The Field That Stays at Home
April 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
FCC 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
April 1942 QST
Wax nostalgic about and learn from the history of early electronics. See articles
QST, published December 1915 - present. All copyrights 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."
The Field That Stays
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
time 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!)
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.
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:
PROVISIONS GOVERNING THE OPERATION OF LOW POWER RADIO
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
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
(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.
2.103. 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.
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
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.
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
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.
However, it 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
T = radius of transmitting coil in centimeters;
= current through the transmitting loop in milliamperes; and
d = distance between centers of the transmitting and receiving loops
This can be stated to give the current required
in a given loop to produce the maximum permissible field at any
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.
- 850-1500 mica padding condenser with 0.005 μfd. fixed mica
C2 - 500 μμfd. midget mica.
C3 - 0.002
μfd. midget mica.
R1 - 50,000 ohms, 1 watt.
RFC - 25-mh.
L1 - 20 turns No. 12 antenna wire, 18-in. dia.,
spaced diameter of wire.
L2 - Coil to tune to frequency
in use. For 50-60 kc.: 150 turns No. 22 e., close-wound on 4-in.
4 - Circuit of the Philco control unit.
C1 - 200 μμfd.
C2 - Mica trimmer. C2 - 0.05 μfd.
R1 - 500 ohms.
L1 - 53 turns, 6¼·in. diameter.
S - Filament switch
(on pulsing device; used to key oscillator)
3 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).
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
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 - low.
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 known:
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
The large 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.
The inductance of large-diameter coils is difficult to calculate
exactly, but it can be worked out with fair accuracy by using Nagaoka's
Fig. 5 - Constant K used in calculating inductance of loops
using formula in text, based on the ratio of diameter to length
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 Fig. 5.
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
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.
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
With this type of circuit the gain per stage
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
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
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