High power hybrid switch

High levels of R.F. power are controlled and switched by means of a hybrid switching network that employs an intermediate power level switch matrix in conjunction with a pair of 8.34 (nominal) directional couplers and a phasing network. The two directional couplers are connected in tandem by two equal length transmission lines to form a broadband quadrature 3dB hybrid. Switching is accomplished by selectively inserting a 180.degree. phase shift means into the lower power carrying transmission line. The phase shifting means can be a length of transmission line, a solid state device, or a Schiffman type phase shifter.

BACKGROUND OF THE INVENTION 
This invention relates to hybrid switching circuits and in particular to 
novel switching techniques and apparatus that permit high power switching 
with intermediate power level switching means. 
Many electronic systems require the switching of high current carrying 
lines with bw or intermediate power switching devices. A particular 
example of this, and one in which the present invention finds great 
utility, is the control of the feeds to airborne antennas that are 
remotely switched between two modes of operation. Systems of this type 
commonly employ a pair of antennas that are fed by a switching circuit 
that provides either full power to one antenna (single mode operation) or 
half power to each antenna (double mode operation). It is usually required 
that such antennas transmit several kilowatts of average c.w. power over a 
wide frequency range while maintaining low VSWR characteristics. 
Conventional switching methods used to produce the "dual" or "single" mode 
performance have been found inadequate either because of the adverse 
effect of changing impedance levels on VSWR or because of the size and 
weight limitations of the full power switches. Although 3dB hybrid 
switches have been used to switch between full power to one of two feeds 
and one half power to each of the two feeds they also have limitations 
that render them less than desirable for many applications. In particular, 
the conventional quarter wave 3dB hybrid is restricted in power handling 
capability and manifests performance sensitivity to tolerance variations. 
Such a device also requires "touchy" tuning devices at higher frequencies. 
Other currently available switching means also exhibit the foregoing and 
other undesirable characteristics. Accordingly there currently exists the 
need for a high power, light weight switch suitable for airborne 
applications that is not subject to the above enumerated deficiencies of 
known switching circuits. The present invention is directed toward 
satisfying that need. 
SUMMARY OF THE INVENTION 
The invention comprises a 3dB hybrid switch that is made up of two 8.34dB 
(nominal) directional couplers connected in tandem by two equal length 
transmission lines. The power traveling along one of the transmission 
lines is diminished from the input power by the coupling factor of the 
first directional coupler. A 180.degree. phase shifter is selectively 
insertable into the low power transmission line by means of intermediate 
power level switches Power fed to the 3dB hybrid input splits equally and 
with quadrature phase to the two output ports in the absence of the 
180.degree. phase shifter. All of the power is transmitted to a single 
output port when the 180.degree. phase shifter is inserted into the low 
power carrying transmission line. The phase shifter may be an appropriate 
length of transmission line, a solid state device or a Schiffman phase 
shifter. The Schiffman phase shifter provides a device that is insensitive 
to frequency. 
It is a principal object of the invention to provide a new and improved 
high power hybrid switch. 
It is another object of the invention to provide a 3dB hybrid switch in 
which high power can be switched with intermediate power level switches. 
It is another object of the invention to provide a 3dB hybrid switch that 
is insensitive to frequency. 
It is another object of the invention to provide means for switching power 
to airborne antennas that does not adversely effect system VSWR 
characteristics. 
It is another object of the invention to provide a 3dB hybrid switch that 
is lightweight and suitable for airborne use.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The high power 3dB hybrid switch comprehended by the invention is 
illustrated in schematic form in FIG. 1. Referring thereto a 3dB hybrid is 
formed by connecting 8.34dB directional couplers 7 and 8 in tandem by 
means of equal length transmission line circuits 9 and 10. Each 
directional coupler has four ports with transmission line circuit 9 
connecting port 2 of coupler 7 to Port 1 of coupler 8 and transmission 
line circuit 10 connecting the ports of each coupler designated as port 4. 
Port 1 of directional coupler 7 is an input for the 3dB hybrid and ports 2 
and 3 of directional coupler 8 are the outputs. Port 3 of directional 
coupler 7 is an isolation port and is terminal with a load 13. 
Transmission line circuit 9 includes switches 11 and 12 that are adapted 
to switch a 180.degree. phase shift means 16 into the circuit. The 
180.degree. phase shift means 16 can be a length of transmission line of 
appropriate length, a solid state device, or a Schiffman type phase shift 
network. 
A description of the modes of operation of the 3dB hybrid switch above 
described together with an analytical derivation of equations governing 
its behavior are now presented. Referring to FIG. 1, it is seen that with 
the switches in the .phi..sub.1 position, the total insertion phases of 
the two interconnecting lines (paths A and B) are identically equal and 
the power splits equally between output ports 2 and 3 with quadrature 
phase (vectorially shown in FIG. 2). It can also be shown that if, with 
the switches in the .phi..sub.2 position, the total insertion phase of 
paths A and B differ by .+-.180.degree. so that .phi..sub.A =.phi..sub.B 
.+-.180.degree., all the input power will arrive at output port 2 and no 
power will be present at output port 3. This can be simply shown for the 
exact vector relationship of the 8.34 db couplers at fo by referring to 
FIG. 3. 
The equations governing the distribution of power to output ports 2 and 3 
for varying values of coupling and frequency are derived below for both 
modes of operation: 
For a single coupler with input to port 1, coupled output at port 2', and 
main output at port 3': 
##EQU1## 
and: 
##EQU2## 
where: 
c is the coupling factor or midband value of coupling 
.theta. is the electrical length of the coupled line, and is equal to to 
.pi./2 at midband frequency, fo. The above equations may be written in the 
following form: 
##EQU3## 
For a pair of identical and synchronously tuned tandem couplers, with 
interconnecting lines .phi..sub.A and .phi..sub.B, as shown schematically 
in FIG. 1, where: 
##EQU4## 
and: 
##EQU5## 
A. Dual Mode 
When .beta.=0 an inspection of the above equation shows that the magnitudes 
of the vector sums are: 
##EQU6## 
and: 
##EQU7## 
B. Single Mode 
When .beta.=.pi., the equations reduce to: 
##EQU8## 
and: 
##EQU9## 
Note that the relationships for the case of .beta.=.pi. are independent of 
frequency if .beta. is independent of frequency. For the specific case 
where .beta. is frequency dependent, as in the use of a coaxial cable 
which is .lambda./2 at fo, the above two equations may be reduced to: 
##EQU10## 
where: .beta.=.pi.f/fo 
and: 
##EQU11## 
By way of example the foregoing equations were programmed onto a computer 
and performance was calculated over an approximate octave frequency band. 
Two conditions of phase shift were considered for the case of single 
output only to port 2; namely that the 180.degree. differential phase 
shift is either frequency invariant as approximated by a Schiffman phase 
shifter network or that it varies linearly with frequency as with a 
coaxial cable. The calculated lossless performance of the full network for 
both modes of operation with frequency varying phase shift 
(.beta.=.pi.f/fo) is shown in FIG. 4 and 5. Also, optimized center 
frequency coupling to achieve minimum deviations from equal power split in 
the dual mode was found to be approximately 7.83dB. 
Since a 180.degree. differential phase shift is required between paths A 
and B, but a conventional Schiffman type phase shifter produces a 
90.degree. differential phase shift, a "double" network is required. The 
required condition is shown by the dual Schiffman sections 14, 15 in FIG. 
6. For the dual mode, .phi..sub.A =.phi..sub.B1, and this is achieved as 
before by simply balancing overall line lengths of 10. For the single 
mode, path A has a line length, which is 540.degree. longer than the line 
length (including switches) of path B2. The B2 path is completed with the 
inclusion of and dual Schiffman sections. 
While the invention has been described in one presently preferred 
embodiment it is understood that the words which have been used are words 
of description rather than words of limitations and that changes within 
the purview of the appended claims may be made without departing from the 
scope and spirit of the invention in its broader aspects.