Nov. / Dec. 1941 Radio-Craft
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
When you look at the circuit
board and/or chassis of a radio set - new or old - you see a lot of components
including resistors, semiconductors (and/or vacuum tubes), inductors, capacitors,
transformers, switches , potentiometers, shielded cables, shielded compartments,
displays, indicator lights, connectors, etc. With the possible exception of
some semiconductors (ICs and discretes), the function of just about every component
can be discerned by most people who are at all familiar with radio electronics
by its location in the circuit, with the exception being inductors and transformers
(other than those in the power supply). Inductors and transformers tend to be
the least understood and therefor the most mysterious. They are the least likely
to bear any identifying marking unless they happen to be encapsulated like a
resistor or capacitor. Articles like this one help remove some of unknowns about
Coil Coupling Problems
L. V. Sorensen
The circuit diagrams given to the right are explained in the text and the
important role played by the coupling coil in radio receiving circuits especially
are discussed at length by the author. The loop antennas used so widely on portable
sets as required some fine engineering, to properly solve the problem of the
best type of coil for the purpose.
The author explains in an easily understood manner, some of the problems
met with in radio design, so far as the coils and coupling methods are concerned.
Every student of radio technique will find many valuable pointers in this article.
Some of the topics are - Loop Antennas, Reducing Hum Modulation, Effect of High
Impedance Primaries, Short-Wave Coils and R.F. Amplifiers.
Fig. 1 - Antenna coupling.
Every radio set employs resonant circuits to select the desired signals and
to discriminate against the undesired signals, but the methods by which these
resonant circuits are coupled into the preceding and following tubes or circuits
vary widely as conditions dictate. It is the purpose of this article to discuss
the general methods used to couple tuned circuits into radio receivers, outline
the general characteristics of each method, and to point out the limitations
that influence the type and amount of coupling.
A loop antenna is the outstanding example of a simple method of coupling
a resonant circuit to a tube. The entire tuned circuit is directly connected
between the grid and the cathode return of the first tube. Signal voltage is
introduced into the loop by direct induction from the signal field existing
When the signal from the desired station is too weak for satisfactory reception
on a loop, it is necessary to couple an outside antenna to the receiver. The
most common method of coupling is to connect the antenna to a few turns of wire
wound around the loop. When the loop is of relatively large diameter the coupling
winding is frequently only a single turn, When the loop is small in diameter,
and consequently requires many turns of wire to make up the required inductance,
the coupling winding frequently has several turns in place of the single turn
used on the larger diameter loops. This type of coupling is known as "low-impedance"
coupling. Its characteristics are: low gain at the low-frequency end of the
tuning range, very much higher gain (often 5 to 8 times) at the high-frequency
end of the tuning range, poor image-ratio, and considerable sensitivity (at
the high-frequency end of the tuning range) to variations in antenna capacity.
The principal limitation in the amount of this type of coupling that can
be employed is the amount of capacity "reflected" from the antenna across the
tuned circuit. When this "reflected" capacity becomes large enough to throw
the circuit badly out of alignment with the other circuits in the receiver,
a net loss in sensitivity is caused by further increases in coupling. Under
such a condition of misalignment the image-ratio suffers very badly. In some
sets an isolating condenser of limited capacity, say 50- to 100-mmf., may be
connected in series with the coupling winding to limit the maximum capacity
that may be reflected across the tuned circuit by antennas of unusually high
A fairly common variation of the coupling winding just described is to make
the coupling conductive rather than inductive. In other words, instead of using
a separate winding for the antenna coupling, a part of the secondary is used
for the dual function of coupling winding and for part of the tuned circuit.
In construction, this is accomplished by putting a tap on the loop winding one
or more turns from the low-potential end of the coil, and connecting this tap
to the antenna, either directly or through an isolating condenser. The tapped
loop may present some manufacturing advantages over the separate coupling winding,
but it has the distinct performance disadvantage of sometimes introducing "hum-modulation"
into the received signal. Following the circuit in Figure 1, it can easily be
seen how this method of coupling the antenna to the loop introduces hum modulation
in AC-DC sets.
Reducing "Hum" Modulation: When the line plug is 50 connected that side "A"
is connected to the "ungrounded" side of the power line, the A.C. line voltage
is divided across C1, C2, and C3 in series. Since only C1 has both terminals
in the grid circuit this is the only condenser across which an A.C. voltage
of power-line frequency can introduce hum into the grid circuit. A moment's
consideration of these three condensers in series will show that increasing
the value of C1, decreasing the value of C2 or reducing the capacity of C3 (the
capacity of antenna to ground) will reduce hum-modulation. Of course, the quickest
and easiest method of reducing hum-modulation in the above case would be to
reverse the line plug. Reversing the line plug, however, will have little effect
on hum voltages that the antenna may pick up from high-voltage power lines that
may be in the neighborhood of the antenna. A.C. sets with the same type of coupling
from antenna to loop may experience similar trouble but probably to a lesser
degree, because the power line is not connected to the chassis, but is merely
bypassed to it with a relatively small condenser.
If hum-modulation only when the antenna is connected is a complaint on any
receiver using the type of antenna coupling shown in Figure 1, it is suggested
that the antenna connection be removed from the tap on the loop and that a completely
separate coupling winding be wound on the loop, right over the turns formerly
included in the antenna circuit.
This new winding should have the same number of turns as there were between
the tap and the AVC end of the loop. The AVC end of the new coupling winding
should be connected to chassis on an A.C. set, and to "B" minus on an A.C.-D.C.
set. In the latter case, an isolating condenser should be connected between
the antenna and the high-potential end of the coupling coil while in the former
case no isolating condenser is necessary. It is very important that the coupling
winding be located at the AVC end of the loop, where the capacity between the
coupling winding and the loop adds virtually nothing to the circuit capacity
on the loop. If the coupling winding is located at the grid end of the . loop,
the winding will add so much fixed capacity to the grid circuit, that it may
be impossible to "trim" the loop to resonance with the oscillator at the proper
Another scheme for coupling an antenna to a loop is to connect the antenna
directly to the grid through a very small capacity - some-thing around 2 mmf.
Such a scheme is extremely simple but results in a poor image ratio. To prevent
serious mistracking at the high-frequency end of the band, it is essential that
the wiring from the low-capacity coupling condenser to the antenna terminal
or lead of the set have a relatively high capacity to chassis, say 10 to 20
mmf. With such a capacity coupling from the low-capacity condenser to ground
or chassis, little change is made in the effective grid-circuit capacity when
the antenna is connected to the antenna post, but if the low-capacity coupling
condenser is so placed that its lead to the antenna terminal has negligible
capacity to ground, the maximum mistracking will result when an antenna is connected.
In broadcast radio sets employing conventional antenna coils, the low-impedance
primary was abandoned years ago in favor of high-impedance coupling to provide
better image ratio, flatter gain characteristics from one end of the band to
the other, and greater freedom from the detuning effects of antennas of different
capacities. The same idea holds true in the better grades of sets employing
loop antennas. These sets are employing high-impedance coupling in either of
two forms: (1) a high-impedance primary of large diameter (perhaps 4 to 6 inches)
coupled directly to the loop, or (2) a conventional high-impedance primary of
the usual diameter coupled rather tightly to a small coil that is connected
in series with the loop.
In either case the net result is the same as a conventional high-impedance
antenna coil having the same inductances, Q's and coupling capacities. There
is no question but what such a method of coupling an antenna to a loop gives
better performance than the low-impedance couplings previously described, but
many designers have used the poorer coupling circuit because of its economy,
and because of the fact that relatively few loop sets ever have an external
antenna connected to them.
Large-Loop Capacity Problem:
In some sets that cover a rather large tuning range on each band, the distributed
capacity of a large-diameter loop, comprising the entire inductance of the tuned
circuit, is so high that the tuning range is seriously restricted. In such cases,
an effective reduction in distributed capacity can be obtained by separating
the inductance of the resonant circuit into two parts, putting approximately
half of the required inductance in the loop and the remainder in a small coil
of low distributed capacity connected in series with the high side of the loop.
If this inductance is mistakenly connected in the low side of the loop, virtually
no advantage has been gained because the capacity of the loop to ground is still
connected to the grid. When this capacity is connected to ground, however, with
the small coil connected to the grid, the effective circuit capacity is much
lower and consequently the tuning ratio is greater.
Sets of the better grade employing the above method of stretching the tuning
ratio of the loop circuit, almost always have a high-impedance primary coupled
loosely to the above mentioned small coil. Thus, when an outside antenna is
connected to the receiver the loop circuit partakes of the desirable characteristics
of an antenna coil with high-impedance primary. rather than the less desirable
characteristics of the low-impedance coupling scheme that employs one or more
coupling turns wound around the loop.
In general, all broadcast-band antenna coils of modern receivers employ high-impedance
coupling for the same reasons as given in favor of such coupling in the better
grade loop sets. The limiting factor in the amount of coupling that can be used
is the amount of mistracking caused at the low-frequency end of the tuning range
when antennas of different capacities are used, and by the amount of change
in the apparent inductance of the tuned secondary as the low-frequency end of
the tuning range is approached. This directly affects the tracking of an antenna
coil with an R.F. or oscillator coil.
Effect of High Impedance Primary:
Mathematically, it can be shown that actually a high-impedance primary (with
antenna connected) reflects a large capacity in series with the tuned secondary,
giving much the same effect as a padding condenser in that circuit, and that
the value of that condenser becomes more disturbing to the normal tuning curve
of the circuit as the resonant frequency of the primary circuit (primary inductance
with antenna capacity) approaches nearer to the low-frequency end of the tuning
range, and as the degree of coupling between the circuits becomes greater. A
reasonably good general design specification for coupling in a high-impedance
circuit is 15% magnetic coupling and a primary resonant frequency of about two-thirds
of the lowest frequency to which the secondary will tune.
In some instances, a little capacity coupling is added to the high-impedance
magnetic coupling for the purpose of changing the gain characteristics of the
coil. Since the polarity of the magnetic coupling between two coils can be reversed
by reversing either winding, there are two possible polarities of connections.
With one of these polarities, the magnetic coupling reinforces or aids the
coupling provided by the coupling capacity while, with the other polarity, the
magnetic coupling counteracts or opposes the coupling produced by the coupling
When the magnetic coupling aids the capacity coupling, the general effect
is to raise the gain all over the band, but most effectively at the high-frequency
end. When the magnetic coupling opposes the capacity coupling there is a reduction
of gain over the entire band but most effectively at some one point, which may
accidentally be in the band in a poorly designed set (or one in which the coil
has been accidentally hooked up incorrectly).
This point of minimum response may be made to fall out ide of the band at
the "image" of some important frequency in the band. When the latter is done
the image ratio is improved most at the one point where cancellation of coupling
is greatest but the beneficial effects of this opposition of couplings extend
for a considerable range on either side of the cancellation frequency. This
opposed coupling is most likely to be found in two-gang super-heterodynes where
image ratio is seldom as good as may be desired.
Short wave coils almost universally employ low-impedance coupling for several
good reasons: (1) A high-impedance winding for the range just higher than the
broadcast band resonates in the broadcast band. If this resonance should accidentally
fall at the frequency of a local station, the station, in all probability, would
cause "cross-talk" on any station tuned in on the short wave band in question.
(2) Low-impedance coupling makes a much better impedance match between a doublet
antenna and the first tuned circuit.
The limiting factor in the amount of low-impedance coupling that can be used
on an antenna coil is the amount of mistracking caused by different antennas
at the high-frequency end of the tuning range, and by the broadness of resonance
of the antenna circuit when aligning with the standard 400-ohm dummy antenna.
This is especially important in sets having only a two-gang condenser. If too
many primary turns are used, the resistance of the dummy antenna is reflected
into the tuned circuit in such an amount that the circuit becomes so broad that
it is difficult to distinguish between the desired signal and the image, when
the set is working on frequencies such as 16 to 18 mc. with a 456-kc. intermediate
frequency and virtually impossible to pad at the low-frequency end.
Fig. 2 - Input filter, or "wave trap."
The types of coupling employed in R.F. amplifier stages probably vary more
than the coupling in any other kind of a radio-frequency circuit. First, the
two general classes of R.F. amplifiers, tuned and untuned, determine whether
coils and coil couplings will be employed or not.
The simplest coupling is resistance coupling, in much the same manner as
a resistance-coupled audio amplifier with the exception that the values of the
coupling resistances and condenser are usually considerably smaller than in
audio circuits, and that a trap circuit (tuned to the intermediate frequency)
is usually employed to keep out of the converter tube the noises arising in
the R.F. tube at the intermediate frequency. If these noises are not suppressed
by means of a wave-trap, the "signal-to-noise" ratio suffers. Such a circuit
is shown in Figure 2.
Untuned R.F. amplifiers of more complicated design may be used to cover a
greater band of frequencies, as in some All-Wave receivers.
Fig. 3 - Straight magnetic coupling, magnetic plus capacity
coupling, and capacity coupling (more familiarly known as choke coupling).
The most commonly used tuned R.F. coupling circuits are high-impedance coupling
on the broadcast or long-wave bands and low-impedance coupling on the short-wave
The high-impedance circuit for R.F. coupling has several versions: straight
magnetic coupling, magnetic plus capacity coupling, and capacity coupling (more
familiarly known as choke coupling). These coupling circuits are shown respectively
in Figure 3, together with representative examples of coils employing the coupling
The characteristic of the above type of coupling is a fairly flat curve of
gain vs. frequency, especially in the circuit employing combined magnetic and
capacity coupling. All of these circuits yield better image ratios than low-impedance
coupling, but to of these three the circuit employing only magnetic coupling
yields better image ratios than either the combined magnetic and capacitive
coupling or the straight capacitive coupling (except in the case where the magnetic
coupling opposes the capacitive coupling for the specific purpose of improving
the image ratio).
In the design of such circuits, particular care should be exercised to see
that the primary resonant frequency does not coincide with the intermediate
frequency of the receiver because poor I.F. rejection results when this occurs.
If the intermediate frequency is 456 kc. the primary resonance may be between
the intermediate frequency and the low-frequency end of the broadcast band,
but such a primary resonant frequency requires very close control of coil constants
and wiring capacities to keep the resonance at the desired frequency. More uniform
and more comfortable production will be experienced if the primary resonance
is placed well below the intermediate frequency, where a reasonable variation
of resonant frequency will have virtually no effect on the uniformity of the
The coupling employed in a few broadcast R.F. circuits of low gain is of
the low-impedance type, for reasons of economy and convenience. It is true that
the Image ratio obtained is not as good as that obtained from a high-impedance
circuit, but the image ratio of a complete set employing a stage of such low-impedance
amplification is so much better than that of a set without a tuned R.F. stage
that some designers use such low-impedance coupling for the economy that results.
If they use high-impedance coupling they would unquestionably obtain a still
better image ratio, but at the price of a primary of many turns of fine wire,
and perhaps a mica bypass condenser from the plate of the R.F. tube to plus
"B" to cut the gain of the stage instead of a low-impedance primary of a relatively
few turns (10 to 20) of fairly heavy wire.
The coupling employed on short-wave R.F. coils is almost universally of the
low-impedance type, in many cases with the primary turns wound between the secondary
turns, in an effort to obtain maximum coupling and maximum gain.
The characteristics of this type of coupling on short-wave R.F. coils are:
maximum gain at all frequencies, greatest gain at the high-frequency en d of
the tuning range and image ratio inferior to high-impedance coupling. This type
of coupling is used however, in spite of this limitation, in order to achieve
Fig. 4 - Single-stage oscillator.
Fig. 5 - Plate coupling tuned circuit by a separate winding.
The limiting factor in the amount of coupling so employed is single-stage
oscillation in the R.F. tube and/or the amount of capacity reflected from the
R.F. plate circuit across the following grid circuit. In order to see how single-stage
oscillation is the limiting factor and how to adjust coupling to avoid this
difficulty, first consider the circuit in Figure 4.
Inspection of this circuit shows it to have a tuned circuit directly in its
grid and plate circuits. If the frequency is anywhere in the range from 2 to
18 mc. the gang condenser is of normal size (365 to 410 mmf.), the coils are
of normal "Q" design, and an A.C. tube operating at normal voltages is employed,
this stage will oscillate as a tuned-grid-tuned-plate oscillator in exactly
the same manner as the old tuned-grid-tuned-plate transmitters operated.
Even with coupling between grid and plate circuit wiring reduced to zero
by careful placement of parts, the capacity inside of the tube is enough to
cause oscillation when the circuits are properly tracked. It is necessary to
reduce the effective impedance in either the grid or the plate circuit of the
R.F. tube to stop oscillation. This could be readily done by connecting the
plate to some point part way down on the tuned circuit and, as a matter of fact,
such an arrangement is sometimes used. The far more common method, however,
is to keep the tuned circuit out of the high-voltage D.C. circuit, and merely
to couple the plate to this tuned circuit by a separate winding, such as shown
in Figure 5.
Since the impedance of any tuned circuit is maximum for the highest inductance,
it is obvious that the highest stage gain results on the lowest-frequency band
of a multiband short-wave receiver, unless steps are taken to prevent this inequality.
The steps usually taken are to use a lower percentage of coupling on the lower-frequency
coil. For example, in a certain set the 5.5- to 18-mc. R.F. coil had three-fourths
as many turns on the primary as on the secondary, while on the 1.7- to 5.6-mc:
band the R.F. primary had only one-third as many turns as the secondary.
This article prepared from data supplied by Meissner Mfg. Co.
(To be concluded)
Posted January 28, 2021(original 10/22/2014)