Coil Coupling Problems, November December 1941 Radio-Craft

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 their functions.

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 in space.

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 capacity.

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 dial setting.

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.

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.

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 condenser.

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 bands.

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 methods illustrated.

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 finished sets.

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 high gain.

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.

Coil Coupling ProblemsNovember / December 1941 Radio-Craft

Coil Coupling Problems
November / December 1941 Radio-Craft