millihenries
and Q that of the coil (50-100 for ordinary r.f. chokes, 100-200 for
honeycomb coils and solenoids, 150-300 for good powdered-iron core coils).
For maximum gain the operating frequency should be chosen so that the
coil tunes to resonance with minimum tuning capacity. Thorough shielding
and bypassing of each stage is recommended as high-gain multi-stage
amplifiers of this type are more than a little inclined to instability
unless carefully handled