Antenna Behaviour

in the Presence of Human Body

Ettore Lorenzo Firrao, Anne Johan Annema, Bram Nauta
University of Twente, MESA+ Research Group, IC-Design group
P.O. Box 217, 7500 AE Enschede, The Netherlands
Phone: +31 (0)53 489 42 85, Fax: +31 (0)53 489 10 34
E-mail: e.l.firrao@el.utwente.nl

Abstract – Mobile phones are widely used nowadays. The due to the impossibility of the RF power amplifier to
demand in the new generations of mobile phones is better transmit the required power level. As it is well-known
performance. Typically the mobile phone performance is both of them are a strong function of the load (antenna)
derived assuming an ideal antenna impedance of 50Ω. impedance.
Some work has been done in understanding the behaviour In order to predict the real mobile phone performance
of the antenna impedance in a real environment [1], [2],
and to improve it, it is relevant to understand the antenna
[3]. This work continued the exploration of the difference
between the ideal antenna impedance and the antenna behaviour in presence of human body. The analysis is
impedance in a real environment. In particular the analysis addressed in this article. It is organised as follows.
was carried out measuring the antenna impedance in SECTION 2 starts with a brief analysis of the impact of
presence of the human body and relating it to the position the antenna impedance on a simplified RF front end.
of the hand and of the hand + head. Two types of antennas Then the antenna behaviour is described. The section
were considered: PIFA antennas and ceramic antennas. ends giving a simplified model valid also in case of the
Several positions of the hand and of the hand + head were presence of human body. Finally some conclusions are
considered. These positions were selected after an accurate given in SECTION 3.
study about how people on the street normally handle their
mobile phones. The study consisted of two steps: 1)
observation of the typical positions of the hand and of the
hand + head with respect to the mobile phone; 2) their II ANALYSIS
imitation in the lab for the measurements
The outcome of the investigation was the changing of the A. Motivation
impedance is primary due to a changing of its imaginary
part. The real part is affected but not in dramatic way. A simplified block diagram of an RF front end for dual
band GSM mobile phones is sketched in figure 1. As
Keywords – antenna behaviour, antenna retuning, human mentioned in the introduction part, during the design the
body interaction, mobile phone performance. typical approach is to consider the antenna impedance
equal to 50Ω. The importance of the antenna impedance
can be evaluated deriving the behaviour of the RF power
I INTRODUCTION amplifier under different load conditions. This can be
achieved in a general way using the scattering parameters
In the present days the use of mobile phones has become [1]. In the particular case, when the optimum load
more and more popular. All the selling specifications, impedance of the RF power amplifier is real, the
such as talk time, are derived assuming an ideal scattering parameters can be reduced to normalised
behaviour of the antenna. During the design the typical scattering parameters [1]. In this case
approach is to assume the antenna impedance 50Ω. In
reality, due to the presence of the user, the antenna
ΓRFPA = Γantenna * e − j 2θ (1)
impedance differs from 50Ω resulting in a degradation of
performance (shorter talk time and/or call lost). The
shorter talk time is the result of a lower efficiency of the where θ is the phase shift between the output of the RF
RF power amplifier draining more power from the power amplifier and the antenna connector/input.
battery in order to keep the connection. The call lost is

487
 RX GSM 900

antenna
TX GSM 900

RF output
power matching
amplifier network
RF input
from transceiver
ΓRFPA
RX GSM 1800
Γantenna

TX GSM 1800

RF output Figure 3: ceramic antenna demo used during the

power matching switch measurements.
amplifier network or
switchplexer
C. Antenna measurements
Figure 1: simplified block diagram of an RF front end for
dual band GSM mobile phones.
Several positions of the hand and of the hand + head
were considered. These positions were found after an
accurate investigation about how people normally hand
In relationship with the particular RF power amplifier
their mobile phones. The most common six positions are
chosen the output power and the efficiency can be also
summarised in table 1. For simplicity only the pictures
derived [2], [3], [4]. In some situations with a VSWR =
for the PIFA antenna are sketched. The same positions
2:1 a drop of 3 dB was recorded for the maximum output
were considered for the ceramic antenna and also with
power. The efficiency was also half of the nominal value.
the presence of the hand + head. The antenna
The situation was even worse for higher VSWR.
performance was compared with a reference environment
denoted as “free space” shown in figure 4.
The measurements were performed using the HP 8510
B. Antenna samples
Vector Network Analyser. During the measurements the
following procedure was adopted: measurements of the
In the analysis two antennas widely used in mobile
reflection coefficient at the samples connector and post-
phones were considered: PIFA antennas and ceramic
processing of the data in order to plot the reflection
antennas. The two samples used in the measurements are
coefficient at the antenna connector (removing thus the
shown in figure 2 and figure 3, respectively.
piece of transmission line on the PCB). The RF
properties of the transmission lines were found using the
physical dimensions and the electrical properties of the
PCB.
The outcome of the investigation is summarised in figure
5, figure 6 and figure 7. Figure 5 and Figure 6 are about
the VSWR vs frequency. These two figures clearly show
the detuning effect because of the presence of the human
body. Figure 7 describes the area covered on a Smith
chart by the antenna impedance of the PIFA antenna and
of the ceramic antenna in GSM 900 and in GSM 1800
bands.
The overall ranges for the real and imaginary part of the
antenna impedance were:
Figure 2: PIFA antenna demo used during the
measurements. Rload ∈ [30Ω, 100Ω] (2)

X load ∈ [0Ω, 100Ω] (3)

488
 VSWR as a function of the frequency
free space
10
9 hand + head in
8 position 1
7 hand + head in

VSWR
6 position 2
5 hand + head in
4 position 3
3 hand + head in
2 position 4
1 hand + head in
880 900 920 940 960 position 5
hand + head in
frequency [MHz]
position 6

Figure 5: VSWR vs frequency for the PIFA antenna in

GSM 900 band.

Figure 4: “free space” environment for the antenna

measurements VSWR as a function of the frequency
free space
10
Table 1: positions of the hand 9 hand + head in
8 position 1
7 hand + head in
VSWR

6 position 2
5 hand + head in
4 position 3
3 hand + head in
2 position 4
1 hand + head in
1600 1700 1800 1900 2000 position 5
hand + head in
frequency [MHz]
position 6

hand in position 1 hand in position 2 Figure 6: VSWR vs frequency for the ceramic antenna in
GSM 1800 band.

Im Γ

hand in position 3 hand in position 4

Re Γ
r=2 r=3
r=0.5 r=1

hand in position 5 hand in position 6

Figure 7: overall area covered on a Smith chart by PIFA
and ceramic antennas in presence of the human body.

489
 [7] Mohab A. Mangoud, Raed A. Abd-Alhameed and
D. Antenna model Peter S. Exceel, “Simulation of Human Interaction with
Mobile Telephone Using Hybrid Techniques Over
A typical simplified circuit model for describing the Coupled Domains”, IEEE Transactions on Microwave
antenna impedance behaviour around the resonant Theory and Techniques, Vol. 48, No. 11, November
frequency is an RLC series or parallel tank. For the two 2000;
investigated antennas a proper model is a series RLC [8] Constantine A. Balanis, “Antenna theory: analysis
tank [8]. and design”, John Wiley & Sons, 1982;
The investigation showed that the same model can be
extended to the antenna in presence of the human body.
The only difference between the free space model and
this model is a new set of parameters (new L, C and R
values).

III CONCLUSIONS

The antenna behaviour and the RF front end performance

were analysed. The conventional approach of using an
ideal load is not correct to predict the overall mobile
phone performance in presence of the user/human body.
In fact this investigation shows that the antenna
impedance can significantly differ from the idea value
(50Ω). The outcome of the investigation was the
changing of the impedance is primary due to a changing
of its imaginary part. The real part is affected but not in
dramatic way.

ACKNOWLEDGEMENT

Henk de Vries is acknowledged for the useful

discussions. Philips Semiconductor Nijmegen is
acknowledged for providing the two antenna samples.

REFERENCE

[1] Robert E. Collin, “Foundations for Microwave

Engineering”, McGraw-Hill, 1966;
[2] Steve C. Cripps, “RF power amplifiers for wireless
communications”, Artech House, 1999;
[3] Steve C. Cripps, “Advanced techniques in RF power
amplifier design”, Artech House, 2002;
[4] Mihai Albulet, “RF Power Amplifiers”, Noble
Publishing, 2001;
[5] Gert F. Pedersen, Jesper O. Nielsen, Kim Olesen and
Istvan Z. Kovacs, “Measured Variation in Performance
of Handheld Antennas for a Large Number of Test
Persons”,
[6] R.A. Sadeghzadel and N.J. McEwan, “Prediction of
Head Proximity Effect on Antenna Impedance Using
Spherical Waves Expansions”, Electronics Letters, Vol.
30, No 17, August 18th 1994;

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