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Design of short-range wireless event transmission devices, including, for example, security systems, medical emergency buttons and instrument fault detectors as well as remote keyless entry devices, has become relatively easy, thanks to the availability of complete transmit and receive modules and highly integrated chips. However, estimation of system performance, either during the stage of choosing the appropriate module or during actual system design, remains a difficult task.

One of the most important parameters for rating a wireless system is its range. However, range deduced from published sensitivity or power output specifications of components is likely to be deceiving. Even when the range is stated by the component manufacturer, its value in comparing between different products or in weighing the part's suitability for a particular system is dubious. Rarely does the range specification include the conditions of measurement, such as the characteristics of the transmitting and receiving antennas, orientation, height above ground, and the performance criteria for claiming that communication was effective at that range.

This article describes a relatively simple way to evaluate radiated power and over-the-air sensitivity of wireless event transmitters and receivers, which doesn't require sophisticated test equipment and a special radio test site. In addition to range estimation, the harmonic content of transmitters can be approximated in preparation for conformance testing to the appropriate national or regional standards.

There are two common ways to measure transmitted radiation for low power devices. One corresponds to the test methods described in the FCC regulations in the United States, the second is the specifications in force in the European Union. Most of the radiation limits in the FCC Part 15 regulations specify maximum field strength, in microvolts or millivolts per meter, at a given distance between the device under test and a calibrated measuring antenna. During the test, the device transmits and a test receiver indicates field strength directly.

On the other hand, the European specifications, exemplified by ETSI 300-220 (short-range devices between 25 and 1000 MHz) measure effective radiated power using the substitution method. First, the level of the transmission of the device under test is recorded by a receiver connected to a test antenna 3 or 10 meters away. Then the device under test is replaced by a half-wave dipole antenna driven by a signal generator. The signal generator output is adjusted until the test receiver indicates the same level recorded during transmission of the device under test. The effective radiated power of the tested device is then the power into the substitution antenna plus its gain in dB.

Equivalently, it's the signal generator output power less the attenuation, in dB, of the cable that connects the signal generator to the substitution antenna, plus dipole gain of 2.1 dB.

Although each test method specifies the results in different units, they can be compared (approximately) using the following relationship between field strength at a given distance, and the equivalent isotropic radiated power (eirp):

where E is the field strength in V/m at d meters, and P_eirp is the power in watts.

The equivalent isotropic radiated power is the power that would be radiated by a lossless isotropic antenna, that is, an antenna that radiates equally in all directions, in order to produce the measured field strength of the transmitter being tested. No physical antenna is purely omnidirectional, and both the European and the FCC regulations require finding the direction of maximum radiation by noting the output of the test receiver when the device under test is oriented in all usable positions.

The test is repeated for both horizontal and vertical polarization of the test antenna to find maximum radiated power. Also, for each recorded measurement in both methods, the test antenna height is varied between 1 and 4 meters to find the maximum indication on the receiver.

For receiver sensitivity measurements, the path gain (negative of the pass loss) must be estimated. One way is to use the free space path gain relationship in Equation 4. This method is appropriate when the distance between transmitting and receiving antennas can be measured accurately, and propagation between them indeed approximates free space conditions. Another way is equivalent to the substitution method described above, which doesn't require using the distance between transmitter and receiver. A signal generator feeding a calibrated antenna transmits to a measurement receiver or spectrum analyzer through an antenna with known gain, which replaces the receiver under test. Path gain is found directly from the difference in dB of received and transmitted powers, corrected by subtracting antenna gains.

Each of the two methods has its pros and cons. The European substitution method is not dependent on the distance between the device under test and the test antenna as is the FCC procedure. However, its accuracy depends on placing the center of the substitution antenna, typically a dipole, in the same position as the antenna of the device under test, which it replaces.

The aim of the tests described herein is to approximate radiated power and receiver sensitivity by simple and repeatable procedures that use commonly available test equipment. Free space path gain calculated from a known distance between transmitter and receiver is used in the test routines described.

Expressions for radiation contain terms that are commonly referred to as loss (mismatch loss, cable loss, path loss) or gain (antenna gain). It's easier and less confusing to add up terms of the same type, so all the terms in the expressions below are expressed as gain.

The tests should be carried out in a laboratory or other contained area, free of furniture. For testing systems in the 300 to 1,000 MHz range, the area should have minimum dimensions of 3 by 8 meters.

The transmitter and receiver tests require a test antenna for which gain and impedance are known, to some degree of accuracy, at the test frequencies.

In general, the best antenna for the tests is a half-wave dipole, whose elements are adjustable to be able to resonate at each of the harmonic frequencies. However, such a commercial, frequency adjustable test dipole with support tripod is expensive and not easy to construct. Additionally, this antenna will require a balun to ensure a balanced feed to the center of the dipole for each harmonic frequency, as well as support of the vertical dipole, which, with the coaxial feedline extending perpendicular to the elements is cumbersome.

Therefore, a workable alternative, which is easy to construct, is a quarter-wave antenna mounted above the center of a square metal ground plane. An RF connector, BNC or N Type, is mounted upside down in the center of the ground plane and a stiff wire element soldered to the connector terminal. Two edges are bent down for support. A simple antenna is shown in Figure 1, with dimensions in millimeters for 434 MHz. For other frequencies, the dimensions can be scaled by multiplying each of them by 434/f where f is the desired frequency in MHz.

Antenna gain can be estimated using an antenna evaluation or design program, such as EZNEC. Let's take as an example the antenna of Figure 1. It was modeled in EZNEC as shown in Figure 3 using 15 wires — one wire for the vertical element and 14 wires to approximate the ground plane. The antenna's vertical radiation patterns on the fundamental and second and third harmonic frequencies are shown in Figure 4. The patterns are oriented in the direction of the arrow in Figure 3.

EZNEC gives us the maximum antenna gain, impedance at the feedline connection, and voltage standing wave ratio (VSWR). From the VSWR we can find the power loss in dB, ML, due to impedance mismatch, from the following equation:

where S is the VSWR. The effective gain of the antenna, G_ant, in a 50Ω system, is then the antenna gain minus the matching loss.

The parameters of the example antenna are shown in Table 1.

The antenna is connected to the test equipment by coaxial cable whose losses at the test frequency must be accounted for.

Finding the attenuation of the cable is straightforward. Connect the cable between the signal generator and the spectrum analyzer, and note the difference between the signal generator output and the power input to the spectrum analyzer. Repeat for harmonic frequencies to be used in transmitter radiation tests. Signal generator modulation is turned off and its output set for a convenient value, say 0 dBm. In the expressions below, we use the cable gain, G_cable, which is the negative of the cable attenuation in dB.

Set up the transmitter near one end of the test area, and a spectrum analyzer and test antenna near the other end. A suitable setup may be as in Figure 2.

The tables are wooden and approximately 1 meter high. If the transmitter is a portable remote control device, it may be held by a person. Operate the transmitter and record the power in dBm on the spectrum analyzer at the fundamental frequency. The resolution bandwidth setting of the spectrum analyzer should be several times the transmitted bandwidth. Move the transmitter or the test antenna higher and lower and side to side to get the maximum radiation at the testing distance. Also, change the orientation of the transmitter for maximum response. Repeat the power measurement on harmonic frequencies using the same test antenna.

An estimate of the radiated power of the transmitter can be found using the following relationship of the gains from Figure 5:

P_T = P_R - PG + G_ant + G_cable

The isotropic path gain PG (the negative of path loss in dB) is estimated using the free space relationship:

where f is the transmission frequency in Hertz, d is the distance between the transmitter and the measuring antenna in meters, and c = 3 × 10⁸ meters/second, the speed of light.

Let's examine briefly the sources of error in the path gain that may be expected because our test setup is not under free space conditions. Reflections from the floor, walls, ceiling and furnishings will affect the power reading on the spectrum analyzer. Figure 6 is a simplified representation of the path gain taking into account only the reflected wave from the floor and assuming isotropic antennas that are 1 meter above ground. The path gain oscillates above and below the free space value with change in distance between the antennas. The amplitude of the oscillations depends on the conductivity of the reflecting surfaces and also on the antenna radiation patterns. To assure some consistency, the transmitter is raised and lowered and the highest reading on the spectrum analyzer is recorded.

The radiated power estimation routine using actual data from a remote control security transmitter is demonstrated. The distance between the transmitter and the measuring antenna was 4 meters. Results are presented in the last column of Table 2.

When the radiated power results of the remote control transmitter are compared to the relevant specified limits for certification for marketing in the European Union, we see that the second and third harmonic power levels are far above the allowed levels.

In the 434 MHz band, maximum fundamental radiated power is 10 dBm, while the limit at 868 MHz is -36 dBm and at 1302 MHz it is -30 dBm. Rework of the transmitter must be done in order to make it conform to the regulations. The test procedure described above is a useful tool during the development effort in preparing low power transmitters for certification.

In a somewhat similar manner, using the free space path loss relation of Equation 4, we can estimate the actual sensitivity of a receiver, including its built-in antenna.

More often than not, published receiver sensitivity figures relate to a performance parameter — signal to noise ratio or bit error rate usually — that is not directly relatable to what the receiver is intended to do. In a security system, for example, the receiver actuates an alarm when it receives a coded signal initiated by a remote sensor. In comparing receivers or estimating range, we want to know the received signal threshold for detecting an alarm or actuating a command. Thus, the signal generator that supplies the transmitted signal for the test must be modulated by the baseband code used in the actual system.

General purpose signal generators usually have provision for AM and FM modulation but are not suitable for the equivalent data modulation types — amplitude-shift keying (ASK) and frequency-shift keying (FSK). This is because the external modulation input of the signal generator is usually AC coupled while the data from the system's transmitter will often have a DC component. Even if the data itself is free of a DC offset, such as when Manchester encoding is used, there still will be a DC pulse at the beginning of the transmitted frame while the signal generator modulation input coupling capacitor charges to the average level of the input signal.

Many control and monitoring systems use ASK, so we describe below a suitable modulator to use for sensitivity testing. When FSK is used, the internal modulator will have to be employed, applying the baseband data to the external modulator input. In this case, the actual transmitter frequency deviation must be determined to set up the signal generator for proper modulation.

The ASK modulator shown in Figure 7 uses a remote control transmitter to provide the baseband data that will turn on and off the signal generator output through an RF switch. The digital data can usually be found on one of the pins of the microcontroller or code generator in the transmitter. It's important to disable the transmitter's RF output.

The receiver sensitivity is measured in two ways. First, sensitivity “on the bench” is checked with the antenna removed and the RF signal injected directly into the receiver input. This test is desirable because it allows a sensitivity comparison with the manufacturer's claims for the particular receiver under test, and with other similar receivers. Figure 8 is a block diagram of the bench top receiver sensitivity setup.

When the receiver has a standard 50Ω antenna connector, there's no problem in connecting the output of the modulation fixture to the receiver input. However, if a built-in antenna is used, or a short wire antenna that is soldered internally, that antenna must be temporarily removed and replaced with the coax cable from the modulation fixture. This will probably result in a mismatch to the receiver and attendant loss (assuming that the receiver RF amplifier input was properly matched to the internal antenna). If it can be assumed that the VSWR for the connection between the modulation fixture and receiver input is under 4, then the mismatch loss will be less than 2 dB.

The purpose of the sensitivity tests is to find the minimum signal strength needed for a given receiver response. Event-type receivers often have a relay output terminal or a visual indicator that responds when the desired signal is correctly received. It may be necessary to initially set up the receiver to recognize the transmitting device. When the input signal is on the threshold of the sensitivity, the indicating device may, or may not respond. Therefore, the sensitivity threshold is defined by how many correct responses are to be obtained in a given number of tries (eight out of ten, for example). In the ASK example we are using, the baseband transmitter that is part of the modulator of Figure 7 is actuated in short bursts, and the signal generator output is adjusted to the edge of the defined threshold.

This sensitivity is defined as:

P_RX = P_SG + G_SW + G_CABLE

In a test of a security system receiver, the modulating fixture gain GSW was -0.5 dB and cable gain including mismatch was -2 dB. The signal generator threshold output was -95.5 dBm. The receiver sensitivity, from Equation 5, was -98 dBm.

The over-the-air sensitivity test takes into account the efficiency of the receiver antenna, but it also is affected by the interference background present on the receiving frequency and within the receiver bandwidth. The test setup is shown in Figure 9. It is similar to Figure 5, with the spectrum analyzer replaced by the signal generator and modulator fixture, and the transmitter replaced by the receiver.

The signal generator output is adjusted for threshold level as in the previous test, and the sensitivity is determined from:

P_RX = P_TX + PG + G_ANT + G_CABLE + G_SW

The reduction of sensitivity shown in Table 3, relative to the bench top sensitivity of -98 dBm - 6.3 dB, is due to the negative gain of the receiver's simple built-in wire antenna and the fact that the background noise is greater than the thermal noise that affects the sensitivity measurement when the signal generator is connected by cable to the receiver input.

We can use the receiver sensitivity result P_RX and the transmitter fundamental power output P_TX to estimate the transmission range in an open unobstructed field.

Copying these results: P_RX = -91.7 dBm; P_TX = -3 dBm.

The minimum path gain for threshold performance is: PG = P_RX - P_TX = -88.7 dB.

The graph in Figure 10 shows the isotropic path gain vs. distance over an open field with receiver and transmitter antenna heights of 1.5 meter and vertical antennas. It takes into account the vectoral combination of the direct line-of-sight signal and the signal component reflected from average countryside clay ground. The distance which gives the minimum path gain is found to be 259 meters.

One cannot expect to get the accuracy of transmitter power and receiver sensitivity measurements, as would be possible, in a commercial test laboratory. The main reasons for this are: The test area physical characteristics are not controlled. Reflections from floor, walls, and ceiling and surrounding furnishings will affect the results in an undeterminable manner; the test antenna gain and impedance are not accurately known; and the orientation of the transmitter under test and its height are not controllable to the degree they would be in a commercial laboratory.

Although their results lack high precision, the tests described above are a useful aid for designers of short-range devices as well as those who need to choose commercially available RF modules or integrated circuits for incorporation in wireless communication products.

Bensky, Alan, Short-range Wireless Communication, LLH Technology Publishing, 2000.

Alan Bensky received a Bachelor of Electrical Engineering and Bachelor of Arts degrees from Union College in Schenectady, N.Y., and M.Sc.E.E. from the Technion, Haifa, Israel, specializing in information theory. After many years holding senior engineering and management positions at military communications equipment manufacturing firms, he became an independent consultant, carrying out development projects in short-range radio communication. Recently, he developed algorithms and designed prototypes for short-range distance measuring devices based on frequency hopping spread spectrum modulation, for which three patent applications are pending. Bensky is a senior member of the IEEE. He can be contacted at abensky@ieee.org.