US2515061A

US2515061A - Radio-frequency filter

Info

Publication number: US2515061A
Application number: US718642A
Authority: US
Inventors: Phillip H Smith
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1946-12-27
Filing date: 1946-12-27
Publication date: 1950-07-11
Anticipated expiration: 1967-07-11

Description

2 Sheet'sSheet 1 Filed Dec. 27, 1946 F/Gl.

406/! FREQUENCY TRAN-9M! TTEI? BROADCAST TRANSMITTER rC/ H //v l/EN TOR RH 5714/ TH ATTORNEY July 11, 1950 P. H. SMITH RADIO FREQUENCY FILTER 2 Sheets-Sheet 2 Filed Dec. 27, 1946 I lNl/E/VTOR R H. SM/ TH H III; II

ATTORNEY Patented July 11, 1950 RADIO-FREQUENCY FILTER Phillip H. Smith, Fair Haven,-N. .I., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application December 27, 1946, Serial No. 718,642

9 Claims.

This invention relates to wave transmission and more particularly to wave filters.

An object of the invention is to couple a grounded coaxial transmission line to I an ungrounded coaxial transmission line.

Another object is to transmit freely a desired band of frequencies from one to the other of such coupled lines while suppressing undesired frequencies.

A further object is to prevent the escape of energy when two such transmission lines are coupled together.

In high frequency transmission systems it is sometimes required to connect a grounded coaxial transmission line to an ungrounded transducer, which may be another coaxial line. For example, a coaxial line may be used to connect a high frequency transmitter to an antenna mounted on top of a series-fed, base-insulated standard broadcast tower radiator. As the outer conductor of this line is normally grounded a transmission network of special type must be inserted in the line at the point Where it crosses the base insulator, to prevent grounding the tower and thereby disabling its use as a vertical antenna. Such a tower-base bridging filter must appear as practically an open circuit in the standard broadcast range but must freely transmit, without radiation, a sufliciently wide band of frequencies in the high frequency range.

In accordance with one embodiment of the invention a filter suitable for this purpose comprises a section of coaxial transmission line, two capacitors connected in series with the inner conductor and spaced preferably less than a half wavelength at a frequency to be passed, a circumferential gap in the outer conductor, and a quarter-wave coaxial skirt connected at one end to one side of and encircling the gap. At its inner end the skirt presents a very low impedance at the frequency to be passed. When the capacitors have the proper values the filter will pass a sufficiently wide high frequency band and substantially match the line impedances at each end. The capacitors are preferably made variable and each may comprise a coaxial rod inserted into the end of the hollow inner conductor of the coaxial section. To prevent the escape of energy at the open end of the skirt a second similar skirt may be connected to the connected end of the first skirt so as to encircle it. This second skirt may be made adjustable axially for precise tuning. However, the filter is substantially an open circuit at the standard broadcast frequencies which may be associated with the tower and the ungrounded portion of the line. To provide additional suppression for the energy in this latter range, a short-circuited quarterwave stub line may be shunted across one end of the filter.

The nature of the invention will be more'fully understood from the following detailed descrip-v tion and by reference to the accompanying drawing, in which like referencecharacters refer to similar or corresponding parts and in which:

Fig. 1 shows an antenna system employing a tower-base bridging filter in accordance with the invention;

Fig. 2 is a vertical sectional view, partly schematic, of one embodiment of the filter;

. Fig. 3 shows the equivalent filter circuit at a transmitted frequency; Fig.4 is the equivalent circuit at a suppressed frequency; and I Fig. -5 is a vertical sectional view of the filter showing certain of the feaurtes in greater detail. In Fig. 1 a tower I of conventional design mounted at its base on insulators 2 provides a vertical antenna insulated from ground. The tower I is series-fed from a broadcast transmitter 3 one output lead 4 of which is connected near the base and the other 5 of which is grounded.

On top of the tower I is mounted a high frequency antenna 6 which is fed from a high frequency transmitter I through a coaxial transmission line. The portion 8 of the transmission line which runs up the tower I must be insulated from ground so that the tower will function properly as a vertical radiator for the transmitter 3. However, the portion 9 of the coaxial line extending from the tower I to the transmitter I will ordinarily have its outer conductor grounded, as shown at Ill. Therefore, in accordance with the invention, a tower-base bridging filter II of special design is inserted between the grounded portion 9 and the ungrounded portion 83 of the coaxial feed line.

The transmittert may, for example, be of the amplitude modulation type having a frequency fl falling within the standard broadcast range of 550 to 16 00 kilocycles while the transmitter I employs frequency modulation at a frequency f2 within the range from 88 to I08 megacycles.

As shown somewhat schematically in Fig. 2, one embodiment of the filter I I comprises a section of coaxial transmission line I2 having two capacitors CI and C2, preferably variable, connected in series with its inner conductor I3. The capacitors CI and C2 are spaced apart a distance D preferably less than half a wavelength at the frequency f2 to be transmitted. The

outer conductor I4 of the coaxial section I2 has a circumferential gap I5, preferably opposite the capacitor CI. A conductive coaxial skirt I6 having a length approximately equal to a quarter wavelength at the frequency f2 encircles the gap I5 and is connected at one end to a side thereof through the conductive annular member I1, but is left open at the opposite end I8. A

second similar coaxial skirt l9 of'approximately the same length but of larger diameter is connected to the connected end of the first skirt I6, as shown at 20, so as to encircle the first. skirt. The outer skirt I9 is preferably made so that it can be moved axially along the skirt l6 for adjustment of the efiective length of the coaxial line section 2| formed therebetween. A section of coaxial line 22 short-circuited at its outer end 23 and having a length approximately equal to a quarter wavelength at the frequency 2 is connected in shunt at one end of the line section I2, preferably near the capacitor C2 at the end connected to the grounded portion 9 of the feed line.

The operation of the filter H will now be explained in connection with the circuits of Figs. 3 and 4. As shown in Fig. 3 the equivalent electrical circuit at the transmitted frequency f2 consi'sts only of the section of line I2 of length D with the series capacitors CI and C2 at its ends. The outer conductor I4 and the sleeve IE cons'titute an open quarter-wave line section 24 the impedance of which viewed at the gap I5 is substantially zero. The short-circuited quarterwave line section 2| is adjusted to provide a substantially infinite series impedance at the open end of the line section 24 and thus prevent the escape of any energy of the frequency f2 flowing in the section I2. As viewed at the point of con: nection the stub section 22 has a substantially infinite impedance at the frequency f2 but a low impedance at the frequency fl, and therefore effectively prevents the passage of energy from the. broadcast transmitter 3 through the filter II to the high frequency transmitter I.

The proper adjustment of the capacitors Cl and 02 will now be considered. A series reactance, such as CI, at one point in a uniform, dissipationless transmission line may be adjusted toa value such that the impedance looking into the line at a second point, located a fixed distance D from the first point, is conjugate to the impedance at the first point. Connecting at the second point a second series reactance, such as C2, of the same character and magnitude as the first will make the impedance of the combination just equal to the characteristic impedance of the line. That is, the reflection caused by the first reactance is completely cancelled out by the second reactance. length within wide limits, but to minimize the overall length of the filter, it is preferably made less than a. half wave-length at the frequency 12 to be passed.

When the capacitors CI and C2, are adjusted in the manner explained above, the circuit of Fig. 3 will be. efiectively equivalent to a smooth section of transmission line, that is, a low-loss one to. one transformer having an image impedance at each end equal to the characteristic impedance ofv the line section I2. The filter will, therefore, transmit a wide band of frequencies including f2. and will match in impedance the coaxial lines 8 and 9 connected to theends thereof.

Fig. 4 shows the equivalent circuit for the filter II at the frequency fI as viewed at the open end I8 of the line section 24, which is the only point at which energy of this frequency may enter. There is one path from the skirt It to the ground Ill through the capacitance C3, which represents the capacitance between the skirt I6 and the outer conductor I4. Since this capacitance is comparatively small its reactance will be high. A second path to ground comprises the impedances ZI, CI, C2 and Z2 in series. ZI is the impedance looking from the filter II into the line section 8 and His the impedance looking into the line section 9. The capacitors CI and C2 also have high reactances at the frequency fl. It is seen, therefore, that the filter offers a high impedance to ground for energy of the frequency fl which is being radiated by the tower I. In practice this impedance can be made from 10 to 100 times the impedance to ground across the insulators 2. The inductance L in shunt withZE represents the stub line 22. It may have a reanctance of an ohm or less at fl andthus assists greatly in preventing energy of this frequency from reaching the transmitter I. i

Fig. 5 shows in greater detail the mechanical structure of an embodiment of the filter II. The skirts I6 and I9 and the stub line 22 are substantially the same as those shown in Fig. 2. The coaxial line section I2, however, has a hollow inner conductor 25 spaced from the outerconductor I 4 by the insulating rings 26. Each of the" coaxial lines 8 and 9 connected to the ends of the filter II also has a hollow inner conductor 2?.

Each of the variable capacitors CI and C2 comprises a section of open-circuited coaxial line 28 constituted by the hollow inner conductor 25 and a conductive rod 29 which extends into an end thereof. The other end of the rod 29 extends into the hollow inner conductor 2'5 and is slidably mounted in a conductive chuck 30. The eifective length of the line section 28, and therefore the capacitance, is adjusted by inserting the? endof' the rod 29 a greater or less distance into the conductor 25. The proper values forthe'f capacitors .CI and C2 are most easily found by trial, but they ordinarily will be approximatelyequal. r A cylinder 3| of insulating material attached at one end to the conductor M by means of the annular flanged member 32 and at the other end to the sleeve I6 serves to hold the parts of the filter II. together and also protects the interior of the filter from the weather. The flanges 33 facilitate the connection of other portions of the coaxial feed line.

The distance D may be any The section of line 24 maybe of fixed length and is preferably made a'quarter wavelengthlong' at the high frequency end' of the, range over which the filter II 15130 be tunable. Then, at lower frequencies within the range the'impedance of the line 2 5, at the gap I 5, will always be capacitive and its reactance will be small in magnitude. This capacitance is" in series with Cl and maybe considered to be a part thereof when the gap i5 is opposite the capacitance CI, as shown. Therefore, at the lower frequencies CI maybe slightly increased in value to compensate for the capacitance of the line 2%.

The stub line '22 may also be of fixed length and is preferably made a quarter wavelength long at the center of the tunable range of frequencies. Its reactance will change over the range but is almost exactly equivalent to a' small series reactance at the same position along the line. The effect of this equivalent series reactance may be exactly compensated by an adjustment of C2 provided the stub 22 is connected near the capacitor C2, as shown. This compensation will require the value of C2 to be re-- duced at the upper end of the range and increased at the lower end.

It is evident that the filter II is a weatherproof, gas-tight, non-radiating structure which is relatively low in cost, small in size, light in weight and easy to install, having a sufliciently high impedance at the frequency fl but low loss over a Wide band centered at the frequency f2. Furthermore, for optimum performance at any specified frequency f2 within a wide range only three elements, the rods 29 and the sleeve l9, require adjustment, which is easily made with the aid of a calibration curve and an ordinary ruler.

What is claimed is:

1. A wave filter for connecting a grounded coaxial transmission line to an ungrounded coaxial transmission line comprising a section of coaxial transmission line, two capacitors connected in series with the inner conductor of said section with a spacing therebetween of less than a half wavelength at a frequency to be passed by said filter, a circumferential gap in the outer conductor of said section approximately opposite one of said capacitors, a conductive coaxial skirt connected at one end to one side of and encircling said gap, and a second similar skirt connected to the connected end of and encircling said first skirt, each of said skirts having a length approximately equal to a quarter wavelength at said frequency and said capacitors being adjusted to provide for said filter image impedances which are substantially equal and each of which is substantially equal to the characteristic impedance of said section of line.

2. A filter in accordance with claim 1 in which said capacitors are variable.

3. A filter in accordance with claim 1111 which the inner conductor of said section is hollow and one of said capacitors comprises a conductive rod extending for an adjustable distance into the end of said inner conductor.

4. A filter in accordance with claim 1 which includes a short-circuited section of coaxial transmission line connected in shunt at one end of said filter and having a length approximately equal to a quarter wavelength at said frequency.

5. A filter in accordance with claim 1 which includes a short-circuited section of coaxial transmission line connected in shunt at one end of said filter near the other of said capacitors and having a length approximately equal to a quarter wavelength at said frequency.

6. A filter in accordance with claim 1 in which the inner conductor of said section is hollow and, each of said capacitors comprises a conductive rod extending for an adjustable distance into an end of said inner conductor.

7. A filter in accordance with claim 1 in which said capacitors are approximately equal in capacitance.

8. A filter in accordance with claim 1 in which said second skirt is adjustable axially with respect to said first skirt.

9. A filter in accordance with claim 8 in which said capacitors are variable.

PHILLIP H. SMITH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS