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As operational frequency increases, parameters of lines, circuits, and traces on PCB will play a critical role, because geometrical sizes become comparable with wavelength. Geometrical sizes help users determine their physical sizes (length and width). As an example, waves lengths from 433 MHz to 5.5 GHz (Table 1-1).
Frequency, MHz | Wave length, λ, cm | Wave length, λ/4, cm |
---|---|---|
433 | 69.23 | 17.30 |
868 | 34.53 | 8.63 |
2440 | 12.29 | 3.07 |
5500 | 5.45 | 1.36 |
Lines and traces change their operational behavior while operational frequency increases and have not only an active resistance. Depending on frequency, inductive or capacitive reactance can be dominant and their values varies. For high frequencies, use the impedance or “Z” (meaning active with reactive resistances). Resistance (R) doesn’t change with the frequency, but reactance (±jX) does change. This means the impedance (Z) varies by frequency.
Standing wave ratio ( SWR) is an important antenna parameter. SWR represents an antenna’s impedance matching (tuning) with a transceiver (or cable, line or circuit). Engineers can use the parameter for understanding how the antenna is matched (good or poor).
The lower the SWR value, the better antenna matched (usually 50 Ohm, 75 Ohm for TV networks). Therefore, the maximum transmitting power is radiated by the antenna. The lowest SWR value is 1:1. All of the power radiates and no power reflects back to the transmitter. In reality, the SWR value cannot reach 1:1 because of parasitic components. Optimum SWR for small antenna placed on PCB, considered for each dedicated case, are normally less than 2:1. Operation bandwidth of the antenna also normally measures at SWR level of 2:1.
In addition, there is a parameter called S11 – reflection coefficient (or return loss). It represents how much power reflects back from the antenna to transmitter. Ideally, S11 has infinite value, meaning there is no reflected power and all power is radiated by the antenna. If S11 is 0 dB, there is no radiated power as it all reflects back to transmitter.
For better visibility of how SWR value relates to reflected power from the antenna back to transmitter you can use Table 2-1.
SWR Value | Reflected Power Percentage |
---|---|
1.001:1 | 0.00 |
1.100:1 | 0.22 |
1.200:1 | 0.83 |
1.300:1 | 1.70 |
1.400:1 | 2.77 |
1.500:1 | 4.00 |
1.600:1 | 5.32 |
1.900:1 | 9.62 |
2.000:1 | 11.10 |
2.500:1 | 18.36 |
3.000:1 | 25.00 |
5.600:1 | 48.56 |
7.000:1 | 56.25 |
10.000:1 | 66.94 |
20.000:1 | 81.86 |
Using Table 2-1, users can calculate how much power radiates from the antenna. For example, measured SWR value is 2:1. This means, the antenna will radiate only 88.9% (100% - 11.10%) of transmitter’s power. For battery powered devices, this can be a huge problem because with higher transmitting power, the transmitter is more power hungry. If the antenna was better matched, the lower transmitter’s power can achieve the same range. That is, we can save a battery for longer operation.
As an example, a free space SWR value of matched to a 50-Ohm SMD antenna PULSE W3013 for ISM 868-MHz band is shown in Figure 2-3.
At frequency of 868-MHz SWR value is 1.58:1 (marker M1). Table 2-1 shows that around 5% of transmitter’s power will be lost.
Antennas are often mounted into a plastic case. For modern devices, this can be a real problem as they become smaller in size, mounting the antenna very close to the plastic. The plastic can be a huge load and detune the antenna. The level of detuning depends on plastic’s chemistry and carbon level. Let’s have a look at real example shown in Figure 2-4. It shows the same PULSE W3013 antenna mounted very close to the plastic case (distance of 1 mm).
The resonant frequency shifted from 868 MHz down to 862.875 MHz (marker M2). At 868-MHz frequency, the SWR value is now 2.53:1 (marker M1). The device loses 18% power. Through a small frequency shift of 5.1 MHz, losses increased significantly (also because this particular antenna is narrow band). Users cannot predict how much the plastic will detune the antenna, so take measurements.
Another example is human body devices. Human’s body also affects the antenna. The Figure 2-5 shows an example for the same 868 MHz antenna mounted into the plastic package and placed in hand. The resonant frequency got shifted down to 847 MHz (marker M3) and SWR value at 868 MHz frequency now is 6.61:1. The power loss is around 50%.
For the antenna’s resonant frequency shifting, simply change the physical length. To get a higher operation frequency, make the length shorter (cut off). To get lower operation frequency, make the antenna longer. It’s a good idea to have some extra length during prototyping for cutting if needed. For PCB antenna, use a sharp knife or solder a piece of copper foil.
The following example shows users how to make an experimental helical antenna for 868-MHz band using a copper wire from a regular power cable (Figure 2-6).
Make the antenna’s length a little bit longer to have some extra length for frequency tuning (Figure 2-7). This example used a board from CC-ANTENNA-DK kit as it has SMA connector and matching network to save time connecting to VNA.
Figure 2-8 shows the resonant frequency of the antenna at 744 MHz (marker M1), far from 868 MHz.
To shift the frequency up to 868 MHz, cut very small pieces off the wire until the antenna is tuned to 868 MHz (Figure 2-9 (tracking marker M2)). The SWR value is 1.30:1. Marker M1 is showing the resonant frequency before changing the antenna’s length to represent the shift.
Figure 2-10 shows the final length of the antenna for 868-MHz band.
In high frequency electronics, standard values of impedance are 50, 75, and 100 Ohm. Most manufacturers of test equipment, cables, connectors, and antennas use these values.
Transceiver data sheets or evaluation board schematics usually have a matching circuit for transforming a non-standard transceiver’s impedance to a standard value of 50 Ohm. Rarely, the antenna itself has an impedance value of 50 Ohm. That is why matching components for the antenna are usually on the board. For an example, view the evaluation board schematic for dual band CC1350 transceiver.
Passive components L32, C71, and C23 are for matching 868-MHz antenna impedance (Figure 3-1) to 50 Ohm. Values of 10 pF and 5.1 nH are for this specific antenna on the board. Values can be different for another type of antenna, circuit configuration, and components. Capacitor C15 (right pin) is inline with the 50 Ohm impedance value.
For Figure 2-7 and Figure 2-10, matching components exist (Z111-Z113). Figure 3-2 shows the zoomed in network. This matching network topology is also called the “Pi-network”, but passive components have “L-network” configuration. In addition, “T-network” type can be used.
Texas Instruments also manufactures modules, for example, Wi-Fi module WL1837MOD and CC3220MOD. These modules have 50 Ohm antenna connectors. WLAN antenna should have a 50 Ohm value to get proper SWR level, bandwidth, and communication range.
A main goal of antenna's impedance matching is compensating for capacitive or inductive reactance and transforming the impedance as close to an active 50-Ohm value as possible.
In the past, such calculations were done thru S (Scattering) parameters matrix (S11 is a parameter for an antenna). It was a very complex job, but in 40s of XX century, two independent engineers A. Volpert and Ph. Smith designed graphical charts for plotting impedances (or complex reflections) values in a visible R ± jX format. Most modern vector network analyzers represent the impedance in this type of chart.
A simplified chart is shown in the Figure 4-1. It’s a chart with circles and arcs. Circles represent an active resistance, arcs represent an inductive or capacitive reactance. Depending on where the measurement result is, users can visibly see what mode of impedance of the antenna is under test.
The center of the chart is a totally matched impedance of 50 Ohm (it also can be 75 or 100 Ohm depending on what type of impedance users need) without any reactance. A horizontal axis thru the center of the chart shows an active resistance value from a short circuit (on the left) to open circuit (on the right). Top and bottom parts of the chart represent an inductive or capacitive reactance. Ideally, values should reach the center of chart and minimize or cancel the inductive or capacitive reactance.
Let’s have a look at example of impedance of a real 868-MHz band antenna shown in the Figure 4-2. Measurements made at three specific frequencies. Two of them at SWR level of 2.0:1 and one directly at 868 MHz ( at frequency of interest). An impedance at this frequency is 59 – j21.71 Ohm and has a capacitive mode, as it is placed on the bottom of the chart. This specific antenna has the capacitive mode at all operation frequencies. Another type of antenna can have only an inductive or both modes.
The idea of this examples is to show how the impedance changes versus the operation frequency in a span of 20 MHz and correlates with SWR level.
Users can see marker M1 (at 868 MHz) has the shortest distance to the center of the chart with the lowest SWR value of 1.54:1 (Figure 4-3).