Question about antenna matching (13.56MHz) - Page 1
Forum section: electronics / rf-microwave

author: snx
author_rank: Contributor
timestamp: July 15, 2019, 04:37:40 am
content: I have to do an antenna for a RFID Reader 13.56Mhz. As most IC manufacturers write in their app notes that the impedance matching is cricital, i got into the topic. But there are a certain points i do not completely understand, hopefull i get advice here.First of all, most datasheets and appnotes point out that the cricital points for the systemem are:Antenna Quality (Q): The lower the Q (eg 10) the higher bit-rate is possible, while higher Q's tend to have better field strenght.Impedance Z: This lower the impedance, the higher the current goes trough the antenna, therefore better field strenght.In order to get a working system, Q and Z must be selected and matched.Lets take this example: You only want to read mifare cards, which requires Q<=30. Since your application has access to DC power, you can select the lowest allowed Z, since power is not an issue (compared to battery powered applications.There are a few calculators for the needed parts, but they are not accurate. Usually they output values that are not E-Series. So you use the next available E Series.In our example, we would hit Q=25.68 and Z=22.34Ohm with E-Series values.This would give the following current into the antenna: 3.3V / 21.34Ohm = 154mAIn case we would adjust the Z to 20Ohm, which would result in paralel capacitors, to reach non E-Series Values, we would get 3.3V/20Ohm = 165mA into the antenna. Thats roughly only 10mA more.In a NXP app note, they want to hit 25Ohm Z. And a result is shown as "bad" with 23.75Ohms, and then a corrected one "good" with 25.57Ohms.What is the point in trying to match to a such specific value in the low-Z area? I understand that if you using batteries and you match in the 50-80Ohms are that current savings are vital, but if you match for 25Ohms, im sure your supply can deliver a bit more for 23.75, in fact thats only a few mA!I do not fully understand why its so cricital to match to a certain value, any advice? In case you have DC supply it doesnt matter that much how high the current is by a few mA difference. So it might be better to just accept the result that was done with standart E-Series parts.Any tipps / explanations here?P.S. Does anyone has a link or similar, that explains what exactly the S11 Smith diagram shows, and why its in use? When i matched my antenna, i could extract all parameters from values, i did not need the smith diagram at all.

author: radiolistener
author_rank: Super Contributor
timestamp: July 15, 2019, 06:33:42 am
content: Quote from: snx on July 15, 2019, 04:37:40 amyour definitions are not clear, for impedance it is even not correct at all. Let's explain it:=== Impedace - Z ===The better field strength will occurs when RF energy is delivered with minimum loss and with no reflections from the load. It happens at specific U / I relation only. Note, that U / I = R - this is impedance of the source or load. So, this is why you're need exactly the same R of source and R of load in order to deliver energy with maximum efficiency.There is active R and reactive X components of resistance. Together they are named impedance Z = R + jX.When X < 0 it means that reactive resistance is due to capacitance.When X > 0 it means that reactive resistance is due to inductance.When X = 0 it means that there is no reactive resistance.For example, impedance Z = 50 + j0, means 50 Ohm active resistance and 0 Ohm reactive resistance.In order to deliver RF energy from source to the load, your source and load should havethe same impedance Z. If source impedance is different than load impedance, thenRF energy reflection occursat the border of two different impedance. For example at the point where you connect 50 Ohm coax cable to 75 Ohm antenna.You can evaluate reflection with SWR parameter (Standing Wave Ratio):SWR = (1 + |Г|) / (1 - |Г|)where Г is reflection coefficient:Г = (Z1 - Z2) / (Z1 + Z2)where Z1 and Z2 is impedance of your source and impedance of your load.For active impedance (Z = R + j0, when circuit doesn't have reactive component), you can simplify and calculate it in the following way:SWR = max(R1, R2) / min(R1, R2)where R1 and R2 is your source and load resistance.- The best efficiency happens when SWR = 1.00.- The value SWR < 1.5 is accepted for typical radio equipment, but it's better to keep it below 1.2.- The value SWR > 2 is not accepted, you're need to use impedance matching circuit in such case.Let's look what happens when you connect 50 Ohm coax cable to the antenna with different Z:1) If antenna impedance is 25 Ohm it leads to SWR = 50 / 25 = 2. This is not acceptable, you're need impedance matching circuit2) If antenna impedance is 50 Ohm it leads to SWR = 50 / 50 = 1.00. The best result, your antenna will works with maximum efficiency.3) If antenna impedance is 100 Ohm it leads to SWR = 100 / 50 = 2. This is not acceptable, you're need impedance matching circuitSo, you're need to use SWR = 1.00 in order to get the best efficiency. If SWR is higher than 1.5, you're needs to use impedance matching circuit (antenna tuner) in order to match different impedance of signal source and load.=== Quality factor - Q factor ===Quality factor of LC circuit shows how many oscillation cycles will occurs when you apply an impulse.Smaller energy loss means higher Q factor. Higher energy loss means lower Q factor.Since LC keeps oscillating for Q cycles after an impulse, it reduces frequency bandwidth which you can transfer through this circuit with minimum energy loss.LC circuit works as bandpass filter, it accumulates energy around resonant frequency during Q cycles and then needs Q cycles to spend it. So, it's behavior is similar with average function by N elements. But average function works around DC, and LC bandpass filter works around it's resonant frequency.The bandwidth of LC circuit (antenna) is:Bandwidth = Fc / Qwhere Fc is the center frequency of interest, in your case Fc = 13.56 MHz.So, you can evaluate maximum allowed Q factor for your system:Q = Fc / BandwidthFor example, if your device requires 2.5 kHz bandwidth at 13.56 MHz, then the maximum allowed Q:Q = 13560000 / 2500 = 5424This is too high. Usual half wave-length dipole has about Q=10..13. High Q factor resonant antenna such as magnetic loop may reach value of Q about 2000, it requires thick copper pipes and even silver plating to reduce energy loss. Antenna with high Q factor has too small bandwidth and needs to be tuned very often. But it has better efficiency in comparison with the same size antenna with lower Q.Too small Q factor of antenna will leads to worse antenna radiation/reception efficiency, but it can be used for more wide bandwidth.If you add proper LC circuit between your device and antenna, it allows you to increase Q factor and match antenna impedance with your device. This is how impedance matching circuit (for example antenna tuner) works.

author: RoGeorge
author_rank: Super Contributor
timestamp: July 15, 2019, 06:46:55 am
content: Quote from: snx on July 15, 2019, 04:37:40 amLink and page?

author: radiolistener
author_rank: Super Contributor
timestamp: July 15, 2019, 08:40:00 am
content: Here is also relation which couple together SWR, Q and BW.BW = (SWR - 1) / (Q * sqrt(SWR))where BW is bandwidth for Fc = 1.For Hz, you can use this one:BW = Fc * (SWR - 1) / (Q * sqrt(SWR))SourceYou can measure Q factor of your antenna with SWR meter or vector analyzer.Just measure your antenna bandwidth by SWR level = 2.618 (it equals to 3 dB attenuation) and use:Q = Fc / BWFor example, according to SWR meter, your antenna has bandwidth 400 kHz at 3600 kHz, measured at level SWR=2.618.Q = 3600 / 400 = 9Let's check it:BW = (2.618 - 1) / (9 * sqrt(2.618)) = 0.11110.1111 * 3600 kHz = 399.96 kHzSo, in such case your antenna has Q = 9You can find exact SWR value for 3 dB level in the following way:BW = (SWR - 1) / (Q * sqrt(SWR))Q = 1/BW=>(SWR - 1) / sqrt(SWR) = 1=>SWR = 3 / 2 + sqrt(5) / 2 = ~2.618034...

author: snx
author_rank: Contributor
timestamp: July 15, 2019, 02:45:00 pm
content: I understand this principle for transmission antennas and stuff, but not for an NFC reader, where there is no "50Ohm output " or so. The current that goes into the antenna is only limited by thermal and silicon limits. The transmitter will just switch VCC to the antenna via a fet or so.Here is a particial qoute from NXP AN11019The target impedance must be defined. For the maximum power transfer, it should be 20 Ω (as indicated in section 4.1.1). In such case the driver current ITVDD gets close to the maximum allowed limit of ITVDDmax = 250 mA....The impedance target might be different for different applications. An impedance target of Rmatch = 50…80 Ω might be better in battery powered devices, where the current consumption must be minimum while at the same time the required maximum operating distance can be lower.This tells me, that every impedance between 20 and 80 Ohms has to work, its just a tradeoff between consumption and reading distance.About the Q the following can found in the app noteTable 1. Q-factorValues for the passive linear antenna circuit only.QCondition10Start value for the matching calculation for NFC Reader design, supporting higher bit rates (212, 424, 848 kbit/s).15Start value for the matching calculation for NFC Reader design, supporting higher bit rates (212, 424 kbit/s).20Start value for the matching calculation for typical NFC Reader design.25Start value for the matching calculation for typical NFC Reader design, limited to 106 kbit/s.30Nominal value for MIFARE Classic communication.In case your measured antenna is Q=30, and you want to use 848kbits, you have to adjust down to 10. If your antenna is Q=23 and you only want MIFARE, its fine!My worries are at another point, all app notes focus on Q and Z, rather than Return-Loss.I have a sample antenna that has -25db return loss, and the peak is right at 13.56Mhz, another antenna has a return loss of -10db, but the peak is at around 14.5Mhz (-30db). So i guess it would be good that the peak is right at 13.56MHz and as deep as possible, right?Why do appnotes not tell you how to shift that peak rather than focussing on matching Z to a tight value.On AN11019, Page 29, they adjust impedance from 23.75 to 25.57 Ohms, and the Smith Chart does not change very much...