The very mention of RF design is usually enough to scare all but the most confident designer. However, wireless specialists such as the company I work for, Nordic Semiconductor, have worked hard to ensure it's no longer solely the domain of the RF expert. Performance optimized transceivers, and the availability of development kits and reference layouts, make it possible for competent electronics design engineers to incorporate wireless hardware into their latest products.
Nonetheless, although the availability of off-the-shelf components and reference designs has made wireless system design a little easier, the designer still needs to acquire some fundamental knowledge about the parameters that influence a wireless link's performance.
A critical issue is communication reliability: how will parameters such as sensitivity, output power, adjacent channel selectivity, and operating frequency influence system performance? In other words, what is the probability of transmitting/receiving an error free data packet in the presence of other radio sources that could interfere with the signal?
A second equally critical issue is range. The designer has to ensure that the radio can operate over its stated range in a number of different operating environments. Given output power and sensitivity, what other parameters affect range? Environmental factors such as air humidity, obstacles such as people and furniture, building materials, and metal film sun-screened windows limit useful range and choice of antenna implementation.
The designer should also be very wary of taking the data sheets of some wireless component vendors at face value. For example, if sensitivity is measured for a lower data-rate than the stated maximum, then it's worth asking why. In a second example, enquire as to whether the effective data-rate stated for systems requires Manchester encoding. Checking just a few fundamental characteristics such as these may save much time and frustration later when realizing that the circuit chosen does not comply with the system specifications.
Fundamentals of a wireless link
A wireless link includes a transmitter with antenna, a transmission path, and a receiver with antenna. The key performance parameters of this simple link are the output power (Pout) of the transmitter and the sensitivity of the receiver (see Figure 1).
Figure 1: Schematic of a typical point-to-point wireless link
Sensitivity is the minimum received power that results in a satisfactory bit error rate (BER, usually 10-3) or one bit error for every 1000 bits transferred at the received data output (i.e., correctly demodulated).
The difference between received signal power and the receiver sensitivity limit is the transmission link margin or "headroom." Headroom is reduced by a number of factors, such as transmission path length, antenna efficiency, carrier frequency, and obstructions in the transmission path (see Figure 2).
Figure 2: Headroom is reduced by a number of factors such as transmission path length, antenna efficiency and obstructions in the transmission path.
Note that sensitivity and output power quoted in the RF-circuit datasheets are given for the load impedance optimized for the input low noise amplifier (LNA) and the output power amplifier. This means that the impedance of the antenna must be equal to the load stated in the datasheet, otherwise a mismatch and subsequent loss of headroom occurs. (A typical matching network introduces around 1 to 3dB of attenuation.)
The antenna transforms the transmitter output power into electromagnetic energy, which radiates from the antenna in a radiation pattern determined by the antenna geometry. For the license-free bands such as 915MHz in the US, 434 and 868MHz in Europe, and the global 2.4GHz band, the maximum output power is expressed as effective isotropic radiated power (EIRP).
An "isotropic radiator" is defined as a hypothetical lossless antenna radiating equally in all directions. This means the regulations governing the license-free bands (issued by ETSI) do not allow transmission range to be boosted by using a directional antenna. If the antenna gain is larger than 1 (0dB) in any given direction, the output power has to be decreased accordingly.
For example, for a radio operating in the 2.4GHz band, the maximum transmission power is 25mW (14dBm) EIRP. A directional antenna that has 10dB gain in a given direction would seem as if it was transmitting +24dBm for a receiver positioned in this direction. Thus, the output power would have to be reduced to +4dBm to fulfill the ETSI requirements. Note that a directional antenna can be used for receivers without any penalty.
Calculating antenna gain and radiation pattern is generally quite complex, and its local environment heavily influences the resulting radiation pattern. Placing the antenna close to conducting surfaces is likely to distort the antenna's radiation pattern and efficiency, but is virtually unavoidable for most practical applications.
Calculating antenna size
The antenna's ability to transform the output power into radiated energy is represented by the parameter Gant. Antenna gain is generally proportional to physical size, in accordance with the following formula from antenna theory:
Gant = (4. π . Ae)/ λ2
where Ae is the effective area of the antenna and λ is the wavelength of the carrier frequency. For the 2.4GHz band, the wavelength is approximately 0.125m. From the formula, the necessary effective area to achieve an antenna gain of 1 (0dB) is 0.00124m2 (12.4 by 12.4cm).
For most practical applications, an antenna this size would be too voluminous and cumbersome, so designers settle on something a bit smaller. Many designers opt for a so-called quarter wave antenna because it is the smallest practical antenna for portable 2.4GHz designs. The theoretical length of a quarter wave antenna would be λ/4, or 3.125cm.
A popular small, low-cost antenna for low power radio applications, often called a "meandering" or "crank-like antenna" (CLA), forms part of the PCB and does not represent a significant cost other than PCB area. The ratio of the length of the antenna to its diameter together with other variables such as the PCB substrate may affect the final antenna length. A CLA is usually a little longer than a quarter-wave antenna, and the CLA's performance is dependent on its geometry and relationship to the PCB's ground plane. It is suggested that the designer starts with an antenna slightly longer than calculated, and then shorten it until resonance occurs.
Figure 3 shows the antenna on the PCB used for the dongle of Nordic's nRF24LU1 compact USB reference design. This antenna has a typical efficiency (or gain) of approximately -20 to -25dB. Notice that by making the antenna much smaller than the unity gain dimensions (12.4 by 12.4cm) the gain is diminished. Consequently, the antenna typically represents a loss in the transmission "budget."
Figure 3. Meandering type antenna (extreme right of PCB) of Nordic's nRF24LU1 compact USB reference design.