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Tackling FM antenna design roadblocks

Posted: 12 Mar 2009 ?? ?Print Version ?Bookmark and Share

Keywords:antenna design FM? sensitivity reception maximizing? portable mobile?

Maximizing efficiency

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, Cres, is used to resonate the antenna at the desired frequency:

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/Zant, where Zant is the impedance of the antenna with the antenna resonance network. Zant is written as:

When the antenna is resonated, the efficiency, , can be written as:

At other frequencies:

At frequencies other than the resonant frequency, ?res, the antenna efficiency, , is lower than the maximum efficiency, res, since the antenna input impedance, Zant, 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.

Accommodating all FM bands

In most countries, the FM broadcast band is in the frequency range of 87.5-108.0MHz. In Japan, the FM broadcast band is 76-90MHz, and, in some eastern European countries, the FM broadcast band is 65.8-74MHz. To accommodate all FM bands worldwide, a 40MHz 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 40MHz, 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.

Fig. 2 shows an efficiency plot for two antennas (a headphone antenna and a short antenna) with fixed resonance at the center of the band (98MHz).

Fig.2: Typical fixed resonance antenna performance in FM band

From Fig.2, 98MHz 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:

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:

where ?c is the resonant frequency, and BW is the 3dB 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.

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