Ground plane size reduction in monopole antennas for ground wave transmission

Conference Paper (PDF Available) · February 2005with414 Reads
DOI: 10.1109/APS.2005.1551578 · Source: IEEE Xplore
Conference: Antennas and Propagation Society International Symposium, 2005 IEEE, Volume: 1B
Abstract
In this paper, the design of a ground plane that does not produce drastic transmission loss despite of its electrically small size is investigated in the context of HF ground wave transmission. To simplify the initial study, a monopole antenna is used to study the effect of a reduced ground plane on transmission loss. All simulations are done by the Numerical Electromagnetic Code (NEC). Several ground plane structures are investigated. Measurements are also performed to verify the simulation. A miniaturized ground plane structure is then proposed for the electrically small antenna
Ground Plane Size Reduction in Monopole Antennas
for Ground Wave Transmission
S. Lim
1*
, R. L. Rogers
2
and H. Ling
1
1
Department of Electrical and Computer Engineering
The University of Texas, Austin, TX 78712 U.S.A.
2
Applied Research Laboratories
The University of Texas, Austin, TX 78713 U.S.A.
Introduction
HF ground-wave transmission is useful for extended non-line-of-sight
communication. Unlike the sky wave, which is unpredictable and depends on
ionospheric conditions, the ground wave is a reliable means of propagation. As
in all HF applications, a key challenge is the design of electrically small
antennas. We have recently reported on an electrically small antenna optimized
by genetic algorithm for ground wave transmission [1]. The size of the antenna
is on the order of 0.03λ. However, the inductively coupled, top-loaded
monopole antenna still requires a sizable ground plane to operate well.
Therefore, it is desirable to design a truly electrically small antenna by also
reducing the ground plane size.
It is well known that most monopole antennas need a ground plane with a
diameter on the order of λ/2 to perform well [2]. The transmission loss
increases rapidly if the diameter of the ground plane is reduced to below λ/10.
Some previous research on methods to reduce the ground plane size has been
reported in [3, 4]. In this paper, the design of a ground plane that does not
produce drastic transmission loss despite of its electrically small size is
investigated in the context of HF ground wave transmission. To simplify the
initial study, a monopole antenna is used to study the effect of a reduced ground
plane on transmission loss. All simulations are done by the Numerical
Electromagnetic Code (NEC). Several ground plane structures are investigated.
Measurements are also performed to verify the simulation. A miniaturized
ground plane structure is then proposed for the electrically small antenna.
Design of an electrically small ground plane
Fig. 1 shows the effect of ground-plane size on the transmission loss simulated
by NEC. For the test antennas, quarter-wavelength monopoles for 30MHz and
100MHz are used. Dry soil condition (ε
r
= 3, σ = 0.0001S/m) is assumed in the
Sommerfeld calculation. Ground radials comprising of 20 arms are used. The
transmitter and the receiver are identical. As Fig. 1 shows, the curves for the
0-7803-8883-6/05/$20.00 ©2005 IEEE
transmission loss drop drastically for ground-plane diameters less than λ/10.
Similar results can be observed when rectangular ground planes are used. As
reported in [5], when the ground plane size of a monopole antenna is reduced
from infinity to zero, the monopole eventually becomes an end-fed dipole. In
going from one extreme to the other, the resonant frequency doubles. In
addition, the directivity changes from 3.2 to 1.6. This means that the directivity
of a monopole changes to that of a dipole. Therefore, a small ground plane
results in a very large mismatch loss at the original frequency of intended
operation. If the mismatch loss can be removed, the drastic drop in the
transmission-loss curve due to the small size of the ground plane can be
overcome.
To remove the mismatch loss, we propose a set of spiral shaped radials as the
ground plane. The spiral ground plane serves to generate large inductance that
helps shift the resonant frequency downward. Fig. 2 depicts a prototype of the
tuned spiral ground plane constructed of 18 gauge copper wires. The diameter
of the spiral ground plane is 43cm (0.043λ at 30MHz). The return loss of the
monopole with the spiral ground plane was simulated and measured. The
measured resonant frequency was 30MHz whereas the simulated frequency was
1MHz higher. The slight discrepancy between the simulated and measured
values is likely due to the difference in the soil characteristics. A balun was
used during the measurement of the small ground plane in order to block the
backward-flowing currents on the outside skin of the cable [6]. Two kinds of
commercial baluns were used in the study (Palomar Engineers, Uniadilla), and
both worked well.
Next, Fig. 3 shows the measured transmission loss results. The test was done on
grassy grounds. A network analyzer was used to measure the transmission loss.
Two identical commercial whips (GLA systems) that operate at 30MHz were
used for the transmitter and the receiver. For the ground plane of the transmitter,
four different cases were measured: (i) no ground plane, (ii) ground radials
comprising of 8 arms with a diameter of 50cm, (iii) ground radials comprising
of 8 arms with a diameter of 120cm, and (iv) a spiral ground plane with a
diameter of 43cm. For the receiver, a 50cm × 50cm rectangular solid ground
plane was used. We can see that the transmission loss achieved by the 43cm-
diameter spiral ground plane is almost the same as that of the 120cm-diameter
ground radials. Moreover, the transmission loss of the 43cm-diameter spiral
ground plane is 6dB better than that of the 50cm-diameter radial ground plane
at the intended operating frequency of 30MHz. These results show that the
spiral ground radial is a good structure for achieving a miniaturized ground
plane.
With the monopole ground plane as our basis, the spiral ground radial idea can
be incorporated into the design of a miniaturized ground plane for our
electrically small monopole antenna [1]. However, the same spiral ground
radials that worked well for the quarter-wave monopole did not give
satisfactory results when the small top-loaded monopole was used. We believe
this is due to the fact that highly complex image currents exist on the infinite
ground plane in the original design. Once ground radials are used, the original
image currents are greatly perturbed and the operation of the top-loaded
monopole is affected. To alleviate this situation, we propose using a solid
ground in the projected area underneath the top loading, and run spiral radials
outside of this area. Fig. 4 shows the conceptual design. More research on this
problem is still ongoing.
Conclusion
Drastic transmission loss occurs when the ground-plane diameter is less than
λ/10. This excessive transmission loss is mainly caused by a mismatch loss as
the resonant frequency of the monopole shifts upward. This mismatch loss,
however, can be removed by using a spiral ground plane. A spiral ground plane
was designed, built, and measured with a 30MHz quarter-wavelength monopole
to maximize transmission into HF ground waves. Outdoor test results showed
that the transmission loss of a 0.043λ-diameter spiral ground plane is almost the
same as that of a radial ground plane that is three times larger than the spiral
ground plane. The same concept will be applied to miniaturized GA antennas
in hope of achieving a truly electrically small package.
Acknowledgments
This work is supported in part by the United States government.
References
[1] Lim, S., Choo, H., Rogers, R. L., Ling, H., “Electrically small antenna for
maximising transmission into HF ground waves”, IEE Electronics Lett.,
vol. 40, pp. 1388 – 1389, Oct. 2004.
[2] Freeman, R. L., “Telecommunications transmission handbook”, Fourth
Edition, John Wiley & Sons, 1998.
[3] McLean, J., Leuvano, M., Foltz, H., “Reduced-size, folded ground plane
for use with low-profile, broadband monopole antennas”, IEEE Radio and
Wireless Conference, pp. 239-242, Aug. 1999.
[4] Collins, S., Antar, Y. M.M., “Antenna size reduction using Yagi-Uda
loops and shorted circular patches”, IEEE Trans. Antennas Propagat.,
vol.52, pp. 855-864, Mar. 2004.
[5] Weiner, M., “Monopole element at the center of a circular ground plane
whose radius is small or comparable to a wavelength”, IEEE Trans.
Antennas Propagat., vol.35, pp. 488-495, May. 1987.
[6] Stutzman, W. L., Thiele, G. A., “Antenna theory and design”, Second
Edition, John Wiley & Sons, 1998.
0 0.5 1 1.5 2 2.5 3
-125
-120
-115
-110
-105
-100
-95
T.L. of 30MHz
T.L. of 100MHz
Transmission Loss [dB]
Diameter [m]
Fig.1 Ground plane size effect
on the transmission loss
Fig.2 Prototype of the spiral
ground plane
25 30 35 40 45
-55
-50
-45
-40
-35
-30
-25
no ground
radials with a diameter of 50cm
radials with a diameter of 120cm
spiral with a diameter of 43cm
Transmission Loss [dB]
Frequency [MHz]
Fig.4 Spiral ground plane with the
two-arm electrically small
antenna [1]
Fig.3 Transmission loss of four
different ground planes
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