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Directional Coupler with improved Directivity for TETRA Radios

IP.com Disclosure Number: IPCOM000010261D
Original Publication Date: 2002-Nov-13
Included in the Prior Art Database: 2002-Nov-13
Document File: 3 page(s) / 40K

Publishing Venue

Motorola

Related People

Cizhong Xu: AUTHOR [+3]

Abstract

The directivity of a directional coupler is improved by making the construction symmetrical. Four different solutions that are easy to manufacture are discussed.

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Directional Coupler with improved Directivity for TETRA Radios

By Cizhong Xu, Klaus Dirksmeier, Bernhard Hammes

Abstract

The directivity of a directional coupler is improved by making the construction symmetrical. Four different solutions that are easy to manufacture are discussed.

Introduction

This paper describes improvements in the field of directional couplers, which offer a higher directivity for use in Cartesian Feedback Systems.

Problem(s) To Be Solved

Transmitters in TETRA radios often use a Cartesian Feedback System to improve the linearity of RF Power Amplifiers (PA) and thereby to reduce the RF power transmitted in the adjacent channels (ACP: Adjacent Channel Power). In a Cartesian Feedback System, a proportional part of the output power is extracted with the use of a directional coupler, and fed to a control circuitry.

Problem:

If the directional coupler does not have sufficient directivity, there is a risk that a mismatch at the antenna and awkward phase conditions may lead to the Cartesian Feedback System being unable to regulate the output power correctly, and that the regulator loop will become unstable. A simple solution for this problem is to build in an isolator (circulator) between the directional coupler and antenna so that the directional coupler is always terminated with the correct line impedance, even in the case of an antenna mismatch, and the coupled power is exactly proportional to the output power flowing towards the antenna.

Disadvantage of using an isolator: An isolator adds additional cost to the product. It requires a certain amount of space, which is a disadvantage, especially in portable radios. The power dissipation of the isolator reduces the efficiency of the transmitter, and at higher output power levels this will cause additional heating up of the product. Therefore the goal is to avoid the use of an isolator.

Proposed Solution to the Problems

The proposed solution is to improve the directivity of the directional coupler, so that it couples only the power flowing towards the antenna, even if there is a mismatch at the antenna. Normally the directional coupler is constructed in micro-strip technology. The conductive tracks of the directional coupler are on one side of the printed circuit board (PCB) surface, as pictured in Figure 1. This structure is simple and can be cost efficiently realized, but it has a crucial disadvantage: The directivity

D = 20 * LOG (PWR3 / PWR4)

is substantially limited. The reason for this is the asymmetrical construction.

Above the two strip lines (Refer to Figure 1) is air, with a dielectric coefficient of approximately 1 (er »1). The substrate material between the two conductive tracks and the ground plane has a dielectric coefficient greater than 1 (er » 4.8 for standard PCB material). Due to the different dielectric coefficients in the regions above and below the conductive tracks the phase velocities (even mode, odd mode) cannot be the same in bot...