Solving FM Antenna Design Challenges
in Portable Devices
Natalian Zhai, Silicon Laboratories Inc.
Introduction
FM radio has witnessed an explosion of interest from the market for its applications in mobile and personal media
players; however, the traditional FM design approach requires a long antenna, such as a wired headphone, which
limits its usefulness and usability. This whitepaper provides a description of an FM radio receiver solution that
enables the antenna to be integrated or embedded inside the portable device enclosure, making the headphone
cable optional. This paper covers techniques for maximizing sensitivity, including maximizing efficiency at the
resonant frequency and maximizing efficiency across the FM band with a tunable matching network.
Maximizing Sensitivity
Sensitivity can be defined as the weakest signal that an FM receiver system can receive while achieving a certain
signal-to-noise ratio (SNR). It is an important parameter of FM receiving system performance and is related to both
signal and noise. The received signal strength indicator (RSSI) indicates only the RF signal strength at a particular
tuned frequency. It does not provide any information about noise or signal quality. The audio signal-to-noise ratio
(SNR) is perhaps a better measure for comparing receiver performance with different antennas. Therefore,
maximizing SNR is essential for listeners to experience good audio quality.
Antennas are the connection between the RF electrical circuits and electromagnetic waves. For FM reception, an
antenna is a transducer that converts energy from electromagnetic waves to a voltage that can be used by an
electrical circuit, such as a Low Noise Amplifier (LNA). The sensitivity of an FM receiving system is directly related
to the electrical voltage received by the internal LNA. To maximize sensitivity, the electrical voltage must be
maximized.
There is a variety of antennas, including headphone, stub, loop, and chip antennas, on the market, but all antennas
can be analyzed using equivalent circuits. Figure 1 shows a generalized equivalent antenna circuit model:
Rrad
X
Rloss
Figure 1: Antenna Equivalent Circuit Model
In Figure 1, X can be either a capacitor or an inductor. The choice of X is determined by the antenna topology,
where the value of the reactance (inductive or capacitive) is related to the antenna geometry. The loss resistance,
Rloss , is related to the power dissipated in the antenna as thermal energy. The radiation resistance, Rrad , is related
to the voltage generated from the electromagnetic wave. For simplicity, we will analyze the loop antenna model in
the remainder of this article. Similar calculations can be made for other antenna types, such as the short monopole
and headphone antennas.
Maximizing Efficiency at the Resonant Frequency
In order to maximize energy from the antenna, a resonant network is used to cancel out the reactive impedance of
the antenna, which would otherwise attenuate the amount of voltage the antenna transfers to the internal LNA. For
an inductive loop antenna, a capacitor, C res , is used to resonate the antenna at the desired frequency:
f res 
1
2 LC res
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The resonant frequency is the frequency at which the antenna most efficiently converts an electromagnetic wave to
voltage. The antenna efficiency is the ratio, of the power through Rrad to the total power collected by the antenna
and can be written as Rrad / Z ant , where Z ant is the impedance of the antenna with the antenna resonance
network. Z ant is written as:

1
Z ant  Rrad  Rloss  j  2fL 
2fC res

When the antenna is resonated, the efficiency,



 , can be written as:
 res 
Rrad
Rrad  Rloss
At other frequencies:
Rrad

Rrad
 Rloss 
2

1
  2fL 
2fC res




2
At frequencies other than the resonant frequency, f res , the antenna efficiency,
 , is lower than the maximum
efficiency,  res , since the antenna input impedance, Z ant , is either capacitive or inductive.
Maximizing Antenna Size
To recover a transmitted radio signal, the antenna must collect as much energy as possible from the
electromagnetic wave and efficiently convert it into voltage through Rrad . The amount of energy collected is limited
by the available space and size of antennas used in portable devices. For traditional headphone antennas, it can
be as long as a quarter wavelength of the FM signal, which collects sufficient energy to convert to a voltage that
can be used by the internal LNA. Consequently, it is less important to maximize the efficiency of the antenna.
Because portable devices are getting smaller and thinner, the space allowed for an embedded FM antenna is very
limited. It is still important to maximize antenna size, but the energy collected by an embedded antenna is small.
Therefore, to use smaller antennas without sacrificing performance, improving antenna efficiency,  , becomes
very important.
Maximizing Efficiency across the FM Band with a Tunable Matching Network
In most countries, the FM broadcast band is in the frequency range of 87.5 to 108.0 MHz. In Japan, the FM
broadcast band is 76 to 90 MHz, and, in some eastern European countries, the FM broadcast band is 65.8 to 74
MHz. To accommodate all FM bands worldwide, a 40 MHz bandwidth is required for an FM receive system.
Traditional solutions usually tune the antenna at the center frequency in the FM band. However, as shown in the
above equations, the efficiency of the antenna system is a function of frequency and reaches its maximum at the
resonant frequency, dropping as the frequency is moved away from the resonant frequency. Again, since the
worldwide FM band can be as wide as 40 MHz, antenna efficiency can decrease significantly at frequencies far
from the resonant frequency.
For example, setting a fixed resonant frequency of 98 MHz gives good efficiency at this frequency point, but
efficiency at other frequencies drops significantly, degrading FM performance the further one moves from the
resonant frequency.
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The graph below shows an efficiency plot for two antennas (a headphone antenna and a short antenna) with fixed
resonance at the center of the band (98 MHz).
Headphone antenna
Short antenna
h
Loss
88
98
Frequency [MHz]
108
Figure 1: Typical Fixed Resonance Antenna Performance in FM Band
From the above graph, 98 MHz achieves the best efficiency, but the efficiency degrades closer to the band edges.
This is not a significant issue for the headphone antenna since the antenna is large enough to collect sufficient
electromagnetic energy to transfer a significant voltage to the RF receiver across the whole band; however, the
short antenna is small and collects less energy compared to a longer headphone antenna, and the efficiency also
rolls off faster as the frequency moves away from resonance. This can present a problem for reception at the band
edges using fixed resonance. This is primarily due to the fact that a short antenna will likely have a higher “Q” than
a headphone, resulting in the sharper drop at the band edges.
The quality factor, Q, is proportional to the energy stored in the antenna network to the energy lost or radiated, per
unit time. For the above antenna equivalent circuit with an antenna resonated network, Q follows below:
2fL 
Q
Rrad
1
2fC res
 Rloss
A headphone antenna has inherently higher radiation resistance, Rrad , than a short antenna due to its larger
geometry, resulting in a lower Q than the short antenna. The issue of efficiency roll-off is very pronounced with the
short high-Q antennas required for embedded implementations.
The antenna’s Q is also related to the bandwidth of the antenna. This relationship is given as:
Q
fc
BW
where f c is the resonant frequency, and BW is the 3 dB bandwidth of the antenna. A short high-Q antenna has a
smaller BW compared to a long headphone antenna and increases losses at the band edges.
To overcome the bandwidth limitations of a high-Q, fixed-resonance antenna, a self-tuning resonant circuit is used
to change “fixed resonance” to “tuned resonance” so that the circuit is always at the resonant frequency for
maximum sensitivity. A higher SNR is achieved with a self-tuning resonant antenna because the gain from the
resonant antenna lowers the system noise figure of the receiver, and the inherent high Q of the embedded antenna
helps filter interference that could mix with harmonics of the local oscillator.
Silicon Laboratories’ Implementation of a Tunable Matching Network
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Figure 2 shows a conceptual block diagram of the Silicon Laboratories’ enhanced FM receiver Si4704/05
architecture used to support a short embedded antenna. The “tuned resonance” is implemented using a patentpending, on-chip tunable varactor and tuning algorithm.
Figure 2: Conceptual Block Diagram of the Si4704/05
The Si4704/05 utilizes a patented mixed-signal, digital, low-IF architecture with a digital signal processor (DSP) to
implement advanced signal processing algorithms including self tuning of short embedded antennas. The antenna
algorithm automatically adjusts the capacitance value of the varactor with each frequency tune of the device for
best performance.
For example, if the user tunes to 101.1 MHz (station 1 in Figure 3), the antenna algorithm will tune the antenna
circuit resonance to 101.1 MHz, thus optimizing the efficiency of the antenna and the resulting Rx performance at
101.1 MHz. When the user tunes to 84.1 MHz (station 2 in Figure 3), the antenna algorithm will retune the antenna
circuit resonance such that it is optimized at 84.1 MHz.
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Figure 3. Tunable Resonance Benefits
Tuning the antenna resonance with the tuned frequency maximizes the received signal strength across the entire
FM band by providing the maximum efficiency at each given frequency. As a result of tuned resonance,
performance using an embedded antenna is improved across the band. Resonating the antenna at the desired
frequency also attenuates interference at other frequencies, significantly increasing the selectivity of the receiver.
As a result, consumers using an Si4704/05 FM receiver with an embedded antenna will have less interference from
unwanted sources. This is extremely important in urban areas with congested FM frequency bands.
Summary
As wireless usage models become more popular in portable devices, more customers are demanding wire-free FM
radio reception with embedded antennas while listening with either a wireless headset or a speaker output. This
paper discussed the goal of maximizing sensitivity to improve FM reception using an embedded antenna and
further discussed the methods to achieve that goal. Based on the limited space available in portable devices for
embedded antennas, a self-tuned resonant network maximizes the sensitivity of the receiver over the FM band and
keeps the short antenna at maximum efficiency on each frequency.
The Si4704/05 is Silicon Laboratories’ latest enhanced FM receiver and implements a self-tuned resonant network
with a patented advanced signal processing algorithm. The antenna algorithm automatically tunes the capacitance
value of the varactor to ensure the antenna stays at its highest efficiency with each frequency tune of the device,
ensuring optimum performance. The Si4704/05 is the world’s first FM receiver that enables wire-free reception of
FM Radio signals to meet the demands of future portable devices. It improves the efficiency of any antenna and
gives board designers more freedom in choosing an antenna.
For more information, please visit www.silabs.com/broadcast.
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