Combiners for R.F. power amplifiers

Directional couplers 7.sup.1 -7.sup.20 are used to couple power from r.f. power amplifier modules 5.sup.1 -5.sup.20 to a transmission line 6 or other type of resonator, from which it is withdrawn by directional coupler 8. The power withdrawn from the continuous transmission line, which is an integral number of wavelengths at the operating frequency of the modules in length, is less than the power circulating around the transmission line 6. The balance load may be removed to enable the output load to be combined with the output of another r.f. power amplifier operating a different frequency so that e.g. sound and vision signals can be combined at an aerial and, in the latter case, means may be provided for improving the resonance of the main loop 6.

BACKGROUND OF THE INVENTION 
1Field of the Invention 
This invention relates to combiners for r.f. power amplifiers. 
2Background Information 
Higher power r.f. power amplifiers for transmitters operating at higher 
frequencies (say HF and upwards) are tending to be made of a number of 
identical lower power r.f. power amplifier modules, typically solid state, 
of e.g. 250 to 1000 watt connected in parallel in typical numbers of up to 
40 or more. 
It has been proposed (FIG. 1) to couple sequentially a number (e.g. 20) of 
power amplifier modules 1.sup.1 . . . 1.sup.20 on to a transmission line 2 
via directional couplers 3, to produce a combined output on load 4. Each 
module receives the same drive input, and the outputs are combined in 
parallel. 
A directional coupler (FIG. 2) has four ports and the property that, 
assuming all ports are properly matched, a proportion of the input signal 
at port A appears across a load connected to port C while none appears 
across a balance load connected to port D (since two equal signals add at 
port C and subtract at port D). The remainder of the input appears across 
the output load. The coupling can be via discrete components or by 
electromagnetic coupling of e.g. two parallel conductors in an outer 
sheath spaced apart from each other by a certain distance over a certain 
length e.g. one quarter wavelength. The effective isolation of port D from 
the input appearing at port A enables the directional coupler to be used 
to combine two inputs e.g. one appearing at port A and one appearing at 
port D. An input appearing at port D in place of the matched load would be 
split between port C and port B. 
In FIG. 1 the couplers are used in this way to combine two appropriately 
phased signals. Thus, as shown in greater detail in FIG. 3 (the coupler 
being shown in its symbolic form, and the phase shift across each of the 
paths AB, CD being 90.degree.) a first signal entering at port A, nominal 
phase 0.degree. is divided into one part leaving port B at nominal phase 
-90.degree. and another part leaving port C nominal phase 0.degree.. A 
second signal entering at port D, nominal phase -90.degree. is divided 
into one part appearing at port C, nominal phase -180.degree. and the 
remainder appearing at port B, nominal phase -90.degree.. It will be 
apparent that an appropriate degree of coupling between the paths AB, CD 
together with appropriate relative voltage amplitudes of the alternating 
signals entering at ports A and D results in cancellation of the signal 
appearing at port C connected to the balance load and summation of the 
signals leaving port B. 
SUMMARY OF THE INVENTION 
A 180.degree. type is an alternative to the quadrature type described, the 
phase difference between ports AB, CD being 180.degree. and the relative 
phase of the signals fed in at ports A, D being 180.degree.. 
The degree of coupling of the coupler is defined by the coupling ratio, 
usually expressed in dBs, given by: 
Coupling ratio=10 log.sub.10 B/D, where D is the power fed into the side 
arm of the directional coupler and B is the power emerging from the 
coupler (being the sum of the power fed in the side arm and the power 
entering on the main line of port A). Different coupling ratios imply 
different separations of transmission lines or different coupling 
components. In the case of the arrangement of FIG. 1, assuming for the 
sake of simplicity that each module produces 1 unit of output power, the 
directional coupler 1.sup.1 has a coupling ratio of 10 log.sub.10 2/1=3.01 
dB. Directional coupler 1.sup.2 has an input power of 2 units, a further 1 
unit fed in from the side arm, and hence an output power of 3 units, and 
thus has a coupling ratio of 10 log.sub.10 3/1=4.77 dB. Given that there 
are 20 modules, the coupling ratio of directional coupler 1.sup.20, having 
an input of 19 units from the main line and an input of 1 unit from the 
coupling arm is 10 log.sub.10 19=013.6 dB. 
A disadvantage of such an arrangement is that a range of couplers is 
required with the coupling ratio extending from 3 dB to 13 dB. 
The invention provides a combiner for a plurality of r.f. power amplifiers 
having the same operating frequency as each other, comprising a respective 
directional coupler for coupling the output of each r.f. power amplifier 
on to a transmission line to combine the outputs, the transmission line 
forming a continuous loop the length of which is an integral number of 
wavelengths of the operating frequency, a directional coupler for 
withdrawing the combined output power from the continuous loop and the 
coupling ratios of the directional couplers being such that in use the 
circulating power in the transmission line is greater than the combined 
output power. 
The use of the continuous loop in conjunction with the directional coupler 
for withdrawing power permits the use of couplers having coupling ratios 
which are closer to each other than to the arrangement of FIG. 1, 
permitting easier manufacture. 
The resonator may be a continuous loop of transmission line, or a cavity or 
a body in which there is a rotating electric field. 
Television transmitters are, in the higher powers, normally constructed 
with separate sound and vision power amplifiers working on adjacent but 
separate frequencies. These two signals are then added together in an 
external combiner. Likewise, a number of T.V. or sound power amplifiers 
each on its own channel may be combined together for connection to a 
common aerial. This may be achieved by using the directional coupler that 
is used for withdrawing the combined output power, for coupling the output 
of a second r.f. power amplifier having a different operating frequency to 
produce a combined output. This combined output e.g. sound and vision, or 
different vision or sound power amplifiers, may then be fed to the aerial. 
Indeed, the combination of outputs of different frequency is possible 
whether or not a plurality of power amplifiers of the same operating 
frequency or just a single power amplifier is coupled to continuous 
transmission line. 
According to another aspect, therefore, the invention also provides a 
combiner for two r.f. power amplifiers having different operating 
frequencies, comprising a directional coupler for coupling the output of a 
first r.f. power amplifier having one operating frequency on to a 
transmission line, the transmission line forming a continuous loop the 
length of which is an integral number of wavelengths of that operating 
frequency, and a directional coupler for withdrawing the output power of 
the r.f. amplifier from the continuous loop as well as for coupling the 
output of a second r.f. power amplifier having a different operating 
frequency to produce a combined output. 
The continuous loop is resonant at the frequency of the first r.f. power 
amplifier, so that the power of the second r.f. power amplifier operating 
at a different frequency cannot couple on to the loop and hence may be 
combined in an output load connected to the directional coupler that is 
used for withdrawing the output of the first r.f. power amplifier from the 
loop. Two ports of the coupler may be connected in the transmission line, 
one may be connected to the output load, and the power of the second r.f. 
power amplifier operating at a different frequency may be fed in the 
fourth port in place of the usual balance load. 
To achieve a sharper resonance in the loop, a secondary ring may be coupled 
to the loop, or other resonators may be used.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 4 the 20 modules 5.sup.1 -5.sup.20 are identical power 
amplifiers, the centre frequency of each of which is 80 MHz and the output 
of each of which 1000 watt. Different frequencies or modules of different 
power output e.g. 500 watt may of course be used instead. Each module is 
coupled to a resonator in the form of a transmission line 6 by means of a 
respective directional coupler 7.sup.1 -7.sup.20. The transmission line is 
the centre conductor of a 50 ohm impedance coaxial cable, the outer sheath 
of which is earthed. 
Each of the directional couplers 7.sup.1 -7.sup.20 consists of a conductor, 
which can be brought to a defined position parallel to and spaced apart by 
a certain distance from the centre conductor, and within the coaxial outer 
sheath. Other forms of directional coupler e.g. discrete components may be 
used instead. The modules may be made so that they can be physically 
plugged into the support for the transmission line 6. 
The inputs of the modules are progressively delayed so that each module's 
output arrives on the resonant ring 6 in phase with the signal already 
there. The delay is the electrical distance around the ring from the first 
coupler 7.sup.1 to each subsequent coupler. The outputs of the modules are 
fed into one port of each directional coupler. The port connected by the 
conductor to this port is connected to a balance load of 50 ohms 
impedance. The other two ports on the transmission line itself are also 
matched to 50 ohms by virtue of the transmission line. 
The combined power of the modules is withdrawn by a further directional 
coupler 8 which may be of similar construction to the couplers 7. Two 
ports are formed by the transmission line, and the two formed by the 
coupled length of cable are connected to an output load and a balance 
load, both matched to the 50 ohms impedance. 
The coupling ratios of the directional couplers are chosen so that there is 
a circulating power in the transmission line greater than the combined 
output of the modules, in this case, four times greater. The coupling 
ratios of the directional couplers are chosen accordingly. Thus, the 
circulating power entering the directional coupler 8 is eighty times 1 kw. 
Only the actual power of the modules can be extracted i.e. 20 kw, so that 
coupling ratio is 6.02 dB=10 log.sub.10 20/80. The coupling ratio of the 
directional coupler 7.sup.1 is 17.85 dB (10 log.sub.10 61/1). The coupling 
ratio of the coupler 7.sup.9 is 18.39 dB (10 log 69/1) that of the coupler 
7.sup.10 is 18.45 dB (log.sub.10 70/1), that of the coupler 7.sup.11 is 
18.51 dB (log.sub.10 71/1), that of the coupler 7.sup.12 is 18.57 dB 
(log.sub.10 72/1) and that of the coupler 7.sup.20 is 19.03 dB 
(log.sub.10 80/1). In order that the power does build up around the loop, 
it is necessary that the length of the loop is an integral number of 
wavelengths of the centre frequency of the modules. 
As the power in the resonant ring of feeder is four times that at the 
output, so the loss is four times that in an equivalent length feeder used 
as a straight line combiner. But as this is a section of high power feeder 
anyway, the actual loss is still very small. 
As the modules are coupled to the main line via directional couplers, it 
would be possible to fit the complete side arm and balancing load to the 
module. This would remove the need for, and hence problems and costs of, 
r.f. plugs and sockets. This would make module testing easier as with the 
module on the test bench the balancing load now becomes the test load. 
Compared to a sequential arrangement of modules on the transmission line, 
the arrangement of the invention has the advantage that the couplers 
7.sup.1 -7.sup.20 all have approximately the same coupling ratio. 
In the second combiner (FIG. 5), advantage is taken of the resonance effect 
arising from the length of the transmission line being an integral number 
of whole wavelengths of the operating frequency of the modules. The second 
combiner is similar to the first, except that the balance load for the 
coupler for withdrawing power is replaced by an r.f. power amplifier 9 
operating at different frequency to that of the modules, and except that a 
secondary resonant ring is coupled to the transmission line 6. Like parts 
are given like reference numerals between FIGS. 4 and 5. The length of the 
secondary resonant ring 10 is an integral number of wavelengths of the 
operating frequency and it is coupled to the main ring by virtue of a 
coupling 11, which is a conductor of short length common to both rings. 
The purpose of the resonant ring is to provide a sharp cut-off of the 
passband of the main ring, which is centered on the operating frequency of 
the modules. The frequency of the r.f. power amplifier 9 lies outside the 
passband of the main resonant ring. Thus, the directional coupler 8 cannot 
operate in the usual manner and split its input between the main loop and 
the output load. Since none of the power can pass around the main resonant 
ring, it is all passed to the output load, where it is combined with the 
combined output of the modules 5.sup.1 -5.sup.20. 
The power amplifier 9 may be for a sound signal and the modules may provide 
the vision signal of a television signal, so that the output load may be a 
transmitting aerial. Alternatively, the power amplifier 9 may have a 
combined sound and vision signal on one channel, which could itself be a 
combined signal from a number of modules, and the modules 5 could each 
carry combined sound and vision signals on another channel, so that the 
load would then carry two combined sound and vision signals on different 
channels. Alternatively, the module 9 on the one hand and the modules 5 in 
the other hand could carry two sound radio signals on different channels. 
Various modifications in both embodiments are possible without departing 
from the scope of the invention. Thus, in both combiners, the circulating 
power could be a different multiple of the combined output powers of the 
modules e.g. three times. The only difference would be in the coupling 
ratios of the directional couplers, and the loss will also be different. 
Also, in both combiners, other resonators around which the r.f. energy can 
circulate may be used in place of the transmission lines. Thus, waveguides 
in the form of a loop may be used, or resonators with a rotating field may 
be used, such as the cavity resonator described in GB-A-1390809 or the 
plate resonator described in GB-A-1605120. In the second combiner, the 
secondary ring 10 or both rings 6, 10 may be of these types. For the 
second combiner of FIG. 5, there need not be a plurality of modules in the 
main loop, and could just be a single module in the main loop. Equally, 
the main loop may have a sharp enough resonance to avoid the need for an 
additional resonant ring or other resonator.