Propagation time equalizer for circular wave guides

The input wave guides are oriented towards a perpendicular output wave gu with a semi-reflecting quarter-wave plane reflector disposed at 45.degree. to two progressive reflectors which cause increasing delay with frequency and which are disposed in the line of the input and output wave guides.

FIELD OF THE INVENTION 
The present invention relates to a propagation time equalizer for circular 
wave guides. 
BACKGROUND OF THE INVENTION 
Circular wave guides are used in telecommunications; they have an internal 
conductive surface in the form of a cylinder of revolution through which 
electromagnetic waves propagate at various frequencies, lying 
conventionally between 30 and 100 Ghz. 
Signals can thus be transmitted over long distances with a very wide 
pass-band. Unfortunately, these signals are progressively distorted 
because the propagation velocity of their various components increases 
with their frequency. This propagation velocity is indeed the group 
velocity of the waves in the guide, it being known that this velocity is 
different from the phase velocity and that it increases with the 
frequency. To obtain signals without distortion, it is therefore necessary 
to place delay equalizers in the signal transmission circuit. Such an 
equalizer should delay the various components of the signals, the delay 
increasing with frequency to compensate the advance acquired by the 
high-frequency components in the guide. 
Delay equalizers are known for rectangular wave guides. These equalizers 
using a set of guides connected together and forming a conventional device 
called a "hybrid T" and constituted by four arms: an input arm, an output 
arm perpendicular to the input arm and two lateral arms alined 
perpendicular to the input arm and to the output arm. There is a 
difference of a quarter of a wavelength between the lengths of these 
lateral arms and each of them is terminated with a "progressive 
reflector", i.e. at a guide having a decreasing cross-section. The 
function of this progressive reflector is to reflect the waves which 
penetrate therein after they have travelled along a path which increases 
with their frequency. It is known that in these conditions the waves 
arriving through the input arm are transmitted to the output arm with a 
delay which increases with their frequency because the higher frequency 
waves have travelled along a longer path in the progressive reflectors. 
Such a delay equalizer for a rectangular wave guide can be used with 
circular wave guides only in conjunction with transition elements between 
the circular wave guides and rectangular wave guides. Such transition 
elements make the telecommunications devices more complex and more 
expensive. 
It is also known to produce delay equalizers for circular wave guides, 
which transpose the frequency of the waves propagated in the guides so as 
to obtain signals at much lower frequencies (medium frequency) which can 
be handled by conventional electronic circuits. These electronic circuits 
are designed to delay the various components of the signals by amounts 
which are greater for components transmitted along the transmission line 
at higher frequencies. Such circuits are complex and expensive. 
Preferred embodiments of the present invention provide cirular wave guide 
delay equalizers which are simple to manufacture. 
SUMMARY OF THE INVENTION 
The present invention provides a delay equalizer for a circular wave guide 
comprising 
a circular input wave guide; 
A circular output wave guide having the same diameter as the input wave 
guide is connected to the input wave guide by a common end, the axes of 
these two guides meeting at an angle. 
A first progressive reflector constituted by a circular wave guide whose 
input diameter is equal to that of the input wave guide and the output 
wave guide and whose cross-section decreases from its input so that the 
waves which enter the first progressive reflector will be reflected after 
having travelled along a path which is longer for increasing frequency is 
placed in the line of the input wave guide beyond said common end to which 
it is connected by its input. 
A second progressive reflector identical to the first is placed in the line 
of the output wave guide beyond said common end to which it is connected 
by its input. 
A plane semi-reflecting plate of the "quarter-wave" type occupies the 
interior cross-section of said wave guides at their common end and is 
disposed so that the axis of the input wave guide will be symmetrical to 
the axis of the output wave guide in relation to this plate. 
The material and the thickness of this plate is chosen so that it will let 
pass half the energy of the waves which it receives with a phase shift of 
a quarter of a wavelength and so that it will reflect the other half of 
this energy. 
An embodiment of a delay equalizer according to the invention and having no 
limiting character is described hereinbelow with reference to the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 1, an input wave guide 2 with a circular cross-section of 50 mm 
diameter is connected at right-angles to an outlet wave guide 4 having the 
same cross-section, the plane of the figure passing through the axes of 
the two wave guides. 
A first progressive reflector 6 and a second progressive reflector 8 are 
disposed coaxially in line with the input wave guide 2 and the output wave 
guide 4, respectively; being connected by their inputs to the common end 
of these two wave guides. These progressive reflectors are identical to 
each other and are each constituted by a circular wave guide whose input 
cross-section is equal to that of the wave guides 2 and 4. Their 
cross-section then decreases progressively. 
It is possible for example to determine the law of variation of diameter D 
of the reflector as a function of the distance x from the input by the 
following hypothesis: 
To(f) + Tr(f) = constant in the frequency band in question where 
To(f) is the propagation time in the line whose delay is to be equalized 
Tr(f) is the propagation time in the reflector f is the frequency in 
question. 
and wherein: 
##EQU1## 
where: n is the square root of the Bessel function characterizing the mode 
of propagation used. 
c is the velocity of light in a vacuum; 
Po is the interior perimeter of the cross-section of the circular wave 
guide whose delay is to be equalized, i.e. its diameter multiplied by the 
number .pi.; 
Lo is the length of the circular wave guide whose delay is to be equalized; 
P(x) is the interior perimeter of the circular cross-section of the 
reflector at the point situated at the distance x from the input of the 
reflector; and 
x1 is the limiting distance from the input of the reflector for the 
frequency and mode being considered. 
A flat plate 10 is disposed at the common end of the wave guides 2 and 4. 
This plate is a semi-reflecting plate, i.e. it reflects half the energy of 
the waves it receives and it is of the "quarter wave" type, i.e. it 
transmits the other half of this energy by causing a phase shift of a 
quarter of the wavelength. This is a property of the choice of the 
material from which it is made, e.g. glass and of its thickness in the 
direction of propagation of the waves, e.g. 0.5 mm. Its plane is 
perpendicular to the plane of the figure and forms an angle of 45.degree. 
with the axes of the wave guides 2 and 4. It is disposed so that the waves 
arriving through the input wave guide 2 will be partly reflected towards 
the second progressive reflector 8. These waves are also partly 
transmitted towards the first progressive reflector 6 with a phase shift 
of a quarter of the wavelength. 
The waves received by the two progressive reflectors are reflected with a 
delay which increases with their frequency. Those which are reflected by 
the reflector 6 are then partly reflected by the plate 10 towards the 
output wave guide 4 and partly transmitted towards the input wave guide 2. 
Those which are reflected by the reflector 8 are then partly reflected by 
the plate 10 towards the input wave guide 2 and partly transmitted towards 
the output wave guide 4. Concerning the input wave guide 2, the waves 
coming from the reflectors 6 and 8 have the same amplitude and are in 
phase opposition, since one set has passed twice through the plate 10 and 
the other set has been reflected twice without any phase shift. Hence no 
energy is reflected into the input wave guide 2. As far as concerns the 
output wave guide 4, the waves coming from the reflectors 6 and 8 are in 
phase coincidence. Hence, neglecting the losses, all the energy arriving 
through the input wave guide 2 is found in the output wave guide 4. The 
only modification which the waves undergo is that the higher frequency 
components have undergone a longer delay in the reflectors 6 and 8. 
In the case where the propagation times in very long wave guides, e.g. wave 
guides of 500 m have to be equalized with a delay equalizer according to 
the invention, circumstances lead to the use of long progressive 
reflectors, e.g. having a length of 2.8 m, which would increase the bulk 
of the delay equalizer very inconveniently. 
That is why it can be useful to connect several such delay equalizers in 
series so that the lengths of the progressive reflectors will be 
superposed. If the lengths of the progressive reflectors of a single delay 
equalizer are L, the lengths of the progressive reflectors of N identical 
delay equalizers connected in series and providing the same corrections 
will be only L/N. It is possible for example to use the disposition shown 
in FIG. 2, in which the wave guide is drawn in thick lines and the 
progressive reflectors are drawn in thin lines. The outlet wave guide of 
the delay equalizer constitutes the input wave guide of the following 
delay equalizer and there is an angle of 90.degree. between the input wave 
guides of two consecutive delay equalizers. 
The successive delay equalizers are designated by the letter C followed by 
the order number of the delay equalizer. The corresponding input wave 
guides are designated by the letter G followed by this order number and 
the first and second corresponding progressive reflectors are designated 
respectively by the letters U and V. 
An input wave guide G1 is horizontal. It constitutes the input wave guide 
of a first delay equalizer C1 provided with progressive reflectors U1 and 
V1. The input wave guide G2 of the second delay equalizer C2 is also 
horizontal. The input wave guide G3 of the third delay equalizer C3 is 
inclined with respect to the horizontal so that the delay equalizer C3 
will be higher than the delay equalizer C2. 
The wave guide G3 is horizontal. The lengths of the wave guides G2 and G4 
are equal and the delay equalizers are oriented so that the delay 
equalizer C4 will be disposed exactly above the wave guide G1, the 
successive input wave guides rotating always in the same direction, e.g. 
anticlockwise. The wave guide G5 is horizontal, the delay equalizer C5 
being disposed above the delay equalizer C1. The wave guide G6 is 
horizontal, the delay equalizer C6 being disposed above the delay 
equalizer C2. In general, the delay equalizers are regularly spaced out on 
four vertical straight lines forming the edges of a prism having a 
rectangular cross-section round which the wave guides wind always in the 
same direction, the superposed wave guides being parallel to one another. 
This disposition makes it possible to connect in series a great number of 
delay equalizers according to the invention with a minimum bulk. The last 
wave guide GN is constituted by the output wave guide of the last delay 
equalizer.