A Dual Polarization Phased Array with Connected Spirals
R. Guinvarc'h*(1), Randy L. Haupt (2)
(1) Supelec/SONDRA, Gif Sur Yvette, France
(2) The Pennsylvania State University, Applied Research Laboratory, State
College, PA 16801, USA
E-mail: regis.guinvarch@supelec.fr
Introduction
A spiral antenna is a broadband element exhibiting circular polarization: righthand circularly polarized (RHCP) or left hand circularly polarized (LHCP)
depending on its winding. An array of spirals with dual polarization must
therefore have spirals winding both right and left. A simple configuration
alternates every other RHCP element with a LHCP element. In that case, the
spacing is such that the frequency at which the grating lobes appear (so the
maximum frequency of the array) is lower than the minimum frequency of the
spiral (and, so, of the array, with a return loss less than -10dB).
This paper describes how to modify a spiral array to greatly improve its behavior
at low frequency, without modifying the spirals themselves. The technique
consists of connecting adjacent cross polarized spirals. Thus, the currents travel
from one spiral to the next instead of being reflecting from the end of the arms. In
addition, the currents transmitted to the neighboring spirals radiate the same
polarization, due to their geometry.
An Alternating Array
At high frequency, the radiating zone of a center fed spiral antenna is limited to a
small region about the feed. It then radiates a wave circularly polarized according
to its winding; at that point both axial ratio (AR) and return loss (RL) are good.
The radiation zone increases as the frequency decreases. At the lowest frequency,
the current reaches the end of the arms of the spiral and reflects. The reflected
current radiates the cross polarization (because of the winding). If the frequency
decreases enough, some of the reflected current reaches the generator and results
in a standing wave. For an Archimedean spiral, the latter frequency is
theoretically equal to [1] c/πD, where c is the speed of light in the vacuum and D
the diameter of the spiral.
This small insight into the functioning of a spiral shows that the primary
limitation to bandwidth in an array occurs at the lowest frequency. In that case,
the current reaches the end of the arms. Typical solutions to increase the lower
limit of the bandwidth are thus based on the modification of the spiral to limit the
current at the end of the arms. Some loads can be added to dissipate the energy
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(resistive for instance [2]) or the shape of the spiral is modified with some
meanders [3] to increase its length. The new solution proposed in the paper is to
connect neighboring spirals at the end of their arms as illustrated in Fig. 1.
Figure 1: Array of alternating spirals connected with straight segments.
Technique description and results
Connecting the spirals has two advantages. The first is a consequence of the
above explanation: the current will no longer be reflected at the end of the arms. It
will thus affect neither the RL nor the AR. The second advantage is that the
neighboring spiral will also be fed. As it will be externally fed, it will actually
radiate the same polarization as the fed spiral. It will therefore contribute to
radiate more energy with the co polarization.
Figure 2: S11 of a middle antenna, with and without the connections.
Two simulations were conducted with a Method of Moment code for a 16 spiral
array (8 RHCP and 8 LHCP). There was no connection in the first simulation
while the second has used straight segments to connect the neighboring spirals, as
depicted in Fig. 1. Fig. 2 shows the VSWR (S11) for one of the middle antennas
when the main beam points at broadside, and the element is matched to 250 ohms.
The bandwidth (S11 < -10 dB) improves from less than 0.2 GHz at 3.2 GHz to
2.15 to 3.1 and 3.3 to 3.5 GHz. If the main beam is steered to 30°, then the grating
lobes appears at 2.94 GHz.
Fig. 3 shows the AR when the main beam is at broadside. Even though the
resulting AR bandwidth (2.9 to 3.7 GHz for AR > -3 dB) associated with the
connected spirals is very narrow, it is a distinct improvement over the
unconnected array of spirals. The same apply for a beam steered at 30°, as plotted
on Fig. 4 (bandwidth 2.8 to 2.95 GHz).
Figure 3: Axial ratio of an array of 16 alternating spirals (8 RHCP and 8
LHCP), with and without connections. The beam is at broadside.
Conclusions
This paper has introduced a new technique to design dual polarization phased
array. It consists of connecting adjacent cross polarized spirals. It has been
applied to an array of alternating spirals. This array does not function well without
the connections, but the addition of the connection has a distinct improvement on
both the AR and the RL. A recent paper [2] by the same authors has presented a
technique to improve the grating lobes limit by interleaving the two subarrays. It
must be interesting to consider the use of both techniques.
Figure 4: axial ratio of an array of 16 alternating spirals (8 RHCP and 8
LHCP), with and without connections. The beam is steered at +30°.
References
[1] J. Kraus and R. Marhefka, Antennas for All Applications. McGraw Hill, 2002.
[2] R. Guinvarc'h and R. L. Haupt, “Dual polarization interleaved spiral antenna
phased array with an octave bandwidth”, IEEE Trans. Antennas and Propag.,
accepted for publication, 2009.
[3] D. Filipovic and J. Volakis, “Broadband meanderline slot spiral antenna,”
Microwaves, Antennas and Propagation, IEE Proceedings, vol 149, No. 2, pp
98-105, Apr 2002.
[4] FEKO Suite 5.4, EM Software and Systems (www.feko.info), 2008.