Receiver With Input Phase Control Between Antenna And Chassis

Abstract

A receiver system including a differential input of which one end is connected to the antenna and the other end is grounded to the receiver chassis and a phase control device for adjusting the phase relationship between the antenna signal and the parasitic signal extracted from space by the receiver chassis.

Parent Case Text

This is a continuation of application Ser. No. 648,298 filed June 23, 1967.

Description

This invention concerns the elimination of signal cancellation problems due to randomly phased multiple signal entries into a receiver. Such multiple entries are not limited to the case of a dipole antenna feeding differential signals into the input terminals of a receiver, but also occur in a seemingly single-ended input which, in reality, is differential in nature due to the incidental or parasitic signal extraction from space by the receiver chassis. Thus, the performance of a simple monopole antenna, connected in single-ended fashion to a receiver's input, can be substantially diminished, if the signal which it delivers to the ungrounded terminal is nearly in phase and nearly equal in amplitude with the signal appearing at the "grounded" terminal, resulting from the capacitive and inductive linkage of the chassis with space. This undesirable phenomenon is more frequently observed with small, portable, battery-operated receivers and is usually explained as the result of an "insufficient counterpoise." This insufficiency may be interpreted not so much as the result of a lack of "mass" but as the result of an ability, possessed by the counterpoise, to act as an antenna itself, and to phase itself unfavorably.

Another frequently observed example of signal cancellations due to antenna action of a "grounded" chassis occurs in small, line-operated FM and television receivers. Intentional or incidental, capacitive grounding of the power cords to the chassis can, at certain frequencies, greatly increase the ability of the chassis to develop large signal amplitudes. These signals can have any random phase relationship with reference to the antenna signals, thereby either effectively enhancing them or cancelling them.

In accordance with the present invention, such accidental signal cancellation, can be avoided if one of the two signal sources, preferably the antenna is equipped with a phase control, which may be adjusted for optimum or acceptable performance, whenever spurious cancellation of signals degrades the performance of the receiver.

The phase control may assume various forms. In some forms it may resemble, and even be physically identical with conventionally frequency-tuning or trimming controls. But the present phase control differs functionally from conventional controls. The conventional frequency-tuning control may be distinguished from the present phase control by means of a simple test. According to this test, the phase control is kept in a fixed position, and either the amplitude or phase, or both, of the unwanted signal reference, i.e., the false ground, or "counterpose," is changed by suitable means which will be described later. If a change is observed in the signal amplitude anywhere within the receiver, the ground or counterpoise is sufficiently unstable to make itself an inadvertent participant in signal interception (parasitic dipole effect). Thus it is seen that the dominant function of the present tuning control is phase shifting, while frequency selection, for reasons explained in the introduction, is of secondary importance.

Another common cause of parasitic phase shifts in seemingly single-ended (monopole) antenna systems is found in the various reactive properties of differential input circuits and coupling elements, and, more particularly, in their non-symmetries. Single-ended or monopole reception with differential input terminals, one of which is "grounded" to the chassis, is commonly used in receivers that are equipped for alternate reception by dipole roof antenna and by indoor monopole antenna. Portable FM and television receivers are typical examples of this dual mode of antenna operation. Severe phase-errors are commonly encountered in such receivers when changing over from dipole to monopole operation. A phase control, in accordance with our invention, can be employed to re-establish proper phase relationships, and thus avoid spurious signal cancellations.

The principles of the present invention will be more fully understood by reference to the preferred embodiments set forth in the following detailed description and illustrated in the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a receiver system illustrating the parasitic antenna properties of an imperfect counterpoise.

FIG. 2 shows the vectorial subtraction of the antenna signal with a parasitic counterpoise signal having a random phase relation to the antenna signal.

FIG. 3 shows the vectorial subtraction of the antenna signal with a parasitic counterpoise signal in phase opposition to the antenna signal.

FIG. 4 shows the vectorial subtraction of the antenna signal with a parasitic counterpoise signal in phase identity with the antenna signal.

FIG. 5 shows a receiver system with a power cord which tends to increase the parasitic antenna-action of the chassis.

FIG. 6 shows an electronic differential input for a receiver and a phase control according to the present invention.

FIG. 7 shows a receiver with differential input transformer and a phase control.

FIGS. 8-11 show receiver systems in which adjustable single-step L/C circuits are used to control the phase relationship between the principal and parasitic antenna signals.

FIG. 12 shows a receiver including a dual purpose phase control circuit which is simultaneously effective in two widely separated frequency bands.

FIGS. 13-15 show receiver systems in which adjustable positive delay lines are employed for the purpose of phase control.

FIGS. 16-18 show receiver systems in which adjustable negative delay lines are employed for the purpose of phase control.

FIG. 1 shows, on its left side, the principal components of a receiver, as they are related to our invention, and on the right side a vertical E field potential distribution curve and its linkage with the receiver. The abscissa of this curve, though horizontal, represents the vertically polarized E-field intensity, referred to ground, in a horizontal, equipotential plane which is parallel to the earth or ground contour 1. The lumped mass of the receiver's chassis 2 is shown as having an equivalent antenna height 3, above earth, causing it to assume, at a given moment, a signal potential e.sub.1, with reference to ground. A second, in this instance larger, potential e.sub.2 is developed by the antenna 5 at its equivalent height 4. In order to simplify the analysis of potential distribution through the remaining shown components of the receiver, it shall be assumed that they are so close to each other potential differences between them can be neglected, or that their linkage with the vertical E-field has already been taken into consideration by selection of the chassis' effective antenna height 3.

Power source 6 which may be, for example, a battery, energizes the receiver 7. Input coupler 8 may be, for instance, an antenna transformer. Connected between the ungrounded input terminal 9 of input coupler 8 and the output terminal 10 of antenna 5 is the adjustable phase control 11, which may be shorted out by switch 12, to demonstrate its effectiveness as explained above.

Various types of phase control devices may be employed within the scope of the invention, such as, for example, adjustable positive or negative delay lines.

The function of phase control 11 can best be understood by assuming that shorting switch 12 is first in its closed position, as shown, and then is opened. When phase control 11 is shorted, antenna 5 is connected directly to the ungrounded terminal 9 of input coupler 8. The effective input voltage appearing between terminal 9 of input coupler 8 and chassis 2 would normally be assumed to be e.sub.2 -e.sub. 1, as indicated by the bracket on the right side of FIG. 1. In reality this voltage is subject to both the difference between the magnitudes of e.sub.2 and e.sub.1 and their relative phase displacement.

If the two vectors e.sub.1 and e.sub. 2 are displaced in phase, as shown in FIG. 2, the resulting differential signal e.sub.2 -e.sub.1 increases with increasing phase displacement. The differential signal e.sub. 2 -e.sub. 1 reaches its maximum when the vectors e.sub.1 and e.sub.2 are in phase opposition as shown in FIG. 3.

At the other extreme, a signal minimum, or total signal cancellation, can occur when the two vectors are equal in length and in phase, as illustrated in FIG. 4. This extreme does not occur as rarely as may first be assumed, although its occurrence presumes equal signal extraction from space by both the antenna and the chassis complex. The following mechanisms, jointly or individually, can bring this about:

a. Wave-tilting, caused by re-radiators or reflectors in the receiver's vicinity, can put the chassis-complex on an "equal footing" with the antenna, causing both to extract identical and in-phase signals from space.

b. Since the antenna will only rarely be located directly in line with the chassis' electrical center line and the direction of polarization of the incoming signal wave, many odd or random relationships between these three geometrical parameters can, accidentally, bring about the signal cancellations which are so often observed.

c. Time delays between the signals delivered by the antenna and the chassis in a non-tilted field can, accidentally, be such that, in a tilted field, they cancel themselves against extra time delays between antenna and chassis, created by wave tilting.

d. An originally horizontally polarized wave has a tendency to tilt itself vertically as it covers large distances near earth. It can therefore, in the course of its travel, assume almost any polarization angle which may then cause the observed cancellations.

Although there are many other causes for signal cancellation, the foregoing examples should suffice to illustrate how the signal extracting capabilities of the receiver chassis can cause serious cancellations of the useful antenna signal.

It should be noted however that the signal extracting capabilities of the chassis may be greatly enhanced by combination with a power cord as shown in FIG. 5. Since the power cord 22 effectively extends the maximum effective height or length of the chassis 21, the overall dimensions of the combination antenna A.sub.2, can, at certain frequencies, be half a wave length longer than monopole antenna A.sub.1, thus creating a phase shift of 180.degree. with reference to antenna A.sub.1. Accidental vertical and horizontal placement of the power cord 22 provides a larger number of random phase and amplitude variations of the parasitic signal. According to the present invention phase control 23, in series with monopole antenna A.sub.1, may be used to avoid accidental signal cancellation between the signals from antenna A.sub.1 and chassis-power cord combination A.sub.2 or, better, to bring about signal addition.

FIG. 6 shows a phase control in accordance with the present invention which is used to avoid accidental signal cancellation in a receiver having an electronic differential input for alternate monopole or dipole antenna operation. A dual triode 31 provides the differential input, its own grids 32 and 33 being connected to input terminals 34 and 35 via blocking or coupling condensers 36 and 37. The input circuit is shown with change-over provisions for dipole roof antenna and built-in monopole antenna operation through two separate pairs of connecting lugs 38, 39 and 40, 41 which connect either the dipole roof antenna 42 and its transmission line 43 or the built-in monopole antenna 44 and chassis ground 45 to input terminals 34 and 35. The dual tuning circuitry includes selector switch decks 46 and 47 which tap inductors 48 and 49 and tuning capacitors 50 and 51. Phase control trimmer-capacitor 52 makes it possible to shift the phase of the signal in the left tuning circuit 48, 50 with reference to the right tuning circuit 49, 51, as required by the random phase shifts between the signals from antenna 44 and chassis 53 in order to avoid signal cancellation, and to increase the signal between chassis 53 and antenna 44 as much as possible.

FIG. 7 shows a receiver similar to that shown in FIG. 6 and using identical component identification numbers wherever applicable, except that the differential input triode 31 of FIG. 6 has been replaced by a differential input antenna transformer 61, the primary 62 of which is connected to input terminals 34 and 35, while its secondary 63 is grounded to the chassis 53 at point 64 and supplies a single-ended input signal to the grid 32 of the triode amplifier 31A. Primary 62 may or may not be grounded at its center 65, as indicated by switch 66. Trimmer-capacitor 52 serves as the phase control.

In a receiver with a differential input transformer the phase control of the present invention may require different reactive components and circuitry than shown in FIGS. 6 and 7, since the reactive characteristics of an input transformer can vary in many ways. If the antenna transformer was an ideal transformer, without stray reactance, stray capacities, hysteresis or eddy current losses (as in ferrite transformers) etc. the reactive properties of the input stage which is connected to the transformer secondary, would simply be reflected directly in the primary, if turn ratio is 1:1, or modified by the square of the turn ratio if other than 1:1. Practical antenna transformers fall short of the ideal. Practical transformers can present nearly purely inductive loads at their input terminals, nearly pure capacitive or nearly pure ohmic loads. In addition, one input terminal of a practical differential transformer can present a different type of load than the other. This odd relationship may often change from frequency band to frequency band, thus requiring more sophisticated phase control systems. FIGS. 8-11 illustrate receiver systems including more complex phase control.

In the receiver of FIG. 8 a choice of two monopole antennas is provided by switch 81. In its upper position 82 switch 81 connects a vertical monopole antenna 83 to the upper transformer input terminal 84. In its lower position 82A, switch 81 connects the power cord which can act as an antenna to terminal 84. Either of the two antennas can appear to the transformer 86 like a resistor 87 or 97, an inductor 88, or 98, or like a capacitor 89, or 99. The input 84 of transformer 86 can also appear to the two antennas, whichever is being used, as a resistor 101, an inductor 102, or a condenser 103. Assuming first that the transformer 86 acts as a resistor 101, the phase of the incoming signal voltage at input terminal 84, with reference to signal voltage in the chassis 111 and the lower "grounded" transformer input terminal 112, will depend, among other operating parameters, such as wave tilt and arrival-delay between chassis and antenna, upon whether the selected antenna, at the particular frequency, acts as as resistor, an inductor or a capacitor. If the antenna acts as a resistor, and its resistance can be considered as being lumped directly next to the transformer input with no travelling delay, no phase shift will occur between the antenna and the transformer input 84, yet the signal may have any phase relationship with the signal delivered by the chassis 111. If the antenna acts as an inductor, the voltage drop it creates at the input 84 of the transformer 86 which acts as before, as a resistor is advanced, because the current flowing through the inductance is delayed. If the antenna acts as a capacitor, voltage is delayed because the current is advanced. These relationships are of interest only in reference to the phase of the chassis signal which may, or may not, tend to cancel the antenna signal.

Since a change of the reactive characteristics of the antenna will change the phase relationship between the antenna signal and the chassis signal, an inductor or capacitor, in series with the antenna or in parallel with it, can be used to adjust the phase relationship, in order to develop maximum amplitude between the antenna transformer input terminals 84 and 112, or at least to prevent communication loss due to signal cancellation.

FIG. 8 shows a receiver system including an adjustable inductor 121 in series with the antenna lead, to create a desired phase shift.

FIG. 9 shows a receiver system including a series trimmer-capacitor 122 in the same circuit location, acting as a phase control.

FIG. 10 shows a receiver system similar to that shown in FIG. 8 except that the phase control is an adjustable inductor 123 in parallel with the transformer input.

FIG. 11 shows a similar receiver system in which a parallel capacitor 124 acts as a phase control.

The foregoing explanation of the phase relationships between the antenna and chassis signals and the changes in these phase relationships, as reactive components in the antenna leads are adjusted, was made with the assumption that the input transformer 86 acts as a resistor. But the transformer may also act as an inductor or as a capacitor as previously indicated. If so, new phase-combinations are formed but the basic principle of phase adjustment by means of series or parallel adjustable inductive or capacitive reactors which are attached to the antenna lead remains the same.

An important aspect of the present invention is that the phase control circuits shown in FIGS. 8-11 are not only alternative choices to fulfil a single purpose, but may be used simultaneously, if the phase shifts encountered cover so wide a range that they cannot be handled by one phase control circuit alone. Further, in accordance with the present invention combinations of phase control circuits may include coupling and decoupling devices, to separate frequency bands, as will become apparent from the following description.

The circuit illustrated in FIG. 12 has been demonstrated to be capable of simultaneously preventing major signal-cancellations, in two widely separated frequency bands, such as, for example, the 60 mc band and the 180 mc band. These are typical television frequencies at which signal cancellations of the type described are likely to occur. The circuit uses two antennas, one of which is the power cord 201, while the other is the relatively short monopole 202 which may or may not be capacitively terminated at its end 203. At 60 mc, and within a broad band around this frequency, the power cord antenna 201 can be made to operate near resonance where minor tuning changes create large phase shifts by action of its own inductance and adjustable "extension inductor" 121. At 180 mc, and within a broad band around that frequency, antenna 202 can be made self-resonant by reducing its inherent, excessive inductance, augmented by the input inductance of transformer 205, by means of series-capacitor 206 which may or may not be adjustable, as shown here, or may even be a stray-capacity across inductor 204. It should be noted that the large inductance of the power cord 201 decouples the power cord 201 from the higher frequency (180 mc) circuit, while low-frequency inductor 204 ineffectively shunts capacitor 206. At the same time, when operating at 60 mc, capacitor 206 ineffectively shunts dominating inductor 204, and antenna 202, due to its small physical dimensions in comparison with the power cord 201, has little influence upon performance at that lower frequency.

While the foregoing examples of phase control circuits involved essentially single, lumped inductors or capacitors, the principles of the present invention can be effectively carried out by the use of delay line phase control circuits including adjustable positive or negative delay lines which can be made to cover a much wider phase-range than the simple, single-step phase control circuits shown in FIGS. 5-12 are capable of covering. Further, since a multi-step delay line can create large phase shifts with negligible amplitude loss, this type of phase control may, in some instances, be preferred over the simple, single-step L/C, L/R and C/R phase controls, previously described.

In the receiver system shown in FIG. 13 power cord 301 acts as the principal antenna, while chassis 302 acts as a second parasitic antenna. Phase relations between the two antennas can be corrected, for optimum performance or at least to avoid signal cancellations, by means of adjustable L/C delay line 303, which is a positive (phase retarding) delay line, here shown as a string of five inductors in series with the power cord antenna 301 and five capacitors connecting the junctions between the inductors to the chassis 302.

FIG. 14 shows a receiver system similar to that of FIG. 13, except that adjustable delay line 303 is inserted in the lower, previously chassis-grounded transformer input lead, while the upper transformer input lead 305 is directly connected to the power cord antenna 301. In the receiver of FIG. 14 phase correction is achieved in the parasitic signal path and can be as effective as in the receiver of FIG. 13.

FIG. 15 shows a receiver system wherein the principal antenna is monopole 306. Delay line 306 is, in this instance, inserted between two sections of the parasitic antenna system, namely, power cord 301 and chassis 302. The monopole antenna 306 may or may not be terminated at its free end 307.

FIG. 16 shows a receiver system similar to that of FIG. 13, except that the positions of the inductors and capacitors of delay line 303 of FIG. 13 have been exchanged, thus making delay line 308 of FIG. 16 a negative (phase advancing) delay line which may be equally useful for the purpose of eliminating signal cancellations and bringing about the optimum phase relationship between the signals from the principal and parasitic antennas.

FIG. 17 shows a receiver system similar to that of FIG. 14 except that negative delay line 308 has been substituted for the positive delay line 303 of FIG. 14.

FIG. 18 shows a receiver system similar to that of FIG. 15 except that negative delay line 308 has been substituted for the positive delay line 303 of FIG. 15.

While the principles of the present invention have been illustrated by reference to a number of examples of specific receiver systems employing various phase control devices, it will be appreciated by those skilled in the art that the present invention is not limited to those examples. It will further be apparent to those skilled in the art that other modifications and adapatations of the disclosed systems may be made without departing from the spirit and scope of the invention as set forth with particularity in the appended claims.

Claims

What is claimed is:

1. A receiver system comprising a power cord; a differential input device having a pair of input terminals said power cord being connected to one of said input terminals to serve as a monopole antenna structure; a parasitic antenna structure including the receiver chassis connected to the other of said input terminals, the signals from said parasitic antenna structure having a random phase relationship with the signals from said power cord antenna structure; and an adjustable phase control device connected between one of said antenna structures and its associated input terminal for adjusting the phase of the signal from said antenna structure to bring about an additive phase relationship with the signal from the other of said structures.

2. The receiver system of claim 1, wherein said adjustable phase control device is connected between said power cord antenna structure and its associated input terminal.

3. The receiver system of claim 2, wherein said adjustable phase control device comprises a variable inductor.

4. The receiver system of claim 2, wherein said adjustable phase control device comprises a variable capacitor.