BROADBAND SIX-PORT CIRCUIT FOR
DIRECTION-FINDING APPLICATIONS
F. P. Casares-Miranda, E. Ma ´ rquez-Segura, Pablo Otero, and
C. Camacho-Pen ˜ alosa
Departamento de Ingenierı ´a de Comunicaciones
E.T.S. Ingenierı ´a de Telecomunicacio ´n
Universidad de Ma ´ laga
29071 Ma ´ laga, Spain
Received 31 May 2004
ABSTRACT: A new six-port circuit configuration based on branch-line
hybrids and Wilkinson power dividers is presented. The phase difference
between two input signals is estimated using this new structure. The
results are compared with the most widely used six-port structures and
a notable improvement in broadband performance is shown. Simulations
are carried out in order to assess this improvement. © 2004 Wiley Peri-
odicals, Inc. Microwave Opt Technol Lett 43: 457– 458, 2004;
Published online in Wiley InterScience (www.interscience.wiley.com).
DOI 10.1002/mop.20500
Key words: direction finding; microwave circuit; six-port circuit
INTRODUCTION
The six-port circuit is able to measure the phase difference be-
tween two input signals by means of power readings at the four
output ports. The angle of arrival of an incoming signal can be
estimated from the phase difference , as it is shown in Figure 1.
In this paper, a broadband six-port circuit to be used in direc-
tion finding systems is presented. An analysis comparing the
proposed design with several well known six-port circuits [1–3], in
terms of frequency band performance, has been conducted.
THE PROPOSED SIX-PORT CIRCUIT
The proposed six-port circuit, shown in Figure 2, consists of two
Wilkinson power dividers, two branch-line hybrids, and a delay
line. The input power at ports 1 and 2 are equally divided to reach
the branch-line hybrids, which properly combine the input signals
in order to get the desired six-port output signals. This circuit has
a good broadband behavior, mainly due to the fact that the two
Wilkinson dividers can operate in a wider frequency band than the
branch-line hybrids [4]. The assembly of the Wilkinson divider
and the 90° line section has, at the design frequency, the same
scattering matrix that a branch-line with one matched port.
The phase difference between the input signals a
1
and a
2
is
defined as:
= arg
a
2
a
1
. (1)
From the six-port output wave amplitudes b
i
, shown in Figure 2,
the phase difference can be calculated using the following
expression:
= arctan
|b
4
|
2
- |b
3
|
2
|b
5
|
2
- |b
6
|
2
= arctan
P
4
- P
3
P
5
- P
6
, (2)
where P
i
is the measured output power level at port i , with i = 3
to 6.
COMPARISON WITH OTHER SIX-PORT CIRCUITS
The six-port circuit shown in Figure 2 has been compared to the
structures proposed in [1–3], which have been the most commonly
used so far. The six-port circuits have been simulated by means of
their transmission line models for different phase differences at
their inputs. The phase difference of the circuit found in [1] is
also computed using Eq. (2). For the six-port in [2], which consists
of three branch-line hybrids and a ring hybrid, the phase difference
can be calculated using
= arctan
P
5
+ P
6
- P
3
- P
4
2 P
5
- P
6
. (3)
For the circuit found in [3], which implements a five-port ring and
a directional coupler, the phase difference can be calculated
using
= arctan
2 P
5
- P
4
- P
6
3 P
4
- P
6
. (4)
The figure of merit used to compare the proposed six-port circuit
with those in [1–3] is the phase-difference error, defined as
error
= |
real
-
calc
|, (5)
where
calc
is calculated by means of Eqs. (2), (3), or (4), depend-
ing on the six-port structure. Figure 1 Direction-finding system with a six-port circuit
Figure 2 Proposed six-port circuit
MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 43, No. 6, December 20 2004 457