US2248751A - Transmission modifying network - Google Patents

Description

y 3, 1941 s. FRANKEL 2,248,751

TRANSMISSION MODIFYING NETWORK Filed Feb. 10, 1939 FEB}.

l2 F 7 i I J I?) 21 k H/G FREQUENCY HIGH FREQUENCY SOURCE 4040 I /50 I INVENTOR 52 53 SIQIVEY FRA/V/fEL ATTORNEY Patented July 8, 1941 TRANSMISSION MODIFYDIG NETWORK Sidney Frankel, New York, N. Y., assignor to Federal Telegraph Company, Newark, N. J., a corporation of California Application February 10, 1939, Serial No. 255,590

6 Claims.

My invention relates to high frequency transmission systems, and more particularly to transmission modifying or filter networks designed to reject certain high frequency wave components from a transmission line.

It is desirable that transmission lines for operating at high frequencies with a balanced load be properly balanced with respect to the earth or ground. These arrangements are particularly useful for connecting a radio frequency receiver or transmitter to its antenna by transmission lines designed to carry voltages which are balanced with respect to ground. Because of spurious pick-up from various sources, unbalanced current may flow in the conductors or feeders of the transmission line. The current in the feeders ar resolvable into two setsof components, the series current, for which the line acts as a loop, and the parallel current, for which the line acts as two halves in parallel. For convenience the component producing the loop effect will hereinafter be referred to as the series current or loop current, and the component which produces the parallel effect, will be referred to as the parallel current. The parallel current in the transmission line represents the degree of unbalance of the line, and when this parallel current is zero the line is balanced.

It can thus be seen that as far as parallel currents are concerned the feeders of the transmission line behave like a single wire, the return circuit being the image conductor in the ground. The impedance of the transmission line to this current may be made very large by the use of a network in accordance with this invention.

It is an object of my invention to provide a transmission modifying network for reducing the parallel current in a two-conductor transmission line.

It is a further object of my invention to provide a transmission system utilizing a filter network for reducing parallel components of the harmonic frequency of the transmission line, as well as the fundamental frequency thereof.

It is a still further object of my invention to provide a filter network comprising an arrangement bridged across a two-conductor transmission line for reducing parallel eifects to substantially zero.

Other features and objects of my invention will become apparent from the particular description made in connection with the accompanying drawing, in which Fig 1 represents a diagrammatic embodiment of a network in accordance with my invention,

Fig. 2 illustrates a network in accordance with my invention applied to a transmission system, and

Figs. 3, 4 and 5 show alternative forms of the network.

As set forth in an application of Andrew Alford, Serial No. 118,886, filed January 2, 1937, which issued as U. S. Patent No. 2,190,131 February 13, 1940, a reentrant network comprising a conductor branched from a transmission line at one point and reconnected to it at another point, if properly dimensioned will operate as a cut-off filter at a particular frequency. If we designate the distance along the transmission line as 01, and the length of the reentrant conductor connected to the line at the spaced point as 02, the cut-off filtering action occurs when a2-01=180 or a multiple thereof electrically termed a difference filter, or when I91+02=360 or a multiple thereof electrically termed a sum filter. Furthermore, by satisfying both of these conditions simultaneously a better cut-oif at a particular frequency may be achieved. My invention utilizes the principles of the reentrant network filter outlined in the abovereferred to application 118,886, and for a more complete understanding of the filter, reference is made to that application.

In Fig. 1, I0 and II represent two conductors of a transmission line spaced apart a distance d. Across the line at one point is bridged a short circuited transmission line section I2, having an electrical length equal to a quarter of a Wavelength or odd multiple thereof with respect to the fundamental frequency to be transmitted over the transmission line, Spaced a distance X from the bridging point of transmission line section [2 is another section 13, also electrically a quarter of a wavelength or an odd multiple thereof in length at the operating frequency. Since sections I2 and 13 are electrically a quarter of a wavelength long and are short circuited, they will not effect the loop current in the transmission line, since they will offer substantially infinite impedance at this frequency. Between the sections I2 and I3 is provided a connecting conductor I4. This conductor is connected between these networks preferably at a point midway of the length of the short circuiting bars on the transmission line section. This network then furnishes a reentrant network in so far as the parallel current in transmission line I 9II is concerned, while offering substantially no impedance to the loop current in the system. This reentrant network may be defined as having one arm 61 equal to X and the other arm 62 equal to i Y-l- Y In order to fulfil the conditions for producing a cut-off wave filter we may place 62-01 equal to Substituting the values of 01 and 02 given above we have This evaluation is not strictly correct since a quarter wave section such as l2 or [3 is not exactly a quarter wavelength long in physical dimensions because of the so called end effect of the short circuiting bar. It is therefore necessary to make the requisite allowance in the length of the conductor Y to take care of this discrepancy, so that the electrical effective length of sections 12 and I3 and connector IQ for the parallel currents will be 180 different from the distance X. Having made these connections the network will serve as cut-off filter. Accordingly, a parallel wave traveling along the transmission line I illl will be stopped by the blocking filter. There may be small amount of interaction due to the fact that Y differs in length from X and for this reason the network will not be perpendicular to the line. However, if the line spacing is small compared to i, the sections I2--l3 are almost perpendicular to the transmission line Iii-l I and produce little interaction.

This device has been set up in a crude form in the field and a very superficial test shows a very great decrease in percentage unbalance.

t is evident that for the second harmonic loop current sections l2l3 represents half-wavelength short circuited transmission lines and therefore will act substantially as a short circuit at the second harmonic frequency, thus filtering out the second harmonic loop current.

Since, as pointed out above, the quarter-wave sections produce the necessary difference to form the half-wavelength difference in length between the two paths 02 and 61, it is clear that the spacing X between the sections may be chosen as any desired value. If this distance X is chosen equal to and Y is of similar effective length, we get the additional result that l This choice of constants is permissible so far as the filtering action of the section in reducing the fundamental parallel currents is concerned. For the second harmonic then, the wavelength may be designated as and the (is for the second harmonic may be des ignated as 01' and 02', respectively. Thus a multiple of 360. The filter network will thus serve as a sum filter to reduce the second harmonic parallel current as well as a difference filter for the fundamental.

If, on the other hand, the spacings X and Y are each made substantially equal to at the fundamental frequency, then the perimeter of the network or 01+02=A at the fundamental frequency. Under these conditions we have both a sum and difference filter at the fundamental frequency. At the second harmonic frequency 01'+02'=2)\ a multiple of 360 and similarly at each of the other harmonic frequencies the corresponding sum of 01 and Hz is equal to a multiple of 360 so that the arrangement will, under these conditions produce a cut-off not only of the parallel current at the fundamental frequency, but of these parallel currents at all of the harmonic frequencies. Simultaneously the quarter wave sections will also act as a rejection filter for the second harmonic loop current.

The application of this filter network for reduction of parallel currents in a high frequency transmission system is illustrated in Fig. 2. In this figure, 20 represents a high frequency source connected over a two-conductor transmission line 2| to a high frequency load 22. At a point in transmission line 2|, is provided a network 23 of the type similar to that illustrated in Fig. 1. The high frequency voltages impressed upon the line from the source 20, are conducted over the line 2| to the load. The balanced or loop currents of the fundamental frequency in transmission line 2| are not impeded by the presence of network 23. However, network 23 is designed as outlined above, so that the parallel currents at the fundamental frequency are filtered out, as pointed out above. Likewise, the network 23 may be so designed, as outlined above, as to act as a rejection filter for the various harmonic frequencies. The high frequency source 23, may be, for example, a radio transmitter in which case the load 22 may be represented as an antenna. Alternatively the high frequency source 20 may be a receiving antenna which is connected over a balanced line 2| to a radio receiver constituting load 22. Furthermore, it is clear that the filter network in accordance with my invention need not be applied only to radio transmitting and receiving arrangements, but may be used in any circuit in which a high frequency balanced line is desired.

The network may have various forms other than that shown in Figs. 1 and 2, if desired. The preferred form of Fig. 1 however, is easier to adjust since the short circuiting bar may be moved to vary the tuning. In Fig. 3 is shown an arrangement where the networks 32 and 33 corresponding to 2 and 13 of Fig. 1 are made triangular in shape. These networks or sections are bridged across transmission line 30 and are interconnected by conductor. Sections 32 and 33 may be adjusted in a known manner so as to present to series currents in line 30 substantially infinite impedance. The other adjustments may then be made in accordance with the teaching of this application so the completed network circuit operates as a cut-off filter.

In Fig. 4 an arrangement similar to Fig. 3 is shown except that the sections 42 and 43 are directly connected together at 44, instead of using a separate connecting conductor. This network, like the others, may be properly adjusted to operate as a rejection filter. In designing the sections 42 and 43, the effect of their reaction on the line must be considered but this is not of any great importance if the angle made with the line is maintained greater than 45.

A still further alternative structure is shown in Fig. 5. Here each of the sections 42, 43 are two conductor parallel transmission lines similar to those shown in Fig. 1. These lines are brought together so that a common short circuiting bar 44 may be used and no additional conductor for interconnecting them is required. In this, as in the other modified structures, the variation from perpendicular relationship must be taken into consideration in designing the network. Furthermore. the sections will have to be electrically an odd multiple of quarter wavelengths long instead of simply one quarter wavelength in order that the other dimensions for the cut-off filter may be realized.

It should be understood that although I have illustrated only a few preferred forms of a network in accordance with my invention, the network may be made in many other forms, it being only necessary that the sections operate as substantially infinite impedances at the fundamental frequency.

Should it be found that radiation from the network itself is troublesome and prevents a complete balance, any of the known means for avoiding such trouble may be provided. For example, the network may be provided with a shield, the network may be folded to cancel out radiation, or known compensating means may be used.

While I have described particular embodiments of my invention for purposes of illustration, various modifications and adaptations thereof, occurring to one skilled in the art may be made within the spirit of the invention, as set forth in the accompanying claims.

What I claim is:

1. A high frequency transmission network for reducing parallel currents in a two-conductor transmission line while permitting free passage of balanced loop current, comprising impedance means offering substantially infinite impedance to the working frequency bridged across said conductors, a second impedance means of similar characteristics to said first impedance means bridged across said transmission line at a point spaced from said first section, and a conductor connecting together said impedance means, the overall electrical length of said impedance means and said connecting conductor differing from said spacing by a quarter wavelength or odd multiple of said working frequency, and the sum of overall length and said spacing being equal to one wavelength or an integral multiple of the wavelength of said working frequency.

2. A high frequency transmission net work for reducing parallel currents in a two-conductor transmission line while permitting free passage of balanced loop current, comprising impedance means offering substantially infinite impedance to the working frequency bridged across said conductors, a second impedance means of similar characteristics to said first impedance means bridged across said transmission line at a point spaced from said first section, and a conductor connecting together said impedance means, the overall electrical length of said impedance means and said connecting conductor differing from said spacing by a quarter wavelength or odd multiple of said working frequency.

3. A high frequency transmission network for reducing parallel currents in a two-conductor transmission line while permitting free passage of balanced loop current, comprising impedance means offering substantially infinite impedance to the Working frequency bridged across said conductors, a second impedance means of similar characteristics to said first impedance means bridged across said transmission line at a point spaced from said first section, and a conductor connecting together said impedance means, the overall electrical length of said impedance means, said connecting conductor, and said spacing being equal to one wavelength or an integral multiple of a wavelength at said working frequency.

4. A high frequency transmission system comprising a two-conductor transmission line, a high frequency energy source coupled to said line, and means for reducing parallel currents in the conductors of said transmission line comprising impedance elements effectively equal to a short circuited transmission line an odd number of quarter wavelengths long bridged across said transmission line conductors at spaced points and a conductive connection intermediate the ends of said impedance elements.

5. A high frequency transmission network for reducing parallel currents in a two-conductor transmission line while permitting free passage of balanced loop current, comprising a short circuited quarter wavelength section of transmission line bridged across said conductors, a second short circuited quarter wavelength section bridged across said transmission line at a point spaced from said first section, and a conductive connection intermediate the ends of said short circuited sections.

6. A high frequency transmission network designed to pass the fundamental frequency loop current and to act as a blocking filter for the second harmonic frequency loop current and the fundamental and harmonic frequency parallel currents, comprising a first short circuited section of transmission line a quarter wavelength or an odd multiple of said fundamental frequency in length, bridged across said transmission line, a second short circuited section of transmission line, a quarter wavelength or an odd multiple of said fundamental frequency long, bridged across said transmission line at a point spaced substantially a quarter wavelength or an odd multiple at said fundamental frequency from said first named section, and a conductor approximately a quarter of a wavelength or an odd multiple thereof long interconnecting said short circuited sections.

SIDNEY FRANKEL.

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