Microwave transformer

Details Microwave transformer Microwave transformer

1988-10-24

1989-08-29

Hille, Rolf

Communications: radio wave antennas

Antennas

With coupling network or impedance in the leadin

343895, 333 26, H01Q 136, H03H 742

Patent

active

048621897

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
This invention relates to microwave `balun` transformers, so called because of the transition they provide between balanced and unbalanced lines or systems.
2. Description of Related Art
A particular application of such transformers concerns cavity-backed antennas, in which, for example, a double spiral conductor mounted on a dielectric plate is backed by a cavity to take up power radiated backwards from the spiral. The cavity may be of such dimensions that a reflecting wall opposite to the spiral reflects the backward signal with such phase as to reinforce the forward transmission. Since such a design tends to limit the operating frequency it is known to absorb the reverse wave with a coating of absorbent material of some kind, e.g. graphite, to dissipate the reverse power rather than reflect it.
The spiral, or rather, double spiral, is fed by a balanced line, a twin pair, each of which is connected to a respective spiral termination.
It is known to mount the resulting cavity-backed antenna on a balun as shown in FIG. 1 of the accompanying drawings to convert the balanced twin line of the antenna feed to an unbalanced coaxial terminal port for connection to a transmitter/receiver. While the balun is satisfactory over a limited frequency range it is always desirable to extend the range of operation and/or generally improve the response.


SUMMARY OF THE INVENTION

It is therefore an object of the present invention to improve the frequency response of a microwave balun transformer, and particularly in use with a cavity-backed spiral antenna.
According to one aspect of the present invention, a microwave balun transformer comprises a dipole extending through a cavity formed between end walls of a conductive housing, at least one arm of the dipole comprising a coaxial line to a terminal port, the arms of the dipole being connected at their junction to the respective conductors of a balanced line which extends through the housing to provide a second terminal port, and a reflector being positioned close to each end of the dipole extending across the cavity transverse to the dipole arms, each reflector being substantially transparent at the frequency for which the length of each dipole arm is a quarter wavelength but being a substantial reflector at higher frequencies so that the effective length of each dipole arm remains closer to a quarter wavelength over a range of frequencies.
The reflector may comprise a conductive layer mounted on the front of a dielectric plate, the dielectric plate increasing the average permittivity of the cavity and thus reducing the frequency for which the effective length of each dipole arm is one half a wavelength. Each reflector may comprise an array of radial conductors extending from a conductive ring embracing the coaxial line.
A layer of radar absorbent material is preferably mounted on each end wall of the cavity to suppress the effect of imaging of the reflectors in the end walls.
According to another aspect of the invention, in a microwave antenna comprising a spiral conductor array mounted on a dielectric plate which in turn forms the closure to an antenna cavity, the cavity is mounted on the conductive housing of a transformer as aforesaid, the balanced line extending through the antenna cavity to feed the spiral array.


BRIEF DESCRIPTION OF THE DRAWINGS

A microwave balun transformer as incorporated in a cavity-backed spiral antenna, will now be described, by way of example, with reference to the accompanying drawings, of which:
FIG. 1 is a sectional elevation of a cavity-backed antenna and balun of conventional form according to the Prior Art;
FIG. 2 corresponds to FIG. 1, modified by the addition of two reflectors shown in FIG. 3;
FIG. 3 is a perspective diagram of an auxiliary reflector used to modify the conventional design;
FIG. 4 shows return loss characteristics for the conventional balun of FIG. 1 and the improved balun of FIG. 2;
FIG. 5 shows insertion loss characteristics for the two designs;
and FIG. 6 sho

REFERENCES:
patent: 3019439 (1962-01-01), Reis et al.
patent: 3786372 (1974-01-01), Epis et al.
patent: 4636802 (1987-01-01), Middleton
patent: 4658262 (1987-04-01), DuHamel

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